Classical Genetics

Class 12 | Biology — Botany | Chapter 2

Class 12 Unit 2 Biology

Chapter 2: Classical Genetics

Learning Objectives

The Learner will be able to:

• Differentiate classical and modern genetics.
• Understand the concepts of principles of inheritance.
• Describe the extensions of Mendelism.
• Explain polygenic inheritance and Pleiotropy.
• Analyze extra chromosomal inheritance in cytoplasmic organelles.

Chapter Outline

• 2.1 Heredity and Variation
• 2.2 Mendelism
• 2.3 Monohybrid, Dihybrid cross, Backcross and Testcross
• 2.4 Interaction of Genes
• 2.5 Polygenic inheritance in Wheat kernel colour, Pleiotropy – Pisum sativum
• 2.6 Extra chromosomal inheritance – Cytoplasmic inheritance in Chloroplast
• 2.7 Atavism

Genetics is the study of how living things receive common traits from previous generations. No field of science has changed the world more, in the past 50 years than genetics. The scientific and technological advances in genetics have transformed agriculture, medicine and forensic science etc.

Genetics – The Science of Heredity (Inheritance)

"Genetics" is the branch of biological science which deals with the mechanism of transmission of characters from parents to offsprings. The term Genetics was introduced by W. Bateson in 1906.

The four major subdisciplines of genetics are:

1. Transmission Genetics / Classical Genetics – Deals with the transmission of genes from parents to offsprings. The foundation of classical genetics came from the study of hereditary behaviour of seven genes by Gregor Mendel.
2. Molecular Genetics – Deals with the structure and function of a gene at molecular level.
3. Population Genetics – Deals with heredity in groups of individuals for traits which is determined by a few genes.
4. Quantitative Genetics – Deals with heredity of traits in groups of individuals where the traits are governed by many genes simultaneously.

What is the reason for similarities, differences of appearance and skipping of generations?

Genes – Functional Units of Inheritance

The basic unit of heredity (biological information) which transmits biochemical, anatomical and behavioural traits from parents to offsprings.

2.1 Heredity and Variation

Genetics is often described as a science which deals with heredity and variation.

Heredity

Heredity is the transmission of characters from parents to offsprings.

Variation

The organisms belonging to the same natural population or species that shows a difference in the characteristics is called variation. Variation is of two types: (1) Discontinuous variation and (2) Continuous variation.

1. Discontinuous Variation

Within a population there are some characteristics which show a limited form of variation.

Example: Style length in Primula, plant height of garden pea.

In discontinuous variation, the characteristics are controlled by one or two major genes which may have two or more allelic forms. These variations are genetically determined by inheritance factors. Individuals produced by this variation show differences without any intermediate form between them and there is no overlapping between the two phenotypes. The phenotypic expression is unaffected by environmental conditions. This is also called as qualitative inheritance.

2. Continuous Variation

This variation may be due to the combining effects of environmental and genetic factors. In a population most of the characteristics exhibit a complete gradation, from one extreme to the other without any break. Inheritance of phenotype is determined by the combined effects of many genes (polygenes) and environmental factors. This is also known as quantitative inheritance.

Example: Human height and skin color.

Importance of Variations

• Variations make some individuals better fitted in the struggle for existence.
• They help the individuals to adapt themselves to the changing environment.
• It provides the genetic material for natural selection.
• Variations allow breeders to improve better yield, quicker growth, increased resistance and lesser input.
• They constitute the raw materials for evolution.

2.2 Mendelism

The contribution of Mendel to Genetics is called Mendelism. It includes all concepts brought out by Mendel through his original research on plant hybridization. Mendelian genetic concepts are basic to modern genetics. Therefore, Mendel is called as Father of Genetics.

2.2.1 Father of Genetics – Gregor Johann Mendel (1822 – 1884)

The first Geneticist, Gregor Johann Mendel unraveled the mystery of heredity. He was born on 22nd July 1822 in Heinzendorf Silesia (now Hyncice, Czechoslovakia), Austria. After school education, later he studied botany, physics and mathematics at the University of Vienna. He then entered a monastery of St. Thomas at Brunn in Austria and continued his interest in plant hybridization. In 1849 Mendel got a temporary position in a school as a teacher and he performed a series of elegant experiments with pea plants in his garden. In 1856, he started his historic studies on pea plants. 1856 to 1863 was the period of Mendel's hybridization experiments on pea plants.

Mendel discovered the principles of heredity by studying the inheritance of seven pairs of contrasting traits of pea plant in his garden. Mendel crossed and catalogued 24,034 plants through many generations. His paper entitled "Experiments on Plant Hybrids" was presented and published in The Proceedings of the Brunn Society of Natural History in 1866. Mendel was the first systematic researcher in the field of genetics.

Mendel was successful because:

• He applied mathematics and statistical methods to biology and laws of probability to his breeding experiments.
• He followed scientific methods and kept accurate and detailed records that include quantitative data of the outcome of his crosses.
• His experiments were carefully planned and he used large samples.
• The pairs of contrasting characters which were controlled by factor (genes) were present on separate chromosomes.
• The parents selected by Mendel were pure breed lines and the purity was tested by self crossing the progeny for many generations.

Mendel's Experimental System – The Garden Pea

He chose pea plant because:

• It is an annual plant and has clear contrasting characters that are controlled by a single gene separately.
• Self-fertilization occurred under normal conditions in garden pea plants.
• Mendel used both self-fertilization and cross fertilization.
• The flowers are large hence emasculation and pollination are very easy for hybridization.

2.2.2 Mendel's Experiments on Pea Plant

Mendel's theory of inheritance, known as the Particulate theory, establishes the existence of minute particles or hereditary units or factors, which are now called as genes. He performed artificial pollination or cross pollination experiments with several true-breeding lines of pea plants.

A true breeding line (Pure-breeding strains) means it has undergone continuous self pollination having stable trait inheritance from parent to offspring. Matings within pure breeding lines produce offsprings having specific parental traits that are constant in inheritance and expression for many generations. Pure line breed refers to homozygosity only.

Fusion of male and female gametes produced by the same individual that is pollen and egg are derived from the same plant is known as self-fertilization. Self pollination takes place in Mendel's peas.

The experimenter can remove the anthers (Emasculation) before fertilization and transfer the pollen from another variety of pea to the stigma of flowers where the anthers are removed. This results in cross-fertilization, which leads to the creation of hybrid varieties with different traits.

Mendel's work on the study of the pattern of inheritance and the principles or laws formulated, now constitute the Mendelian Genetics.

The First Model Organism in Genetics – Garden Peas (Pisum sativum)

Seven characters studied by Mendel:

Character	Gene	Dominant Trait	Recessive Trait
Plant height	Le	Tall	Dwarf
Pod shape	V	Inflated	Constricted
Seed shape	R	Round	Wrinkled
Cotyledon Colour	I	Yellow	Green
Flower position	Fa	Axial	Terminal
Flower colour	A	Purple	White
Pod colour	GP	Green	Yellow

Table 2.1: Seven characters of Pisum sativum with genes

Mendel worked at the rules of inheritance and arrived at the correct mechanism before any knowledge of cellular mechanism, DNA, genes, chromosomes became available. Mendel insights and meticulous work into the mechanism of inheritance played an important role which led to the development of improved crop varieties and a revolution in crop hybridization. Mendel died in 1884. In 1900 the work of Mendel's experiments were rediscovered by three biologists, Hugo de Vries of Holland, Carl Correns of Germany and Erich von Tschermak of Austria.

