Chromosomal Basis Of Inheritance

Class 12 | Biology — Botany | Chapter 3

Class 12 Unit 3 Biology

Chapter 3: Chromosomal Basis of Inheritance

The Learner will be able to:

• Understand chromosomal theory of inheritance.
• Analyze the three-point test crosses and appreciate results in linkage map construction.
• Differentiate mutation types with examples.

Chapter Outline

• 3.1 Chromosomal theory of Inheritance
• 3.2 Linkage - Eye colour in Drosophila and Seed colour in Maize
• 3.3 Crossing over, Recombination and Gene mapping
• 3.4 Multiple alleles
• 3.5 Mutation - types, mutagenic agents and their significance

In the previous chapter you have learned about Mendelian genetics, now you are going to be study with deviations of concepts related to Mendelian genetics and chromosomal theory of inheritance. You must recall the structure of chromosome and cell division from eleventh standard.

3.1 Chromosomal Theory of Inheritance

G. J. Mendel (1865) studied the inheritance of well-defined characters of pea plant but for several reasons it was unrecognized till 1900. Three scientists (de Vries, Correns and Tschermak) independently rediscovered Mendel's results on the inheritance of characters. Various cytologists also observed cell division due to advancements in microscopy. This led to the discovery of structures inside nucleus. In eukaryotic cells, worm-shaped structures formed during cell division are called chromosomes (colored bodies, visualized by staining). An organism which possesses two complete basic sets of chromosomes are known as diploid. A chromosome consists of long, continuous coiled piece of DNA in which genes are arranged in linear order. Each gene has a definite position (locus) on a chromosome. These genes are hereditary units. Chromosomal theory of inheritance states that Mendelian factors (genes) have specific locus (position) on chromosomes and they carry information from one generation to the next generation.

3.1.1 Historical Development of Chromosome Theory

The important cytological findings related to the chromosome theory of inheritance are given below.

Key Historical Milestones

• Wilhelm Roux (1883) postulated that the chromosomes of a cell are responsible for transferring heredity.
• Montgomery (1901) was first to suggest occurrence of distinct pairs of chromosomes and he also concluded that maternal chromosomes pair with paternal chromosomes only during meiosis.
• T. Boveri (1902) supported the idea that the chromosomes contain genetic determiners, and he was largely responsible for developing the chromosomal theory of inheritance.
• W.S. Sutton (1902), a young American student independently recognized a parallelism (similarity) between the behaviour of chromosomes and Mendelian factors during gamete formation.
• Sutton and Boveri (1903) independently proposed the chromosome theory of inheritance. Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it chromosomal theory of inheritance.

3.1.2 Salient Features of the Chromosomal Theory of Inheritance

Salient Features

• Somatic cells of organisms are derived from the zygote by repeated cell division (mitosis). These consist of two identical sets of chromosomes. One set is received from female parent (maternal) and the other from male parent (paternal). These two chromosomes constitute the homologous pair.
• Chromosomes retain their structural uniqueness and individuality throughout the life cycle of an organism.
• Each chromosome carries specific determiners or Mendelian factors which are now termed as genes.
• The behaviour of chromosomes during the gamete formation (meiosis) provides evidence to the fact that genes or factors are located on chromosomes.

3.1.3 Comparison between Gene and Chromosome Behaviour

Around twentieth century cytologists established that, generally the total number of chromosomes is constant in all cells of a species. A diploid eukaryotic cell has two haploid sets of chromosomes, one set from each parent. All somatic cells of an organism carry the same genetic complement. The behaviour of chromosomes during meiosis not only explains Mendel's principles but leads to new and different approaches to study about heredity.

Figure 3.1: Comparison of Chromosome and Gene Behaviour

[Refer to Figure 3.1 in your textbook]

Table 3.1: Parallelism between Mendelian Factors and Chromosomal Behaviour

Mendelian Factors	Chromosomal Behaviour
Alleles of a factor occur in pair	Chromosomes occur in pairs
Similar or dissimilar alleles of a factor separate during the gamete formation	The homologous chromosomes separate during meiosis
Mendelian factors can assort independently	The paired chromosomes can separate independently during meiosis but the linked genes in the same chromosome normally do not assort independently.

