Classical Genetics

Class 12 | Pure Science — Botany | Chapter 2

Class 12 Unit 2 Pure_Science

Unit VII: Genetics

Learning Objectives

The Learner will be able to:

• Differentiate classical and modern genetics.
• Understand the concepts and 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, Testcross, Backcross and Dihybrid cross
• 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.

Think About It!

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 off springs.

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 off springs.

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)

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 died in 1884. In 1900 the work of Mendel's experiments was rediscovered by three biologists: Hugo de Vries of Holland, Carl Correns of Germany and Erich von Tschermak of Austria.

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 strain) means it has undergone continuous self-pollination having stable trait inheritance from parent to offspring. Matings within pure breeding lines produce off springs 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 in Pisum sativum:

1. Stem height – Tall or Dwarf
2. Seed shape – Round or Wrinkled
3. Seed colour – Yellow or Green
4. Pod shape – Inflated or Constricted
5. Pod colour – Green or Yellow
6. Flower colour – Purple or White
7. Flower position – Axial or Terminal

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's 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.

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, accumulated 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

Key Terms

• Alleles: A gene can exist in alternate forms (versions) for the same trait. Mendel noticed two different expressions of a trait – Example: Tall and dwarf.
• Homozygous: If an individual has two identical alleles of a gene, it is called as homozygous (e.g., TT).
• Heterozygous: An individual with two different alleles is called heterozygous (e.g., Tt). Mendel's non-true breeding plants are heterozygous, called as hybrids.
• Dominant allele: Symbolized with capital letter (T = Tall).
• Recessive allele: Symbolized with small letter (t = Dwarf).
• Homozygous dominant: Two dominant alleles (TT) – tall pea plants.
• Homozygous recessive: Two recessive alleles (tt) – dwarf pea plants.
• Heterozygous tall: One dominant and one recessive allele (Tt) – non-true breeding tall pea plants.

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 — that is 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.

Key Terms in Monohybrid Cross

• Genotype: The genetic constitution of an individual.
• Phenotype: The observable characteristic of an organism.
• Punnett's Square: Named after British Geneticist Reginald C. Punnett. It is a graphical representation to calculate the probability of all possible genotypes of offsprings in a genetic cross.

Reciprocal Cross

In one experiment, the tall pea plants were pollinated with the pollens from a true-breeding dwarf plant — 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. These matings done in both ways 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 gene for plant height has two alleles: Tall (T) × 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. Mendel's analytical approach is truly an outstanding scientific achievement. His meticulous work and precisely executed breeding experiments proposed that discrete particulate units of heredity are present and they are transmitted from one generation to the other. Now they are called as genes.

Mendel's experiments were well planned to determine the relationships which govern hereditary traits. This rationale is called an empirical approach. Laws that were arrived from an empirical approach are known as empirical laws.

2.3.2 Test Cross

Test cross is crossing an individual of dominant phenotype but unknown genotype with a homozygous recessive. In Mendel's monohybrid cross all the plants are tall in F₁ generation. In F₂ tall and dwarf plants were in the ratio of 3:1. Mendel self-pollinated dwarf F₂ plants and got dwarf plants in F₃ and F₄ generations. So, he concluded that the genotype of dwarf was homozygous (tt).

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. Test cross is used to identify whether an individual is homozygous or heterozygous for dominant character.

Do You Know?

Why are Mendel's pea plants 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 (Active gibberellin).
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. The allele is dominant even if it is two or single (Le le), it produces gibberellins and the pea plants are tall. Dwarf pea plants have two recessive alleles (le le) which code for non-functional protein; hence they are dwarf.

2.3.3 Back Cross

Back Cross

• Back cross is a cross of F₁ hybrid with any one of the parental genotypes. The back cross is of two types: 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.

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 and green) as the two characters.

Dihybrid Cross – Details

• In seed shape: Round (R) is dominant over wrinkled (r).
• In cotyledon colour: Yellow (Y) is dominant over green (y).
• Pure breeding round yellow parent genotype: RRYY
• Pure breeding green wrinkled parent genotype: rryy

During gamete formation the paired genes of a character assort out independently of the other pair. The resultant gametes thus will be genetically different and they are of the following four types in F₂:

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.