Do You Know?

Can you identify Mendel's gene for regulating white colour in peas?

In 2010, the gene responsible for regulating flower colour in peas were identified by an international team of researchers. It was called Pea Gene A which encodes a protein that functions as a transcription factor which is responsible for the production of anthocyanin pigment. So the flowers are purple. Pea plants with white flowers do not have anthocyanin, even though they have the gene that encodes the enzyme involved in anthocyanin synthesis.

Researchers delivered normal copies of gene A into the cells of the petals of white flowers by the gene gun method. When Gene A entered in a small percentage of cells of white flowers it is expressed in those particular cells, accumulate anthocyanin pigments and became purple. In white flowers the gene A sequence showed a single-nucleotide change that makes the transcription factor inactive. So the mutant form of gene A do not accumulate anthocyanin and hence they are white.

2.2.3 Terminology Related to Mendelism

Mendel noticed two different expressions of a trait – Example: Tall and dwarf. Traits are expressed in different ways due to the fact that a gene can exist in alternate forms (versions) for the same trait is called alleles.

• If an individual has two identical alleles of a gene, it is called as homozygous (TT).
• An individual with two different alleles is called heterozygous (Tt).
• Mendel's non-true breeding plants are heterozygous, called as hybrids.
• When the gene has two alleles the dominant allele is symbolized with capital letter and the recessive with small letter.
• When both alleles are recessive the individual is called homozygous recessive (tt) – dwarf pea plants.
• An individual with two dominant alleles is called homozygous dominant (TT) – tall pea plants.
• One dominant allele and one recessive allele (Tt) denotes non-true breeding tall pea plants – heterozygous tall.

2.2.4 Mendelian Inheritance – Mendel's Laws of Heredity

Mendel proposed two rules based on his observations on monohybrid cross, today these rules are called laws of inheritance. The first law is The Law of Dominance and the second law is The Law of Segregation. These scientific laws play an important role in the history of evolution.

The Law of Dominance

The characters are controlled by discrete units called factors which occur in pairs. In a dissimilar pair of factors one member of the pair is dominant and the other is recessive. This law gives an explanation to the monohybrid cross:

• (a) the expression of only one of the parental characters in F1 generation
• (b) the expression of both in the F2 generation

It also explains the proportion of 3:1 obtained at the F2.

The Law of Segregation (Law of Purity of Gametes)

Alleles do not show any blending, both characters are seen as such in the F2 generation although one of the characters is not seen in the F1 generation. During the formation of gametes, the factors or alleles of a pair separate and segregate from each other such that each gamete receives only one of the two factors. A homozygous parent produces similar gametes and a heterozygous parent produces two kinds of gametes each having one allele with equal proportion. Gametes are never hybrid.

2.3 Monohybrid Cross

Monohybrid inheritance is the inheritance of a single character (i.e. plant height). It involves the inheritance of two alleles of a single gene. When the F1 generation was selfed Mendel noticed that 787 of 1064 F2 plants were tall, while 277 of 1064 were dwarf. The dwarf trait disappeared in the F1 generation only to reappear in the F2 generation.

The term genotype is the genetic constitution of an individual. The term phenotype refers to the observable characteristic of an organism.

In a genetic cross the genotypes and phenotypes of offspring, resulting from combining gametes during fertilization can be easily understood with the help of a diagram called Punnett's Square named after a British Geneticist Reginald C. Punnett. It is a graphical representation to calculate the probability of all possible genotypes of offsprings in a genetic cross. The Law of Dominance and the Law of Segregation give suitable explanation to Mendel's monohybrid cross.

Figure 2.6: Monohybrid Cross

Generation	Genotype	Phenotype
Parent generation	TT (True Tall) × tt (True Dwarf)	Tall × Dwarf
F1 generation	Tt, Tt, Tt, Tt	All Tall
F1 Selfed (Tt × Tt)	–
F2 generation	TT : Tt : tt	Tall : Tall : Dwarf
Genotypic Ratio	1 TT : 2 Tt : 1 tt
Phenotypic Ratio	3 Tall : 1 Dwarf

Reciprocal Cross

In one experiment, the tall pea plants were pollinated with the pollens from a true-breeding dwarf plants, the result was all tall plants. When the parental types were reversed, the pollen from a tall plant was used to pollinate a dwarf pea plant which gave only tall plants. The result was the same – All tall plants.

Tall (T) × Dwarf (D) and Dwarf (D) × Tall (T) matings are done in both ways which are called reciprocal crosses. The results of the reciprocal crosses are the same. So it was concluded that the trait is not sex dependent. The results of Mendel's monohybrid crosses were not sex dependent.

The gene for plant height has two alleles: Tall (T) and Dwarf (t). The phenotypic and genotypic analysis of the crosses has been shown by Checker board method or by Forkline method.

2.3.1 Mendel's Analytical and Empirical Approach

Mendel chose two contrasting traits for each character. So it seemed logical that two distinct factors exist. In F1 the recessive trait and its factors do not disappear and they are hidden or masked only to reappear in ¼ of the F2 generation. He concluded that tall and dwarf alleles of F1 heterozygote segregate randomly into gametes. Mendel got 3:1 ratio in F2 between the dominant and recessive trait. He was the first scientist to use this type of quantitative analysis in a biological experiment. Mendel's data is concerned with the proportions of offspring.

2.3.2 Test Cross

Test cross is crossing an individual of unknown genotype with a homozygous recessive.

If Heterozygous Tall – Test Cross

Generation	Genotype	Phenotype
Parental (P)	Tt × tt	F1 Heterozygous Tall × Homozygous Dwarf
Gametes	T, t × t, t	–
F2 Offspring	Tt : tt	Tall : Dwarf
Genotypic Ratio	1 : 1
Phenotypic Ratio	1 Tall : 1 Dwarf

The genotypes of tall plants TT or Tt from F1 and F2 cannot be predicted. To determine the genotype of a tall plant Mendel crossed the plants from F2 with the homozygous recessive dwarf plant. This he called a test cross. The progenies of the test cross can be easily analysed to predict the genotype of the plant or the test organism.

Thus in a typical test cross an organism (pea plants) showing dominant phenotype (whose genotype is to be determined) is crossed with the recessive parent instead of self crossing. Test cross is used to identify whether an individual is homozygous or heterozygous for dominant character.

Do You Know?

Why Mendel's pea plants are tall and dwarf? Molecular characterization of Mendel's gene for plant height.

The plant height is controlled by a single gene with two alleles. The reason for this difference in plant height is due to the following facts:

1. The cells of the pea plant have the ability to convert a precursor molecule of gibberellins into an active form (GA1).
2. Tall pea plants have one allele (Le) that codes for a protein (functional enzyme) which functions normally in the gibberellin-synthesis pathway and catalyzes the formation of gibberellins (GA1). The allele is dominant even if it is two (Le Le) or single (Le le), it produces gibberellins and the pea plants are tall.
3. Dwarf pea plants have two recessive alleles (le le) which code for non-functional protein, hence they are dwarf.