Table 3.2: Number of Chromosomes

Organism	Number of Chromosomes (2n)
Adder's tongue fern (Ophioglossum)	1262
Horsetail (Equisetum)	216
Giant sequoia	22
Arabidopsis	10
Sugarcane	80
Apple	34
Rice	24
Potato	48
Maize	20
Onion	16
Haplopappus gracilis	4

Nobel Prize

Thomas Hunt Morgan (1933) received Nobel Prize in Physiology or Medicine for his discoveries concerning the role played by chromosomes in heredity.

Do You Know?

Fossil Genes: Some of the junk DNA is made up of pseudo genes, the sequences presence in that was once working genes. They lost their ability to make proteins. They tell the story of evolution through fossilized parts.

3.2 Linkage

The genes which determine the character of an individual are carried by the chromosomes. The genes for different characters may be present either in the same chromosome or in different chromosomes. When the genes are present in different chromosomes, they assort independently according to Mendel's Law of Independent Assortment. Biologists came across certain genetic characteristics that did not assort out independently in other organisms after Mendel's work. One such case was reported in Sweet pea (Lathyrus odoratus) by William Bateson and Reginald C. Punnet in 1906. They crossed one homozygous strain of sweet peas having purple flowers and long pollen grains with another homozygous strain having red flowers and round pollen grains. All the F1 progenies had purple flower and long pollen grains indicating purple flower long pollen (PL/PL) was dominant over red flower round pollen (pl/pl). When they crossed the F1 with double recessive parent (test cross) in results F2 progenies did not exhibit in 1:1:1:1 ratio as expected with independent assortment. A greater number of F2 plants had purple flowers and long pollen or red flowers and round pollen. So they concluded that genes for purple colour and long pollen grain and the genes for red colour and round pollen grain were found close together in the same homologous pair of chromosomes. These genes do not allow themselves to be separated. So they do not assort independently. This type of tendency of genes to stay together during separation of chromosomes is called Linkage.

Figure 3.2: Arrangement of Linked and Unlinked Genes on Chromosome

[Refer to Figure 3.2 in your textbook]

Genes located close together on the same chromosome and inherited together are called linked genes. But the two genes that are sufficiently far apart on the same chromosome are called unlinked genes or syntenic genes. Such condition is known as synteny. It is to be differentiated by the value of recombination frequency. If the recombination frequency value is more than 50% the two genes show unlinked. When the recombination frequency value is less than 50%, they show linked. Closely located genes show strong linkage, while genes widely located show weak linkages.

3.2.1 Coupling and Repulsion Theory

Coupling and Repulsion

• Coupling (cis configuration): The two dominant alleles or recessive alleles occur in the same homologous chromosomes, tend to inherit together into same gamete.
• Repulsion (trans configuration): If dominant or recessive alleles are present on two different, but homologous chromosomes they inherit apart into different gamete.

3.2.2 Kinds of Linkage

T.H. Morgan found two types of linkage. They are complete linkage and incomplete linkage depending upon the absence or presence of new combination of linked genes.

Complete Linkage

If the chances of separation of two linked genes are not possible those genes always remain together as a result, only parental combinations are observed. The linked genes are located very close together on the same chromosome such genes do not exhibit crossing over. This phenomenon is called complete linkage. It is rare but has been reported in male Drosophila. C.B Bridges (1919) discovered that crossing over is completely absent in some species of male Drosophila.

Incomplete Linkage

If two linked genes are sufficiently apart, the chances of their separation are possible. As a result, parental and non-parental combinations are observed. The linked genes exhibit some crossing over. This phenomenon is called incomplete linkage. This was observed in maize. It was reported by Hutchinson.

3.2.3 Linkage Groups

The groups of linearly arranged linked genes on a chromosome are called Linkage groups. In any species the number of linkage groups corresponds to the number haploid set of chromosomes.

Table 3.3: Linkage Groups in Some Organisms

Name of Organism	Linkage Groups
Mucor	2
Drosophila	4
Sweet pea	7
Neurospora	7
Maize	10

Linkage and crossing over are two processes that have opposite effects. Linkage keeps particular genes together but crossing over mixes them. The differences are given below.