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 small 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.

2.3.5 The Dihybrid Test Cross

Dihybrid Test Cross

In a dihybrid test cross, the F₁ dihybrid (RrYy) is crossed with the homozygous recessive parent (rryy). This test cross helps determine the genotype of the individual showing the dominant phenotype and confirms the independent assortment of two pairs of genes.

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, gene interactions can be classified as:

• Intragenic gene interactions (Intra allelic or allelic interactions)
• Intergenic gene interactions (Inter allelic or non-allelic interactions)

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. Lethal genes
4. Pleiotropic genes

1. 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. 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: 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. There is no blending of genes. If blending had taken place, the original pure traits would not have appeared and all F₂ plants would have pink flowers.

Molecular Explanation – Incomplete Dominance

Gene expression is explained in a quantitative way:

• R₁R₁ (Red): Wild-type allele present in two copies produces a functional enzyme which synthesizes red pigments (100% of functional protein).
• R₂R₂ (White): Mutant allele in two copies produces an enzyme which cannot synthesize red pigments. White flower is due to complete loss of function.
• R₁R₂ (Pink – F₁ intermediate phenotype): Has one copy of allele R₁. Produces 50% of the functional protein resulting in half of the pigment of red flowered plant, so it is pink. The intermediate phenotype with 50% of functional protein is not enough to create the red phenotype homozygous, which makes 100% of the functional protein.

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.

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

In human beings, Iᴬ and I^B alleles of I gene are codominant which follows Mendel's law of segregation. In Gossypium hirsutum and Gossypium sturtianum, their heterozygote (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 — showing the presence of both types of proteins similar to their parents. The progeny in a phenotypic and genotypic ratio of 1:2:1.

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 – Genotype: CC
2. Yellowish green (aurea) plants with carotenoids – Genotype: Cc
3. White plants without any chlorophyll – Genotype: cc

When two aurea plants are crossed, the F₁ progeny has phenotypic and genotypic ratio of 1:2:1 (1 Green (CC) : 2 Aurea (Cc) : 1 White (cc)). Since the white plants lack chlorophyll pigment, they will not survive. So, the ratio is modified into 1:2. In this case the homozygous recessive genotype (cc) is lethal.

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 genotypic ratio will be 2:1 or 1:2 respectively.

d. Pleiotropy – A Single Gene Affects Multiple Traits

In Pleiotropy, the single gene affects multiple traits and alters the phenotype of the organism. The pleiotropic gene influences a number of characters simultaneously. These 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 were all inherited together as a single unit. Another example is sickle cell anemia.

2.4.2 Intergenic Interactions

Interlocus interactions take place between the alleles at different loci — that is between alleles of different genes.

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.

• Epistatic gene: The gene that suppresses or masks the phenotypic expression of a gene at another locus.
• Hypostatic gene: The gene whose expression is interfered by nonallelic genes and prevents from exhibiting its character.

In the summer squash, the fruit colour locus has:

• Dominant allele 'W' for white colour
• Recessive allele 'w' for coloured fruit
• Hypostatic allele 'G' for yellow fruit
• Recessive allele 'g' for green fruit

When the white fruit (WWgg) is crossed with yellow fruit (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.

• Homozygous recessive ww genotypes only can give the coloured fruits (4/16).
• Double recessive wwgg will give green fruit (1/16).
• Plants having only 'G' in genotype (wwGg or wwGG) will give yellow fruit (3/16).

Summary Table – Intragenic (Allelic) Interactions

S.No.	Gene Interaction	Example	F₂ Phenotypic Ratio
1	Incomplete Dominance	Flower colour in Mirabilis jalapa	1 : 2 : 1
1	Incomplete Dominance	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 in shepherd's purse (Bursa bursa-pastoris)	15 : 1

2.5 Polygenic Inheritance in Wheat (Kernel Colour)

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.