Figure 2.8: Gene for plant height in Peas

2.3.3 Back Cross

Back cross is a cross of F1 hybrid with any one of the parental genotypes. The back cross is of two types; they are dominant back cross and recessive back cross. It involves the cross between the F1 offspring with either of the two parents.

When the F1 offsprings are crossed with the dominant parents all the F2 develop dominant character and no recessive individuals are obtained in the progeny. If the F1 hybrid is crossed with the recessive parent individuals of both the phenotypes appear in equal proportion and this cross is specified as test cross. The recessive back cross helps to identify the heterozygosity of the hybrid.

2.3.4 Dihybrid Cross

It is a genetic cross which involves individuals differing in two characters. Dihybrid inheritance is the inheritance of two separate genes each with two alleles.

Law of Independent Assortment

When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent to the other pair of characters. Genes that are located in different chromosomes assort independently during meiosis. Many possible combinations of factors can occur in the gametes. Independent assortment leads to genetic diversity.

If an individual produces genetically dissimilar gametes it is the consequence of independent assortment. Through independent assortment, the maternal and paternal members of all pairs were distributed to gametes, so all possible chromosomal combinations were produced leading to genetic variation. In sexually reproducing plants/organisms, due to independent assortment, genetic variation takes place which is important in the process of evolution.

The Law of Segregation is concerned with alleles of one gene but the Law of Independent Assortment deals with the relationship between genes.

The crossing of two plants differing in two pairs of contrasting traits is called dihybrid cross. In dihybrid cross, two characters (colour and shape) are considered at a time. Mendel considered the seed shape (round and wrinkled) and cotyledon colour (yellow & green) as the two characters. In seed shape round (R) is dominant over wrinkled (r); in cotyledon colour yellow (Y) is dominant over green (y). Hence the pure breeding round yellow parent is represented by the genotype RRYY and the pure breeding green wrinkled parent is represented by the genotype rryy. During gamete formation the paired genes of a character assort out independently of the other pair.

Figure 2.10: Dihybrid Cross in Garden Peas

Generation	Genotype	Phenotype
Parent Generation	RRYY × rryy	Round Yellow × Wrinkled Green
Gametes	RY × ry	–
F1 Generation	RrYy	All Round Yellow
F1 Selfed (RrYy × RrYy)	Four types of gametes: RY, Ry, rY, ry
F2 Phenotypic Ratio	9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green

During the F1 by F1 fertilization each zygote with an equal probability receives one of the four combinations from each parent. The resultant gametes thus will be genetically different and they are of the following four types:

1. Yellow round (YR) – 9/16
2. Yellow wrinkled (Yr) – 3/16
3. Green round (yR) – 3/16
4. Green wrinkled (yr) – 1/16

These four types of gametes of F1 dihybrids unite randomly in the process of fertilization and produce sixteen types of individuals in F2 in the ratio of 9:3:3:1.

Mendel's 9:3:3:1 dihybrid ratio is an ideal ratio based on the probability including segregation, independent assortment and random fertilization.

Do You Know?

How does the wrinkled gene make Mendel's peas wrinkled?

The protein called starch branching enzyme (SBEI) is encoded by the wild-type allele of the gene (RR) which is dominant. When the seed matures, this enzyme SBEI catalyzes the formation of highly branched starch molecules. Normal gene (R) has become interrupted by the insertion of extra piece of DNA (0.8 kb) into the gene, resulting in r allele. In the homozygous mutant form of the gene (rr) which is recessive, the activity of the enzyme SBEI is lost resulting in wrinkled peas.

The wrinkled seed accumulates more sucrose and high-water content. Hence the osmotic pressure inside the seed rises. As a result, the seed absorbs more water and when it matures it loses water as it dries. So, it becomes wrinkled at maturation. When the seed has at least one copy of normal dominant gene heterozygous, the dominant allele helps to synthesize starch, amylopectin an insoluble carbohydrate, with the osmotic balance which minimises the loss of water resulting in smooth structured round seed.

Figure 2.11: Molecular explanation of round and wrinkled peas.

2.3.5 The Dihybrid Test Cross

The F1 hybrid plant (round yellow peas) is crossed with homozygous double recessive genotype, wrinkled green peas (rryy). This is called dihybrid test cross with the ratio of 1 : 1 : 1 : 1.

Figure 2.12: Dihybrid Test Cross

Cross	Genotype	Phenotype
Parental (P)	RrYy × rryy	F1 Heterozygous Round Yellow × Homozygous Wrinkled Green
F2 Offspring 1	RrYy	Round Yellow
F2 Offspring 2	Rryy	Round Green
F2 Offspring 3	rrYy	Wrinkled Yellow
F2 Offspring 4	rryy	Wrinkled Green
Genotypic Ratio	1 : 1 : 1 : 1
Phenotypic Ratio	25% : 25% : 25% : 25%

2.3.6 Extensions of Mendelian Genetics

Apart from monohybrid, dihybrid and trihybrid crosses, there are exceptions to Mendelian principles, that is, the occurrence of different phenotypic ratios. The more complex patterns of inheritance are the extensions of Mendelian Genetics. There are examples where phenotype of the organism is the result of the interactions among genes.

2.4 Gene Interaction

A single phenotype is controlled by more than one set of genes, each of which has two or more alleles. This phenomenon is called Gene Interaction. Many characteristics of the organism including structural and chemical which constitute the phenotype are the result of interaction between two or more genes.

Mendelian experiments prove that a single gene controls one character. But in the post Mendelian findings, various exceptions have been noticed, in which different types of interactions are possible between the genes. This gene interaction concept was introduced and explained by W. Bateson. This concept is otherwise known as Factor hypothesis or Bateson's factor hypothesis.

According to Bateson's factor hypothesis, the gene interactions can be classified as:

• Intragenic gene interactions or Intra allelic or allelic interactions
• Intergenic gene interactions or inter allelic or non-allelic interactions

Gene Interactions – Classification

I. Intralocus interactions (Allelic interactions)

1. Dominant relationship:
   • a. Complete dominance – Example: Tall and dwarf pea plants
   • b. Incomplete dominance
   • c. Codominance
   • d. Over dominance
2. Lethal Genes:
   • a. Dominant lethals
   • b. Recessive lethals
   • c. Conditional lethals
   • d. Sex linked lethals
   • e. Balanced lethals
3. Multiple alleles

Figure 2.13: Gene Interaction

2.4.1 Intragenic Interactions

Interactions take place between the alleles of the same gene that is, alleles at the same locus is called intragenic or intralocus gene interaction. It includes the following:

1. Incomplete dominance
2. Codominance
3. Multiple alleles
4. Pleiotropic genes

These are common examples for intragenic interaction.

a. Incomplete Dominance – No Blending of Genes

The German Botanist Carl Correns's (1905) Experiment – In 4 O'clock plant, Mirabilis jalapa, when the pure breeding homozygous red (R1R1) parent is crossed with homozygous white (R2R2), the phenotype of the F1 hybrid is heterozygous pink (R1R2). The F1 heterozygous phenotype differs from both the parental homozygous phenotype. This cross did not exhibit the character of the dominant parent but an intermediate colour pink.