Table 3.4: Differences between Linkage and Crossing Over

Linkage	Crossing Over
The genes present on chromosome stay close together	It leads to separation of linked genes
It involves same chromosome of homologous chromosome	It involves exchange of segments between non-sister chromatids of homologous chromosome.
It reduces new gene combinations	It increases variability by forming new gene combinations, lead to formation of new organism

3.3 Crossing Over

Crossing over is a biological process that produces new combination of genes by interchanging the corresponding segments between non-sister chromatids of homologous pair of chromosomes. The term 'crossing over' was coined by Morgan (1912). It takes place during pachytene stage of prophase I of meiosis. Usually crossing over occurs in germinal cells during gametogenesis. It is called meiotic or germinal crossing over. It has universal occurrence and has great significance. Rarely, crossing over occurs in somatic cells during mitosis. It is called somatic or mitotic crossing over.

3.3.1 Mechanism of Crossing Over

Crossing over is a precise process that includes stages like synapsis, tetrad formation, cross over and terminalization.

(1) Synapsis

Intimate pairing between two homologous chromosomes is initiated during zygotene stage of prophase 1 of meiosis I. Homologous chromosomes are aligned side by side resulting in a pair of homologous chromosomes called bivalents. This pairing phenomenon is called synapsis or syndesis. It is of three types:

1. Procentric synapsis: Pairing starts from middle of the chromosome.
2. Proterminal synapsis: Pairing starts from the telomeres.
3. Random synapsis: Pairing may start from anywhere.

(2) Tetrad Formation

Each homologous chromosome of a bivalent begin to form two identical sister chromatids, which remain held together by a centromere. At this stage each bivalent has four chromatids. This stage is called tetrad stage.

(3) Cross Over

After tetrad formation, crossing over occurs in pachytene stage. The non-sister chromatids of homologous pair make a contact at one or more points. These points of contact between non-sister chromatids of homologous chromosomes are called Chiasmata (singular - Chiasma). At chiasma, cross-shaped or X-shaped structures are formed, where breaking and rejoining of two chromatids occur. This results in reciprocal exchange of equal and corresponding segments.

(4) Terminalization

After crossing over, chiasma starts to move towards the terminal end of chromatids. This is known as terminalization. As a result, complete separation of homologous chromosomes occurs.

Activity Solve This

Consider two hypothetical recessive autosomal genes a and b, where a heterozygote is testcrossed to a double homozygous mutant. Predict the phenotypic ratios under the following conditions:

(a) a and b are located on separate autosomes.

(b) a and b are linked on the same autosome but are so far apart that a crossover occurs between them.

(c) a and b are linked on the same autosome but are so close together that a crossover almost never occurs.

3.3.2 Importance of Crossing Over

Importance of Crossing Over

Crossing over occurs in all organisms like bacteria, yeast, fungi, higher plants and animals. Its importance is:

• Exchange of segments leads to new gene combinations which plays an important role in evolution.
• Studies of crossing over reveal that genes are arranged linearly on the chromosomes.
• Genetic maps are made based on the frequency of crossing over.
• Crossing over helps to understand the nature and mechanism of gene action.
• If a useful new combination is formed it can be used in plant breeding.

3.3.3 Recombination

Crossing over results in the formation of new combination of characters in an organism called recombinants. In this, segments of DNA are broken and recombined to produce new combinations of alleles. This process is called Recombination.

Calculation of Recombination Frequency (RF)

The percentage of recombinant progeny in a cross is called recombination frequency. The recombination frequency (cross over frequency) (RF) is calculated by using the following formula:

RF = (Total number of Recombinations / Total number of Offspring) × 100

3.3.4 Genetic Mapping

Genes are present in a linear order along the chromosome. They are present in a specific location called locus (plural: loci). The diagrammatic representation of position of genes and related distances between the adjacent genes is called genetic mapping. It is directly proportional to the frequency of recombination between them. It is also called as linkage map. The concept of gene mapping was first developed by Morgan's student Alfred H Sturtevant in 1913. It provides clues about where the genes lies on that chromosome.

Map Distance

The unit of distance in a genetic map is called a map unit (m.u). One map unit is equivalent to one percent of crossing over. One map unit is also called a centimorgan (cM) in honour of T.H. Morgan. 100 centimorgan is equal to one Morgan (M).

Example: A distance between A and B genes is estimated to be 3.5 map units. It is equal to 3.5 centimorgans or 3.5% or 0.035 recombination frequency between the genes.