Nilsson-Ehle's Experiment

• Dark red parent genotype: R₁R₁R₂R₂
• White parent genotype: r₁r₁r₂r₂
• F₁ generation: Medium red – genotype R₁r₁R₂r₂
• F₁ wheat plant produces four types of gametes: R₁R₂, R₁r₂, r₁R₂, r₁r₂

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

• Four R genes: Dark red kernel colour
• Three R genes: Medium-dark red kernel colour
• Two R genes: Medium-red kernel colour
• One R gene: Light red kernel colour
• Absence of R gene: White kernel colour

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

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

Conclusion

The loci 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.

Nilsson-Ehle showed that blending inheritance was not taking place in the kernel of wheat. In F₂ generation plants have kernels with a 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.

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: 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 vice versa, 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.

Cytoplasmic Genetic Male Sterility

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)
• S (sterile)

The genes for these are found in mitochondrion. There are also restores of fertility (Rf) genes. These 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.

• N cytoplasm with rfrf and S cytoplasm with RfRf → Plants with fertile pollens
• S cytoplasm with rfrf → Male sterile plants only

2.7 Atavism

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

Summary

Chapter 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 : 1 recessive (Mendelian ratio).
• Parents transmit discrete information about the traits to their offspring which Mendel called "factors".
• To test his experimental results Mendel devised a powerful procedure called the test cross.
• 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.
• Incomplete dominance is not an example for blending inheritance. It 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 A. Multiple Choice Questions

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 other alleles of same loci d) Interaction between two alleles of same loci

13. In a test cross involving F₁ dihybrid fly, 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 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 – 1863 b) 1850 – 1870 c) 1857 – 1869 d) 1870 – 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 B. 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 C. 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 alters 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 off-springs 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, 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: A test cross involves crossing the F1 hybrid with a double recessive genotype (aabb) to determine the different types of gametes produced by the plant of dominant phenotype.

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, and heterozygous only at locus B. During gamete formation, only locus B segregates, producing two kinds of gametes: 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: Human height and skin colour are examples of polygenic inheritance, where the inheritance of phenotype is determined by the combined effects of many genes (polygenes) and environmental factors.

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

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

Explanation: In a dihybrid cross, the F2 generation produces four phenotypic classes in the ratio 9:3:3:1. Round yellow (9), round green (3), wrinkled yellow (3), and wrinkled green (1) are all expected among the F2 phenotypes.

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 dominant phenotype but unknown genotype with a homozygous recessive individual to determine the genotype of the dominant phenotype organism.

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: When a heterozygous yellow (Yy) is crossed with homozygous green (yy), this is a test cross. The progeny will be 1 Yy (yellow) : 1 yy (green), which equals a 50:50 ratio.

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 show very few recombinations/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 F1 generation resembles both parents because both alleles are simultaneously expressed.

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 the summer squash, the fruit colour locus has a dominant allele 'W' for white colour which masks the expression of any colour, giving a phenotypic ratio of 12 white : 3 yellow : 1 green — this is dominant epistasis.

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: Mendel studied seven contrasting characters in pea plants which did not include pod length. Pod length is not among the seven characters studied by Mendel.

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 other alleles of same loci d) Interaction between two alleles of same loci

Answer: b) Interaction between two alleles of different loci

Explanation: Epistasis involves interlocus interactions — interactions between alleles at different loci. One gene at one locus interferes and suppresses or masks the phenotypic expression of a different pair of alleles at another locus.

13. In a test cross involving F1 dihybrid fly, 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: When more parental types are produced than recombinant types in a test cross, it indicates that the two genes are linked (located on the same chromosome), as genes on separate chromosomes assort independently.

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: According to the chapter text and Figure 2.4, Mendel's seven characters in Garden Peas are shown on the plant's seven chromosomes, but the genes are located on four different chromosomes.

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 are distributed to gametes, so all possible chromosomal combinations are produced, leading to genetic variation and new trait combinations not seen in either parent.

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: The Law of Segregation states that during the formation of gametes, the factors (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 gene's 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 phenotypic expression of a different pair of alleles 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 that in a dissimilar pair of factors one member of the pair is dominant and the other is recessive, resulting in the expression of only the dominant character (tall) in F1 and a 3:1 ratio in F2.