When one allele is not completely dominant to another allele it shows incomplete dominance. Such allelic interaction is known as incomplete dominance. F1 generation produces intermediate phenotype pink coloured flower. When pink coloured plants of F1 generation were interbred in F2 both phenotypic and genotypic ratios were found to be identical as 1 : 2 : 1 (1 red : 2 pink : 1 white). Genotypic ratio is 1 R1R1 : 2 R1R2 : 1 R2R2.

From this we conclude that the alleles themselves remain discrete and unaltered proving the Mendel's Law of Segregation. The phenotypic and genotypic ratios are the same. There is no blending of genes. In the F2 generation R1 and R2 genes segregate and recombine to produce red, pink and white in the ratio of 1 : 2 : 1.

• R1 allele codes for an enzyme responsible for the formation of red pigment.
• R2 allele codes for defective enzyme.
• R1 and R2 genotypes produce only enough red pigments to make the flower pink.
• Two R1R1 are needed for producing red flowers.
• Two R2R2 genes are needed for white flowers.

If blending had taken place, the original pure traits would not have appeared and all F2 plants would have pink flowers. It is very clear that Mendel's particulate inheritance takes place in this cross which is confirmed by the reappearance of original phenotype in F2.

Figure 2.14: Incomplete dominance in 4 O'clock plant

b. Codominance (1 : 2 : 1)

This pattern occurs due to simultaneous (joint) expression of both alleles in the heterozygote. The phenomenon in which two alleles are both expressed in the heterozygous individual is known as codominance.

Example: Red and white flowers of Camellia, inheritance of sickle cell haemoglobin, ABO blood group system in human beings.

In human beings, IA and IB alleles of I gene are codominant which follows Mendel's law of segregation. The codominance was demonstrated in plants with the help of electrophoresis or chromatography for protein or flavonoid substance.

Example: Gossypium hirsutum and Gossypium sturtianum, their F1 hybrid (amphiploid) was tested for seed proteins by electrophoresis. Both the parents have different banding patterns for their seed proteins. In hybrids, additive banding pattern was noticed. Their hybrid shows the presence of both the types of proteins similar to their parents. The heterozygote genotype gives rise to a phenotype distinctly different from either of the homozygous genotypes. The F1 heterozygotes produce a F2 progeny in a phenotypic and genotypic ratios of 1 : 2 : 1.

Do You Know?

How are we going to interpret the lack of dominance and give explanation to the intermediate heterozygote phenotype? How will you explain incomplete dominance at the molecular level?

Gene expression is explained in a quantitative way.

• Wild-type allele which is a functional allele when present in two copies (R1R1) produces a functional enzyme which synthesizes red pigments.
• The mutant allele which is a defective allele in two copies (R2R2) produces an enzyme which cannot synthesize necessary red pigments. The white flower is due to the mutation causing complete loss of function.
• The F1 intermediate phenotype heterozygote (R1R2) has one copy of the allele R1. R1 produces 50% of the functional protein resulting in half of the pigment of red flowered plant and so it is pink.
• The intermediate phenotype pink heterozygote with 50% of functional protein is not enough to create the red phenotype homozygous, which makes 100% of the functional protein.

c. Lethal Genes

An allele which has the potential to cause the death of an organism is called a "Lethal Allele". In 1907, E. Baur reported a lethal gene in snapdragon (Antirrhinum sp.). It is an example for recessive lethality.

In snapdragon there are three kinds of plants:

1. Green plants with chlorophyll (CC)
2. Yellowish green plants with carotenoids – referred to as pale green, golden or aurea plants (Cc)
3. White plants without any chlorophyll (cc)

The genotype of the homozygous green plants is CC. The genotype of the homozygous white plant is cc. The aurea plants have the genotype Cc because they are heterozygous of green and white plants.

When two such aurea plants are crossed the F1 progeny has identical phenotypic and genotypic ratio of 1 : 2 : 1 (that is, 1 Green (CC) : 2 Aurea (Cc) : 1 White (cc)).

Since the white plants lack chlorophyll pigment, they will not survive. So the F2 ratio is modified into 1 : 2. In this case the homozygous recessive genotype (cc) is lethal.

The term "lethal" is applied to those changes in the genome of an organism which produces effects severe enough to cause death. Lethality is a condition in which the death of certain genotype occurs prematurely. The fully dominant or fully recessive lethal allele kills the carrier individual only in its homozygous condition. So the F2 genotypic ratio will be 2 : 1 or 1 : 2 respectively.

Figure 2.15: Lethal genes

d. Pleiotropy – A Single Gene Affects Multiple Traits

In Pleiotropy, the single gene affects multiple traits and alter the phenotype of the organism. The Pleiotropic gene influences a number of characters simultaneously and such genes are called pleiotropic gene. When a variety of peas with purple flowers, dark-coloured seeds and a leaf axil spot were crossed with a variety of peas having white flowers, light coloured seeds and no spot on the axils of the leaves, the three traits for flower colour, seed colour and a leaf axil spot all were inherited together as a single unit. Another example is: sickle cell anemia.

2.4.2 Intergenic Gene Interactions

Interlocus interactions take place between the alleles at different loci i.e. between alleles of different genes. It includes the following:

Dominant Epistasis

It is a gene interaction in which two alleles of a gene at one locus interfere and suppress or mask the phenotypic expression of a different pair of alleles of another gene at another locus. The gene that suppresses or masks the phenotypic expression of a gene at another locus is known as epistatic. The gene whose expression is interfered by non-allelic genes and prevents from exhibiting its character is known as hypostatic. When both the genes are present together, the phenotype is determined by the epistatic gene and not by the hypostatic gene.

In the summer squash the fruit colour locus has a dominant allele 'W' for white colour and a recessive allele 'w' for coloured fruit. 'W' allele is dominant that masks the expression of any colour. In another locus hypostatic allele 'G' is for yellow fruit and its recessive allele 'g' for green fruit. In the first locus the white is dominant to colour whereas in the second locus yellow is dominant to green.

When the white fruit with genotype WWgg is crossed with yellow fruit with genotype wwGG, the F1 plants have white fruit and are heterozygous (WwGg). When F1 heterozygous plants are crossed they give rise to F2 with the phenotypic ratio of 12 white : 3 yellow : 1 green.

Phenotype	Ratio (F2)
White fruit	12/16
Yellow fruit	3/16
Green fruit	1/16

Since W is epistatic to the allele's 'G' and 'g', the white which is dominant, masks the effect of yellow or green. Homozygous recessive ww genotypes only can give the coloured fruits (4/16). Double recessive 'wwgg' will give green fruit (1/16). The Plants having only 'G' in its genotype (wwGg or wwGG) will give the yellow fruit (3/16).