Uses of Genetic Mapping

• It is used to determine gene order, identify the locus of a gene and calculate the distances between genes.
• They are useful in predicting results of dihybrid and trihybrid crosses.
• It allows the geneticists to understand the overall genetic complexity of particular organism.

3.4 Multiple Alleles

A given phenotypic trait of an individual depends on a single pair of genes, each of which occupies a specific position called the locus on homologous chromosome. When any of the three or more allelic forms of a gene occupy the same locus in a given pair of homologous chromosomes, they are said to be called multiple alleles.

Check Your Grasp: There may be multiple alleles within the population, but individuals have only two of those alleles. Why?

3.4.1 Characteristics of Multiple Alleles

Characteristics

• Multiple alleles of a series always occupy the same locus in the homologous chromosome. Therefore, no crossing over occurs within the alleles of a series.
• Multiple alleles are always responsible for the same character.
• The wild type alleles of a series exhibit dominant character whereas mutant type will influence dominance or an intermediate phenotypic effect.
• When any two of the mutant multiple alleles are crossed the phenotype is always mutant type and not the wild type.

3.4.2 Self-sterility in Nicotiana

In plants, multiple alleles have been reported in association with self-sterility or Self incompatibility. Self-sterility means that the pollen from a plant is unable to germinate on its own stigma and will not be able to bring about fertilization in the ovules of the same plant. East (1925) observed multiple alleles in Nicotiana which are responsible for self-incompatibility or self-sterility. The gene for self-incompatibility can be designated as S, which has allelic series S1, S2, S3, S4 and S5.

The cross-fertilizing tobacco plants were not always homozygous as S1S1 or S2S2, but all plants were heterozygous as S1S2, S3S4, S5S6. When crosses were made between different S1S2 plants, the pollen tube did not develop normally. But effective pollen tube development was observed when crossing was made with other than S1S2 for example S3S4. When crosses were made between seed parents with S1S2 and pollen parents with S2S3, two kinds of pollen tubes were distinguished. Pollen grains carrying S2 were not effective, but the pollen grains carrying S3 were capable of fertilization. Thus, from the cross S1S2 × S3S4, all the pollens were effective and four kinds of progeny resulted: S1S3, S1S4, S2S3 and S2S4.

Table 3.5: Different Combinations of Progeny in Self-incompatibility

Female Parent (stigma)	Male Parent (pollen source)	Progeny
S1S2	S1S2	SELF STERILE
S1S2	S2S3	S3S2, S3S1
S1S2	S3S4	S1S3, S1S4, S2S3, S2S4
S2S3	S1S2	S1S3, S2S3
S2S3	S2S3	SELF STERILE
S2S3	S3S4	S2S4, S3S4
S3S4	S1S3	S1S4, S3S4
S3S4	S2S3	S2S4, S3S4
S3S4	S2S3, S2S4	SELF STERILE

3.4.3 Sex Determination in Maize

Zea mays (maize) is an example for monoecious, which means male and female flowers are present on the same plant. There are two types of inflorescence:

• The terminal inflorescence which bears staminate florets develops from shoot apical meristem called tassel.
• The lateral inflorescence which develop pistillate florets from axillary bud is called ear or cob.

Unisexuality in maize occurs through the selective abortion of stamens in ear florets and pistils in tassel florets. A substitution of two single gene pairs 'ba' for barren plant and 'ts' for tassel seed makes the difference between monoecious and dioecious (rare) maize plants. The allele for barren plant (ba) when homozygous makes the stalk staminate by eliminating silk and ears. The allele for tassel seed (ts) transforms tassel into a pistillate structure that produce no pollen. Most of these mutations are shown to be defects in gibberellin biosynthesis. Gibberellins play an important role in the suppression of stamens in florets on the ears.