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 (e.g., fruit colour in summer squash), the F1 heterozygous plants when crossed 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) 1856–1863 b) 1850–1870 c) 1857–1869 d) 1870–1877

Answer: a) 1856–1863

Explanation: According to the chapter, 1856 to 1863 was the period of Mendel's hybridization experiments on pea plants. In 1856 he started his historic studies on pea plants.

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: Trichome type (glandular or non-glandular) was not among the seven contrasting characters studied by Mendel in pea plants. Mendel's seven characters included stem height, seed colour, seed shape, pod shape, pod colour, flower colour, and flower position.

II. Answer Briefly

22. Name the seven contrasting traits of Mendel.

Answer: Mendel discovered the principles of heredity by studying the inheritance of seven pairs of contrasting traits of pea plant. These seven characters studied are shown in Figure 2.3 and Figure 2.4 of the chapter: stem height (tall/dwarf), seed colour (yellow/green), seed shape (round/wrinkled), pod shape (inflated/constricted), pod colour, flower colour (purple/white), and flower position (axial/terminal).

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 offspring 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 was 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 involves the cross between the F1 offspring with either of the two parents. The back cross is of two types: dominant back cross and recessive back cross. The recessive back cross helps to identify the heterozygosity of the hybrid.

26. Define Genetics.

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

27. What are multiple alleles?

Answer: A gene can exist in alternate forms (versions) for the same trait, called alleles. When a gene has more than two alternate forms at the same locus, they are called multiple alleles. An example given in the chapter is the ABO blood group system in human beings, where IA and IB alleles of the I gene are codominant multiple alleles.

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

Answer: Mendel was successful because: he applied mathematics, statistical methods and laws of probability to his breeding experiments; he followed scientific methods and kept accurate and detailed quantitative records; his experiments were carefully planned using large samples; the contrasting characters were controlled by genes present on separate chromosomes; and 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 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 the monohybrid cross of tall (TT) × dwarf (tt) pea plants, all F1 plants were tall (Tt) showing only the dominant trait. When F1 plants were selfed, the F2 generation produced tall and dwarf plants in the ratio 3:1. This law explains the expression of only one parental character in F1 and both characters in F2 in the proportion of 3:1.

30. Differentiate incomplete dominance and codominance.

Answer: In incomplete dominance, when one allele is not completely dominant to another allele, the F1 heterozygote shows an intermediate phenotype different from both parents (e.g., red × white = pink in Mirabilis jalapa). In codominance, both alleles are simultaneously expressed in the heterozygote and the F1 resembles both parents (e.g., ABO blood group, Gossypium). Both show a phenotypic and genotypic ratio of 1:2:1 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 involves cytoplasmic organelles (chloroplast and mitochondrion) that act as inheritance vectors, based on independent, self-replicating extra chromosomal units called plasmogenes located in the cytoplasmic organelles.

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 of another gene at another locus. The gene that suppresses is called epistatic and the gene whose expression is prevented is called hypostatic.

In summer squash, fruit colour locus has a dominant allele 'W' for white colour and recessive allele 'w' for coloured fruit. At another locus, dominant allele 'G' is for yellow fruit and recessive 'g' is for green fruit. When white fruit (WWgg) is crossed with yellow fruit

📝 Summary

1. Genetics is the branch of biological science dealing with the mechanism of transmission of characters from parents to offspring, a term introduced by W. Bateson in 1906, and encompasses four major subdisciplines: Transmission 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. Variation is of two types: Discontinuous Variation (qualitative inheritance), controlled by one or two major genes with no intermediate forms between phenotypes, and Continuous Variation (quantitative inheritance), influenced by combined effects of many genes (polygenes) and environmental factors.
4. Classical (Transmission) Genetics forms the foundation of genetic study, originating from Gregor Mendel's study of hereditary behaviour of seven genes, and deals with how genes are transmitted from parents to offspring.
5. Variations are biologically significant as they help individuals adapt to changing environments, provide genetic material for natural selection, allow breeders to improve crop yield and resistance, and constitute the raw materials for evolution.