Figure 2.16: Dominant epistasis in summer squash

Summary Table: Intragenic (Allelic) Interactions

S.No.	Gene Interaction	Example	F2 Phenotypic Ratio
1	Incomplete Dominance	Flower colour in Mirabilis jalapa; Flower colour in snapdragon (Antirrhinum spp.)	1 : 2 : 1
2	Codominance	ABO Blood group system in humans	1 : 2 : 1

Summary Table: Intergenic (Non-allelic) Interactions

S.No.	Epistatic Interaction	Example	F2 Phenotypic Ratio
1	Dominant epistasis	Fruit colour in summer squash	12 : 3 : 1
2	Recessive epistasis	Flower colour of Antirrhinum spp.	9 : 3 : 4
3	Duplicate genes with cumulative effect	Fruit shape in summer squash	9 : 6 : 1
4	Complementary genes	Flower colour in sweet peas	9 : 7
5	Supplementary genes	Grain colour in Maize	9 : 3 : 4
6	Inhibitor genes	Leaf colour in rice plants	13 : 3
7	Duplicate genes	Seed capsule shape (fruit shape) in shepherd's purse Bursa bursa-pastoris	15 : 1

2.5 Polygenic Inheritance in Wheat (Kernel Colour)

Polygenic Inheritance

Several genes combine to affect a single trait. A group of genes that together determine (contribute) a characteristic of an organism is called polygenic inheritance. It gives explanations to the inheritance of continuous traits which are compatible with Mendel's Law.

The first experiment on polygenic inheritance was demonstrated by Swedish Geneticist H. Nilsson-Ehle (1909) in wheat kernels. Kernel colour is controlled by two genes each with two alleles, one with red kernel colour was dominant to white. He crossed the two pure breeding wheat varieties dark red and a white. Dark red genotypes R1R1R2R2 and white genotypes are r1r1r2r2. In the F1 generation medium red were obtained with the genotype R1r1R2r2. F1 wheat plant produces four types of gametes R1R2, R1r2, r1R2, r1r2.

The intensity of the red colour is determined by the number of R genes in the F2 generation:

• Four R genes: A dark red kernel colour is obtained.
• Three R genes: Medium-dark red kernel colour is obtained.
• Two R genes: Medium red kernel colour is obtained.
• One R gene: Light red kernel colour is obtained.
• Absence of R gene: Results in White kernel colour.

The R gene in an additive manner produces the red kernel colour. The number of each phenotype is plotted against the intensity of red kernel colour which produces a bell-shaped curve. This represents the distribution of phenotype.

Other example: Height and skin colour in humans are controlled by three pairs of genes.

Figure 2.16(a): Polygenic inheritance in wheat kernel colour

Figure 2.17(b): The Genetic Control of Colour in Wheat Kernels

F2 Genotype	Number of R Genes	Phenotype
R1R1R2R2	4	Dark red
R1R1R2r2 / R1r1R2R2 (total 4)	3	Medium dark red
R1r1R2r2 / R1R1r2r2 / r1r1R2R2 (total 6)	2	Medium red
R1r1r2r2 / r1r1R2r2 (total 4)	1	Light red
r1r1r2r2	0	White

Conclusion

Finally the loci that was studied by Nilsson-Ehle were not linked and the genes assorted independently. Later, researchers discovered the third gene that also affects the kernel colour of wheat. The three independent pairs of alleles were involved in wheat kernel colour. Nilsson-Ehle found the ratio of 63 red : 1 white in F2 generation – 1 : 6 : 15 : 20 : 15 : 6 : 1 in F2 generation.

From the above results Nilsson-Ehle showed that the blending inheritance was not taking place in the kernel of wheat. In F2 generation plants have kernels with wide range of colour variation. This is due to the fact that the genes are segregating and recombination takes place. Another evidence for the absence of blending inheritance is that the parental phenotypes dark red and white appear again in F2. There is no blending of genes, only the phenotype.

The cumulative effect of several pairs of gene interaction gives rise to many shades of kernel colour. He hypothesized that the two loci must contribute additively to the kernel colour of wheat. The contribution of each red allele to the kernel colour of wheat is additive.

Figure 2.18: Polygenic inheritance in Wheat kernel (Three gene pairs – F2 ratio: 63 red : 1 white)

2.6 Extra Chromosomal Inheritance or Extra Nuclear Inheritance (Cytoplasmic Inheritance)

DNA is the universal genetic material. Genes located in nuclear chromosomes follow Mendelian inheritance. But certain traits are governed either by the chloroplast or mitochondrial genes. This phenomenon is known as extra nuclear inheritance. It is a kind of Non-Mendelian inheritance. Since it involves cytoplasmic organelles such as chloroplast and mitochondrion that act as inheritance vectors, it is also called Cytoplasmic inheritance. It is based on independent, self-replicating extra chromosomal unit called plasmogene located in the cytoplasmic organelles, chloroplast and mitochondrion.

Chloroplast Inheritance

It is found in 4 O'Clock plant (Mirabilis jalapa). In this, there are two types of variegated leaves namely dark green leaved plants and pale green leaved plants. When the pollen of dark green leaved plant (male) is transferred to the stigma of pale green leaved plant (female) and pollen of pale green leaved plant is transferred to the stigma of dark green leaved plant, the F1 generation of both the crosses must be identical as per Mendelian inheritance.

But in the reciprocal cross the F1 plant differs from each other. In each cross, the F1 plant reveals the character of the plant which is used as female plant. This inheritance is not through nuclear gene. It is due to the chloroplast gene found in the ovum of the female plant which contributes the cytoplasm during fertilization since the male gamete contributes only the nucleus but not cytoplasm.

Figure 2.19: Chloroplast inheritance

Figure 2.20: A cellular explanation of the variegated phenotype of the leaves in Mirabilis jalapa

Recently it has been discovered that cytoplasmic genetic male sterility is common in many plant species. This sterility is maintained by the influence of both nuclear and cytoplasmic genes. There are commonly two types of cytoplasm N (normal) and S (sterile). The genes for these are found in mitochondrion. There are also restorers of fertility (Rf) genes. Even though these genes are nuclear genes, they are distinct from genetic male sterility genes of other plants. Because the Rf genes do not have any expression of their own, unless the sterile cytoplasm is present. Rf genes are required to restore fertility in S cytoplasm which is responsible for sterility. So, the combination of N cytoplasm with rfrf and S cytoplasm with RfRf produces plants with fertile pollens, while S cytoplasm with rfrf produces only male sterile plants.

2.7 Atavism

Atavism is a modification of a biological structure whereby an ancestral trait reappears after having been lost through reemergence of sexual reproduction in the flowering plant Hieracium pilosella is the best example for Atavism in plants.

Summary

• Gregor Johann Mendel, father of Genetics unraveled the mystery of heredity through his experiments on garden peas.
• Mendel's laws, analytical and empirical reasoning endure till now guiding geneticists to study variation.
• The monohybrid cross of Mendel proved his particulate theory of inheritance. In F2 the alternative traits were expressed in the ratio of 3 dominant and 1 recessive. The characteristic 3 : 1 segregation is referred to as Mendelian ratio.
• Parents transmit discrete information about the traits to their offspring which Mendel called it as "factors".
• To test his experimental results Mendel devised a powerful procedure called the test cross. Test cross is used to determine the genotype of an individual when two genes are involved.
• In Mendel's dihybrid cross, the two pairs of factors were inherited independently. From the results of dihybrid cross Mendel gave the Law of Independent Assortment.
• Mendel's dihybrid ratio of 9 : 3 : 3 : 1 with the representation of two new recombinations appeared in the progeny, i.e. round green peas or wrinkled yellow peas.
• Molecular explanation of Mendel's gene for monohybrid cross, dihybrid cross were explained.
• Extension of Mendelian Genetics was dealt with examples for interaction among genes.
• Incomplete dominance is not an example for blending inheritance. Incomplete dominance exhibits a phenotypic heterozygote intermediate between the two homozygous.
• In plants codominance can be demonstrated by the methods of electrophoresis or chromatography for protein or flavonoid substances.
• Lethal genes with an example are explained.
• Pleiotropy – a single gene which affects multiple traits was explained with an example of Pisum sativum.
• Dominant epistasis in summer squash with 12 : 3 : 1 ratio was discussed.
• Polygenic inheritance is an example for inheritance of continuous traits which is compatible with Mendel's laws.
• The inheritance of mitochondrial and chloroplast genes were explained with examples which does not follow the rules of nuclear genes.