Table 3.6: Sex Determination in Maize

Genotype	Dominant/Recessive	Modification	Sex
barren/barren, tassel/tassel	Double recessive	Lacks silk on the stalk, but transformed tassel to pistil	Rudimentary female
barren/barren, Tassel+/tassel+	Recessive and dominant	Lacks silk and have tassel	Male
barren dominant/barren dominant, ts recessive/ts recessive	Double dominant	Have both tassel and cob	Monoecious
barren dominant/barren dominant, Tassel/tassel	Dominant and recessive	Bears cob and lacks tassel	Normal female

(Superscript + denotes dominant character)

3.5 Mutation

Genetic variation among individuals provides the raw material for the ultimate source of evolutionary changes. Mutation and recombination are the two major processes responsible for genetic variation. A sudden change in the genetic material of an organisms is called mutation. The term mutation was introduced by Hugo de Vries (1901) while he was studying on the plant, evening primrose (Oenothera lamarkiana) and proposed 'Mutation theory'. There are two broad types of changes in genetic material. They are point mutation and chromosomal mutations. Mutational events that take place within individual genes are called gene mutations or point mutation, whereas the changes occur in structure and number of chromosomes is called chromosomal mutation. Agents which are responsible for mutation are called mutagens, that increase the rate of mutation. Mutations can occur either spontaneously or induced. The production of mutants through exposure of mutagens is called mutagenesis, and the organism is said to be mutagenized.

3.5.1 Types of Mutation

General Classes of Gene Mutation

• Mutations affecting single base or base pair of DNA are called point mutation
• Mutations altering the number of copies of a small repeated nucleotide sequence within a gene

Figure 3.9: Types of Point Mutation

• a. No Mutation: Wild type protein produced.
• b. Transition Mutation: Changes a purine nucleotide to another purine or pyrimidine to another pyrimidine.
• c. Transversion Mutation: Single purine changed to pyrimidine or vice versa.
• d. Missense Mutation: The new codon encodes a different amino acid due to transition mutation.
• e. Nonsense Mutation: The new codon is a stop codon (UAA) due to transition mutation, it leads to premature termination of translation.
• f. Silent Mutation: The new codon encodes the same amino acid after transition mutation.
• g. Frameshift Mutation: Addition or Deletion of one nucleotide, change the reading frame results in completely different translation.

Table 3.7: Major Types of Mutations

S.No.	Basis of Classification	Major Types	Major Features
1	Origin	Spontaneous	Occurs in the absence of known mutagen
1	Origin	Induced	Occurs in the presence of known mutagen
2	Cell type	Somatic	Occurs in non reproductive cells
2	Cell type	Germ line	Occurs in reproductive cells
3	Effect on function	Loss of function (knockout/null)	Eliminates normal function
3	Effect on function	Hypomorphic (leaky)	Reduces normal function
3	Effect on function	Hypermorphic	Increases normal function
3	Effect on function	Gain of function (ectopic expression)	Expressed at incorrect time or inappropriate cells
4	Molecular change	Nucleotide substitution - Transition	A base pair in DNA duplex is replaced with a different base pair; Purine to purine (A→G) or pyrimidine to pyrimidine (T→C)
4	Molecular change	Transversion	Purine to pyrimidine (A→T) or pyrimidine to purine (C→G)
4	Molecular change	Insertion	One or more extra nucleotides are present
4	Molecular change	Deletion	One or more nucleotides are missing
5	Effect on translation	Silent	No change in amino acid encoded
5	Effect on translation	Missense (non synonymous)	Change in amino acid encoded
5	Effect on translation	Nonsense (termination)	Creates translation termination codon (UAA, UAG, or UGA)
5	Effect on translation	Frameshift	Shift triplet reading of codons out of correct phase

Point Mutation

It refers to alterations of single base pairs of DNA or of a small number of adjacent base pairs.

Types of Point Mutations

Point mutations in DNA are categorised into two main types. They are base pair substitutions and base pair insertions or deletions.

• Base substitutions are mutations in which there is a change in the DNA such that one base pair is replaced by another. It can be divided into two subtypes: transitions and transversions.
• Addition or deletion mutations are actually additions or deletions of nucleotide pairs and also called base pair addition or deletions. Collectively, they are termed indel mutations (for insertion-deletion).

Based on these, different types of mutations are given below:

• Synonymous or silent mutations: The mutation that changes one codon for an amino acid into another codon for that same amino acid.
• Missense or non-synonymous mutations: The mutation where the codon for one amino acid is changed into a codon for another amino acid.
• Nonsense mutation: The mutations where codon for one amino acid is changed into a termination or stop codon.
• Frameshift mutations: Mutations that result in the addition or deletion of a single base pair of DNA that changes the reading frame for the translation process as a result of which there is complete loss of normal protein structure and function.