Evaluation

Exercise MCQ – Choose the Correct Answer

1. Extra nuclear inheritance is a consequence of presence of genes in

a. Mitochondria and chloroplasts b. Endoplasmic reticulum and mitochondria c. Ribosomes and chloroplast d. Lysosomes and ribosomes

2. In order to find out the different types of gametes produced by a pea plant having the genotype AaBb, it should be crossed to a plant with the genotype

a. AABb b. AaBB c. AABB d. aabb

3. How many different kinds of gametes will be produced by a plant having the genotype AABbCC?

a. Three b. Four c. Nine d. Two

4. Which one of the following is an example of polygenic inheritance?

a. Flower colour in Mirabilis jalapa b. Production of male honey bee c. Pod shape in garden pea d. Skin colour in humans

5. In Mendel's experiments with garden pea, round seed shape (RR) was dominant over wrinkled seeds (rr), yellow cotyledon (YY) was dominant over green cotyledon (yy). What are the expected phenotypes in the F2 generation of the cross RRYY × rryy?

a. Only round seeds with green cotyledons b. Only wrinkled seeds with yellow cotyledons c. Only wrinkled seeds with green cotyledons d. Round seeds with yellow cotyledons and wrinkled seeds with yellow cotyledons

6. Test cross involves

a. Crossing between two genotypes with recessive trait b. Crossing between two F1 hybrids c. Crossing the F1 hybrid with a double recessive genotype d. Crossing between two genotypes with dominant trait

7. In pea plants, yellow seeds are dominant to green. If a heterozygous yellow seed plant is crossed with a green seeded plant, what ratio of yellow and green seeded plants would you expect in F1 generation?

a. 9:1 b. 1:3 c. 3:1 d. 50:50

8. Select the correct statement from the ones given below with respect to dihybrid cross

a. Tightly linked genes on the same chromosomes show very few combinations b. Tightly linked genes on the same chromosomes show higher combinations c. Genes far apart on the same chromosomes show very few recombinations d. Genes loosely linked on the same chromosomes show similar recombinations as the tightly linked ones

9. Which Mendelian idea is depicted by a cross in which the F1 generation resembles both the parents?

a. Incomplete dominance b. Law of dominance c. Inheritance of one gene d. Co-dominance

10. Fruit colour in squash is an example of

a. Recessive epistasis b. Dominant epistasis c. Complementary genes d. Inhibitory genes

11. In his classic experiments on Pea plants, Mendel did not use

a. Flowering position b. Seed colour c. Pod length d. Seed shape

12. The epistatic effect, in which the dihybrid cross 9:3:3:1 between AaBb × Aabb is modified as

a. Dominance of one allele on another allele of both loci b. Interaction between two alleles of different loci c. Dominance of one allele to another allele of same loci d. Interaction between two alleles of same loci

13. In a test cross involving F1 dihybrid flies, more parental type offspring were produced than the recombination type offspring. This indicates

a. The two genes are located on two different chromosomes b. Chromosomes failed to separate during meiosis c. The two genes are linked and present on the same chromosome d. Both of the characters are controlled by more than one gene

14. The genes controlling the seven pea characters studied by Mendel are known to be located on how many different chromosomes?

a. Seven b. Six c. Five d. Four

15. Which of the following explains how progeny can possess the combinations of traits that none of the parent possessed?

a. Law of segregation b. Chromosome theory c. Law of independent assortment d. Polygenic inheritance

16. "Gametes are never hybrid". This is a statement of

a. Law of dominance b. Law of independent assortment c. Law of segregation d. Law of random fertilization

17. Gene which suppresses other genes activity but does not lie on the same locus is called as

a. Epistatic b. Supplement only c. Hypostatic d. Codominant

18. Pure tall plants are crossed with pure dwarf plants. In the F1 generation, all plants were tall. These tall plants of F1 generation were selfed and the ratio of tall to dwarf plants obtained was 3:1. This is called

a. Dominance b. Inheritance c. Codominance d. Heredity

19. The dominant epistasis ratio is

a. 9:3:3:1 b. 12:3:1 c. 9:3:4 d. 9:6:1

20. Select the period for Mendel's hybridization experiments

a. 1856 to 1863 b. 1850 to 1870 c. 1857 to 1869 d. 1870 to 1877

21. Among the following characters which one was not considered by Mendel in his experimentation on pea?

a. Stem – Tall or dwarf b. Trichome – glandular or non-glandular c. Seed – Green or yellow d. Pod – Inflated or constricted Exercise Short Answer Questions

22. Name the seven contrasting traits of Mendel.

23. What is meant by true breeding or pure breeding lines/strain?

24. Give the names of the scientists who rediscovered Mendelism.

25. What is back cross?

26. Define Genetics.

27. What are multiple alleles?

28. What are the reasons for Mendel's successes in his breeding experiment?

Exercise Long Answer Questions

29. Explain the law of dominance in monohybrid cross.

30. Differentiate incomplete dominance and codominance.

31. What is meant by cytoplasmic inheritance?

32. Describe dominant epistasis with an example.

33. Explain polygenic inheritance with an example.

34. Differentiate continuous variation with discontinuous variation.

35. Explain with an example how single genes affect multiple traits and alter the phenotype of an organism.

36. Bring out the inheritance of chloroplast gene with an example.

Glossary

Alleles (n) Alternative forms of a gene. Back Cross (n) Crosses between F1 offsprings with either of the two parents (hybrid) are known as back cross. F1 / First Filial Generation (n) The second stage of Mendel's experiment is called F1 generation. Gene (n) The determinant of a characteristic of an organism (Mendelian factor). Gene symbols are underlined or italicized. Genetic Code (n) The set of 64 triplets of bases (codons) corresponding to the twenty amino acids in proteins and the signals for initiation and termination of polypeptide synthesis. Genotype (n) The types of alleles in a single individual is called genotype. Genome (n) The total complement of genes contained in a cell. Heterozygous (adj) Diploid organisms that have two different alleles at a specific gene locus are said to be heterozygous. Homozygous (adj) A diploid organism in which both alleles are the same at a given gene locus is said to be homozygous. Hybrid Vigour or Heterosis (n) The superiority of hybrid over either of its parents in one or more traits. Locus (n) The site or position of a particular gene on a chromosome. Phenotype (n) The physical expression of an individual's gene. The physical observable characteristics of an organism. Punnett Square / Checkerboard (n) A sort of cross-multiplication matrix used in the prediction of the outcome of a genetic cross, in which male and female gametes and their frequencies are arranged along the edges.

Book-back Exercises — Questions & Answers

Answers generated strictly from the Samacheer Kalvi textbook content. Attempt each question first, then click "Show Answer" to check.