3.5.2 Mutagenic Agents

The factors which cause genetic mutation are called mutagenic agents or mutagens. Mutagens are of two types, physical mutagen and chemical mutagen. Muller (1927) was the first to find out physical mutagen in Drosophila.

Physical Mutagens

Scientists are using temperature and radiations such as X rays, gamma rays, alfa rays, beta rays, neutron, cosmic rays, radioactive isotopes, ultraviolet rays as physical mutagen to produce mutation in various plants and animals.

Temperature

Increase in temperature increases the rate of mutation. While rise in temperature, breaks the hydrogen bonds between two DNA nucleotides which affects the process of replication and transcription.

Radiation

The electromagnetic spectrum contains shorter and longer wave length rays than the visible spectrum. These are classified into ionizing and non-ionizing radiation.

• Ionizing radiation are short wave length and carry enough higher energy to ionize electrons from atom. X rays, gamma rays, alfa rays, beta rays and cosmic rays which breaks the chromosomes (chromosomal mutation) and chromatids in irradiated cells.
• Non-ionizing radiation: UV rays have longer wavelengths and carry lower energy, so they have lower penetrating power than the ionizing radiations. It is used to treat unicellular microorganisms, spores, pollen grains which possess nuclei located near surface membrane.

Sharbati Sonora

Sharbati Sonora is a mutant variety of wheat, which is developed from Mexican variety (Sonora 64) by irradiating of gamma rays. It is the work of Dr. M.S. Swaminathan who is known as 'Father of Indian green revolution' and his team.

Castor Aruna

Castor Aruna is mutant variety of castor which is developed by treatment of seeds with thermal neutrons in order to induce very early maturity (120 days instead of 270 days as original variety).

Chemical Mutagens

Chemicals which induce mutation are called chemical mutagens. Some chemical mutagens are mustard gas, nitrous acid, ethyl and methyl methane sulphonate (EMS and MMS), ethyl urethane, magnous salt, formaldehyde, eosin and enthrosine.

Example: Nitrous oxide alters the nitrogen bases of DNA and disturb the replication and transcription that leads to the formation of incomplete and defective polypeptide during translation. Mustard gas (Dichloro ethyl sulphide) used as chemical weapon in world war 1.

Important Researchers in Mutagenesis

• H J Muller (1928) first time used X rays to induce mutations in fruit fly.
• L J Stadler reported induced mutations in plants by using X rays and gamma rays.
• Chemical mutagenesis was first reported by C. Auerback (1944).

Comutagens

The compounds which are not having own mutagenic properties but can enhance the effects of known mutagens are called comutagens.

• Ascorbic acid increases the damage caused by hydrogen peroxide.
• Caffeine increases the toxicity of methotrexate.

3.5.3 Chromosomal Mutations

The genome can also be modified on a larger scale by altering the chromosome structure or by changing the number of chromosomes in a cell. These large-scale variations are termed as chromosomal mutations or chromosomal aberrations. Gene mutations are changes that take place within a gene, whereas chromosomal mutations are changes to a chromosome region consisting of many genes. It can be detected by microscopic examination, genetic analysis, or both. In contrast, gene mutations are never detectable microscopically.

Chromosomal mutations are divided into two groups: changes in chromosome number and changes in chromosome structure.

1. Changes in Chromosome Number

Each cell of living organisms possesses fixed number of chromosomes. It varies in different species. Even though some species of plants and animals are having identical number of chromosomes, they will not be similar in character. Hence the number of chromosomes will not differentiate the character of species from one another but the nature of hereditary material (gene) in chromosome that determines the character of species. Sometimes the chromosome number of somatic cells are changed due to addition or elimination of individual chromosome or basic set of chromosomes. This condition is known as numerical chromosomal aberration or ploidy. There are two types of ploidy:

1. Ploidy involving individual chromosomes within a diploid set (Aneuploidy)
2. Ploidy involving entire sets of chromosomes (Euploidy)

Figure 3.10: Types of Ploidy

Ploidy → two types:

• 1. Aneuploidy
  • Hyperploidy: Trisomy (2n+1), Double trisomy (2n+1+1), Tetrasomy (2n+2), Double tetrasomy (2n+2+2), Pentasomy (2n+3)
  • Hypoploidy: Monosomy (2n−1), Double monosomy (2n−1−1), Nullisomy (2n−2), Double nullisomy (2n−2−2)
• 2. Euploidy
  • Haploidy (n): Monoploidy (x)
  • Diploidy (2n)
  • Polyploidy (2n+n+n): Autopolyploidy (Autotriploid, Autotetraploid), Allopolyploidy

(1) Aneuploidy

It is a condition in which diploid number is altered either by addition or deletion of one or more chromosomes. Organisms showing aneuploidy are known as aneuploids or heteroploids. They are of two types, Hyperploidy and Hypoploidy.

Types of Aneuploidy

Type	Formula	Description
Disomy (normal)	2n	Normal diploid
Monosomy	2n−1	Loss of one chromosome
Double Monosomy	2n−1−1	Loss of two individual chromosomes
Nullisomy	2n−2	Loss of a pair of homologous chromosomes
Trisomy	2n+1	Addition of one chromosome
Double Trisomy	2n+1+1	Addition of two individual chromosomes from different pairs
Tetrasomy	2n+2	Addition of a pair of chromosomes
Pentasomy	2n+3	Addition of three individual chromosomes from different pairs

1. Hyperploidy

Addition of one or more chromosomes to diploid sets are called hyperploidy. Diploid set of chromosomes represented as Disomy. Hyperploidy can be divided into three types:

(a) Trisomy

Addition of single chromosome to diploid set is called Simple trisomy (2n+1). Trisomics were first reported by Blackeslee (1910) in Datura stramonium (Jimson weed). But later it was reported in Nicotiana, Pisum and Oenothera. Sometimes addition of two individual chromosome from different chromosomal pairs to normal diploid sets are called Double trisomy (2n+1+1).

(b) Tetrasomy

Addition of a pair or two individual pairs of chromosomes to diploid set is called tetrasomy (2n+2) and Double tetrasomy (2n+2+2) respectively. All possible tetrasomics are available in Wheat.

(c) Pentasomy

Addition of three individual chromosome from different chromosomal pairs to normal diploid set are called pentasomy (2n+3).

2. Hypoploidy

Loss of one or more chromosome from the diploid set in the cell is called hypoploidy. It can be divided into two types:

(a) Monosomy

Loss of a single chromosome from the diploid set are called monosomy (2n−1). However loss of two individual or three individual chromosomes are called double monosomy (2n−1−1) and triple monosomy (2n−1−1−1) respectively. Double monosomics are observed in maize.

(b) Nullisomy

Loss of a pair of homologous chromosomes or two pairs of homologous chromosomes from the diploid set are called Nullisomy (2n−2) and double Nullisomy (2n−2−2) respectively. Selfing of monosomic plants produce nullisomics. They are usually lethal.

(2) Euploidy

Euploidy is a condition where the organisms possess one or more basic sets of chromosomes. Euploidy is classified as monoploidy, diploidy and polyploidy. The condition where an organism or somatic cell has two sets of chromosomes are called diploid (2n). Half the number of somatic chromosomes is referred as gametic chromosome number called haploid (n). It should be noted that haploidy (n) is different from a monoploidy (x).

Example: The common wheat plant is a polyploidy (hexaploidy) 2n = 6x = 72 chromosomes. Its haploid number (n) is 36, but its monoploidy (x) is 12. Therefore, the haploid and diploid condition came regularly one after another and the same number of chromosomes is maintained from generation to generation, but monoploidy condition occurs when an organism is under polyploidy condition. In a true diploid both the monoploid and haploid chromosome number are same. Thus a monoploid can be a haploid but all haploids cannot be a monoploid.

Polyploidy

Polyploidy is the condition where an organism possesses more than two basic sets of chromosomes. When there are three, four, five or six basic sets of chromosomes, they are called triploidy (3x), tetraploidy (4x), pentaploidy (5x) and hexaploidy (6x) respectively. Generally, polyploidy is very common in plants but rarer in animals. An increase in the number of chromosome sets has been an important factor in the origin of new plant species. But higher ploidy level leads to death. Polyploidy is of two types. They are autopolyploidy and allopolyploidy.

1. Autopolyploidy

The organism which possesses more than two haploid sets of chromosomes derived from within the same species is called autopolyploid. They are divided into two types: Autotriploids and Autotetraploids.