I. Choose the Correct Answer

1. Extra nuclear inheritance is a consequence of presence of genes in

a) Mitochondria and chloroplasts b) Endoplasmic reticulum and mitochondria c) Ribosomes and chloroplast d) Lysosomes and ribosomes

Answer: a) Mitochondria and chloroplasts

Explanation: Extra nuclear inheritance involves cytoplasmic organelles such as chloroplast and mitochondrion that act as inheritance vectors, and is based on independent, self-replicating extra chromosomal units called plasmogenes located in these organelles.

2. In order to find out the different types of gametes produced by a pea plant having the genotype AaBb, it should be crossed to a plant with the genotype

a) AAbb b) AaBB c) AABB d) aabb

Answer: d) aabb

Explanation: Test cross is crossing an individual of unknown genotype with a homozygous recessive (aabb). The dihybrid test cross involves crossing the F1 hybrid with homozygous double recessive genotype to determine the types of gametes produced.

3. How many different kinds of gametes will be produced by a plant having the genotype AABbCC?

a) Three b) Four c) Nine d) Two

Answer: d) Two

Explanation: The genotype AABbCC is homozygous at loci A and C, so only the Bb locus produces different alleles. Therefore, only two types of gametes are produced: ABC and AbC.

4. Which one of the following is an example of polygenic inheritance?

a) Flower colour in Mirabilis Jalapa b) Production of male honey bee c) Pod shape in garden pea d) Skin colour in humans

Answer: d) Skin colour in humans

Explanation: Height and skin colour in humans are controlled by three pairs of genes and are examples of polygenic inheritance where several genes combine to affect a single trait.

5. In Mendel's experiments with garden pea, round seed shape (RR) was dominant over wrinkled seeds (rr), yellow cotyledon (YY) was dominant over green cotyledon (yy). What are the expected phenotypes in the F2 generation of the cross RRYY x rryy?

a) Only round seeds with green cotyledons b) Only wrinkled seeds with yellow cotyledons c) Only wrinkled seeds with green cotyledons d) Round seeds with yellow cotyledons and wrinkled seeds with yellow cotyledons

Answer: d) Round seeds with yellow cotyledons and wrinkled seeds with yellow cotyledons

Explanation: In the dihybrid cross, the F2 generation produces four phenotypic classes in the ratio 9:3:3:1 — round yellow, round green, wrinkled yellow, and wrinkled green. Round yellow and wrinkled yellow are both present in F2.

6. Test cross involves

a) Crossing between two genotypes with recessive trait b) Crossing between two F1 hybrids c) Crossing the F1 hybrid with a double recessive genotype d) Crossing between two genotypes with dominant trait

Answer: c) Crossing the F1 hybrid with a double recessive genotype

Explanation: Test cross is crossing an individual of unknown genotype with a homozygous recessive. An organism showing dominant phenotype is crossed with the recessive parent to determine its genotype.

7. In pea plants, yellow seeds are dominant to green. If a heterozygous yellow seed plant is crossed with a green seeded plant, what ratio of yellow and green seeded plants would you expect in F1 generation?

a) 9:1 b) 1:3 c) 3:1 d) 50:50

Answer: d) 50:50

Explanation: This is a test cross (heterozygous dominant × homozygous recessive). The recessive back cross gives individuals of both phenotypes in equal proportion, resulting in a 1:1 ratio (50:50).

8. Select the correct statement from the ones given below with respect to dihybrid cross

a) Tightly linked genes on the same chromosomes show very few combinations b) Tightly linked genes on the same chromosomes show higher combinations c) Genes far apart on the same chromosomes show very few recombinations d) Genes loosely linked on the same chromosomes show similar recombinations as the tightly linked ones

Answer: a) Tightly linked genes on the same chromosomes show very few combinations

Explanation: Genes that are located in different chromosomes assort independently during meiosis. Tightly linked genes on the same chromosome do not assort independently and therefore show very few recombination combinations.

9. Which Mendelian idea is depicted by a cross in which the F1 generation resembles both the parents?

a) Incomplete dominance b) Law of dominance c) Inheritance of one gene d) Co-dominance

Answer: d) Co-dominance

Explanation: Codominance is the phenomenon in which two alleles are both expressed in the heterozygous individual. The heterozygote shows characters of both parents simultaneously, so F1 resembles both parents.

10. Fruit colour in squash is an example of

a) Recessive epistasis b) Dominant epistasis c) Complementary genes d) Inhibitory genes

Answer: b) Dominant epistasis

Explanation: In summer squash, dominant epistasis occurs where the dominant allele 'W' for white colour masks the expression of colour alleles at another locus, giving the phenotypic ratio of 12 white : 3 yellow : 1 green.

11. In his classic experiments on Pea plants, Mendel did not use

a) Flowering position b) Seed colour c) Pod length d) Seed shape

Answer: c) Pod length

Explanation: The seven characters studied by Mendel in Pisum sativum were plant height, pod shape, seed shape, cotyledon colour, flower position, flower colour, and pod colour. Pod length was not among Mendel's seven studied characters.

12. The epistatic effect, in which the dihybrid cross 9:3:3:1 between AaBb into Aabb is modified as

a) Dominance of one allele on another allele of both loci b) Interaction between two alleles of different loci c) Dominance of one allele to another alleles of same loci d) Interaction between two alleles of same loci

Answer: b) Interaction between two alleles of different loci

Explanation: Interlocus interactions take place between the alleles at different loci, i.e., between alleles of different genes. Epistasis involves one gene at one locus suppressing or masking the phenotypic expression of a different pair of alleles at another locus.

13. In a test cross involving F1 dihybrid flies, more parental type offspring were produced than the recombination type offspring. This indicates

a) The two genes are located on two different chromosomes b) Chromosomes failed to separate during meiosis c) The two genes are linked and present on the same chromosome d) Both of the characters are controlled by more than one gene

Answer: c) The two genes are linked and present on the same chromosome

Explanation: Genes that are located in different chromosomes assort independently during meiosis giving equal parental and recombinant types. When more parental types appear, the genes are linked on the same chromosome.

14. The genes controlling the seven pea characters studied by Mendel are known to be located on how many different chromosomes?

a) Seven b) Six c) Five d) Four

Answer: d) Four

Explanation: The pairs of contrasting characters which were controlled by factors (genes) were present on separate chromosomes. Mendel's seven characters are shown on the plant's seven chromosomes, but they are located on four different chromosomes as some are on the same chromosome.

15. Which of the following explains how progeny can possess the combinations of traits that none of the parents possessed?

a) Law of segregation b) Chromosome theory c) Law of independent assortment d) Polygenic inheritance

Answer: c) Law of independent assortment

Explanation: Through independent assortment, the maternal and paternal members of all pairs were distributed to gametes, so all possible chromosomal combinations were produced leading to genetic variation and new combinations of traits not seen in parents.

16. "Gametes are never hybrid". This is a statement of

a) Law of dominance b) Law of independent assortment c) Law of segregation d) Law of random fertilization

Answer: c) Law of segregation

Explanation: According to the Law of Segregation (Law of Purity of gametes), during the formation of gametes, the factors or alleles of a pair separate and segregate from each other such that each gamete receives only one of the two factors. Gametes are never hybrid.

17. Gene which suppresses other genes' activity but does not lie on the same locus is called as

a) Epistatic b) Supplement only c) Hypostatic d) Codominant

Answer: a) Epistatic

Explanation: The gene that suppresses or masks the phenotypic expression of a gene at another locus is known as epistatic. It interferes with the expression of another gene located at a different locus.

18. Pure tall plants are crossed with pure dwarf plants. In the F1 generation, all plants were tall. These tall plants of F1 generation were selfed and the ratio of tall to dwarf plants obtained was 3:1. This is called

a) Dominance b) Inheritance c) Codominance d) Heredity

Answer: a) Dominance

Explanation: The Law of Dominance explains the expression of only one of the parental characters in F1 generation and the expression of both in F2 generation. It also explains the proportion of 3:1 obtained at the F2 in monohybrid cross.

19. The dominant epistasis ratio is

a) 9:3:3:1 b) 12:3:1 c) 9:3:4 d) 9:6:1

Answer: b) 12:3:1

Explanation: In dominant epistasis, as seen in summer squash fruit colour, when F1 heterozygous plants are crossed they give rise to F2 with the phenotypic ratio of 12 white : 3 yellow : 1 green.

20. Select the period for Mendel's hybridization experiments

a) 1856 to 1863 b) 1850 to 1870 c) 1857 to 1869 d) 1870 to 1877

Answer: a) 1856 to 1863

Explanation: 1856 to 1863 was the period of Mendel's hybridization experiments on pea plants, as stated in the chapter text.

21. Among the following characters which one was not considered by Mendel in his experimentation on pea?

a) Stem – Tall or dwarf b) Trichome – glandular or non-glandular c) Seed – Green or yellow d) Pod – Inflated or constricted

Answer: b) Trichome – glandular or non-glandular

Explanation: The seven characters studied by Mendel were plant height, pod shape, seed shape, cotyledon colour, flower position, flower colour, and pod colour. Trichome type was not among his seven studied characters.

II. Answer Briefly

22. Name the seven contrasting traits of Mendel.

Answer: The seven contrasting traits studied by Mendel in Pisum sativum are: (1) Plant height – Tall / Dwarf, (2) Pod shape – Inflated / Constricted, (3) Seed shape – Round / Wrinkled, (4) Cotyledon colour – Yellow / Green, (5) Flower position – Axial / Terminal, (6) Flower colour – Purple / White, (7) Pod colour – Green / Yellow.

23. What is meant by true breeding or pure breeding lines/strain?

Answer: A true breeding line (pure-breeding strain) means it has undergone continuous self pollination having stable trait inheritance from parent to offspring. Matings within pure breeding lines produce offsprings having specific parental traits that are constant in inheritance and expression for many generations. Pure line breed refers to homozygosity only.

24. Give the names of the scientists who rediscovered Mendelism.

Answer: In 1900, the work of Mendel's experiments were rediscovered by three biologists: Hugo de Vries of Holland, Carl Correns of Germany, and Erich von Tschermak of Austria.

25. What is back cross?

Answer: Back cross is a cross of F1 hybrid with any one of the parental genotypes. It is of two types: dominant back cross and recessive back cross. When F1 offspring are crossed with the dominant parent, all F2 develop dominant characters. When crossed with the recessive parent, individuals of both phenotypes appear in equal proportion; this recessive back cross is also called test cross.

26. Define Genetics.

Answer: Genetics is the branch of biological science which deals with the mechanism of transmission of characters from parents to offsprings. The term Genetics was introduced by W. Bateson in 1906. It is also described as a science which deals with heredity and variation.

27. What are multiple alleles?

Answer: Multiple alleles are a common example for intragenic interaction. A gene can exist in alternate forms (versions) for the same trait called alleles. When a gene has more than two allelic forms existing in a population, they are referred to as multiple alleles. Multiple alleles are a type of intralocus (intragenic) gene interaction.

28. What are the reasons for Mendel's success in his breeding experiment?

Answer: Mendel was successful because: (1) He applied mathematics, statistical methods, and laws of probability to his breeding experiments. (2) He followed scientific methods and kept accurate and detailed quantitative records. (3) His experiments were carefully planned and he used large samples. (4) The pairs of contrasting characters controlled by genes were present on separate chromosomes. (5) The parents selected were pure breed lines tested by self crossing for many generations.

29. Explain the law of dominance in monohybrid cross.

Answer: The Law of Dominance states that the characters are controlled by discrete units called factors which occur in pairs. In a dissimilar pair of factors, one member is dominant and the other is recessive. In monohybrid cross, when tall (TT) is crossed with dwarf (tt), all F1 plants are tall (Tt) showing dominance of tall over dwarf. When F1 is selfed, F2 shows 3 tall : 1 dwarf ratio, where both traits reappear, explaining dominance and the 3:1 phenotypic ratio.

30. Differentiate incomplete dominance and codominance.

Answer: In incomplete dominance, when one allele is not completely dominant to another, the F1 heterozygous phenotype shows an intermediate between both parents (e.g., red × white = pink in Mirabilis jalapa). In codominance, both alleles are simultaneously expressed in the heterozygote without blending (e.g., ABO blood groups in humans, seed proteins in Gossypium). Both show 1:2:1 phenotypic and genotypic ratios in F2.

31. What is meant by cytoplasmic inheritance?

Answer: Cytoplasmic inheritance is a kind of non-Mendelian inheritance where certain traits are governed by chloroplast or mitochondrial genes rather than nuclear genes. It is based on independent, self-replicating extra chromosomal units called plasmogenes located in cytoplasmic organelles (chloroplast and mitochondrion). The inheritance pattern follows the female parent because the male gamete contributes only the nucleus and not the cytoplasm during fertilization.

III. Answer in Detail

32. Describe dominant epistasis with an example.

Answer: Dominant Epistasis is a gene interaction in which two alleles of a gene at one locus interfere and suppress or mask the phenotypic expression of a different pair of alleles at another locus. The gene that suppresses is called epistatic and the gene whose expression is masked is called hypostatic.

Example: Fruit colour in summer squash. The dominant allele 'W' at one locus is for white colour and masks the

📝 Summary

1. Genetics is the branch of biological science that deals with the mechanism of transmission of characters from parents to offspring, a term introduced by W. Bateson in 1906, with four major subdisciplines: Transmission/Classical Genetics, Molecular Genetics, Population Genetics, and Quantitative Genetics.
2. Heredity refers to the transmission of characters from parents to offspring, while variation refers to differences in characteristics shown by organisms belonging to the same natural population or species.
3. Discontinuous variation (qualitative inheritance) shows limited, non-overlapping phenotypic forms controlled by one or two major genes, unaffected by environmental conditions, whereas continuous variation (quantitative inheritance) shows a complete gradation determined by polygenes and environmental factors.
4. Classical Genetics is founded on the study of hereditary behaviour of seven genes by Gregor Mendel, and covers key concepts including Mendelism, monohybrid and dihybrid crosses, backcross, testcross, and gene interactions.
5. Advanced topics in classical genetics include polygenic inheritance (e.g., wheat kernel colour), pleiotropy in Pisum sativum, extra chromosomal/cytoplasmic inheritance in chloroplasts, and the concept of atavism.