• Autotriploids have three set of its own genomes. They can be produced artificially by crossing between autotetraploid and diploid species. They are highly sterile due to defective gamete formation.
  Examples: The cultivated banana are usually triploids and are seedless having larger fruits than diploids. Triploid sugar beets have higher sugar content than diploids and are resistant to moulds. Common doob grass (Cyanodon dactylon) is a natural autotriploid. Seedless watermelon, apple, sugar beet, tomato, banana are man made autotriploids.
• Autotetraploids have four copies of its own genome. They may be induced by doubling the chromosomes of a diploid species.
  Examples: rye, grapes, alfalfa, groundnut, potato and coffee.

2. Allopolyploidy

An organism which possesses two or more basic sets of chromosomes derived from two different species is called allopolyploidy. It can be developed by interspecific crosses and fertility is restored by chromosome doubling with colchicine treatment. Allopolyploids are formed between closely related species only.

Example 1 – Raphanobrassica: G.D. Karpechenko (1927), a Russian geneticist, crossed the radish (Raphanus sativus, 2n = 18) and cabbage (Brassica oleracea, 2n = 18) to produce F1 hybrid which was sterile. When he doubled the chromosome of F1 hybrid he got it fertile. He expected this plant to exhibit the root of radish and the leaves like cabbage, which would make the entire plant edible, but the case was vice versa, so he was greatly disappointed.

Example 2 – Triticale: Triticale is the successful first man made cereal. Depending on the ploidy level Triticale can be divided into three main groups:
(1) Tetraploidy: Crosses between diploid wheat and rye.
(2) Hexaploidy: Crosses between tetraploid wheat Triticum durum (macaroni wheat) and rye.
(3) Octoploidy: Crosses between hexaploidy wheat T. aestivum (bread wheat) and rye.
Hexaploidy Triticale hybrid plants demonstrate characteristics of both macaroni wheat and rye. For example, they combine the high-protein content of wheat with rye's high content of the amino acid lysine, which is low in wheat.

Figure 3.14: Formation of Triticale (Hexaploidy)

Step	Details
Parent generation	Triticum durum (2n = 4x = 28, Tetraploidy) × Secale cereale (2n = 2x = 14, Diploidy)
Gametes	n = 2x = 14     n = x = 7
F1 hybrid (sterile)	2n = 3x = 21 (Triploidy)
Chromosome doubling by colchicine	2n = 6x = 42 → Triticale (Hexaploidy)

Do You Know?

Colchicine, an alkaloid is extracted from root and corms of Colchicum autumnale. When applied in low concentration to the growing tips of the plants it will induce polyploidy. Surprisingly it does not affect the source plant Colchicum, due to presence of anticolchicine.

Activity Solve This

When two plants (A and B) belonging to the same genus but different species are crossed, the F1 hybrid is viable and has more ornate flowers. Unfortunately, this hybrid is sterile and can only be propagated by vegetative cuttings. Explain the sterility of the hybrid and what would have to occur for the sterility of this hybrid to be reversed.

Significance of Ploidy

• Many polyploids are more vigorous and more adaptable than diploids.
• Many ornamental plants are autotetraploids and have larger flowers and longer flowering duration than diploids.
• Autopolyploids usually have higher in fresh weight due to more water content.
• Aneuploids are useful to determine the phenotypic effects of loss or gain of different chromosomes.
• Many angiosperms are allopolyploids and they play a role in the evolution of plants.

2. Structural Changes in Chromosome (Structural Chromosomal Aberration)

Structural variations caused by addition or deletion of a part of chromosome leading to rearrangement of genes is called structural chromosomal aberration. It occurs due to ionizing radiation or chemical compounds. On the basis of breaks and reunion in chromosomes, there are four types of aberrations. They are classified under two groups:

A. Changes in the Number of the Gene Loci

1. Deletion or Deficiency
2. Duplication or Repeat

B. Changes in the Arrangement of Gene Loci

1. Inversion
2. Translocation

1. Deletion or Deficiency

Loss of a portion of chromosome is called deletion. On the basis of location of breakage on chromosome, it is divided into terminal deletion and intercalary deletion. It occurs due to chemicals, drugs and radiations. It is observed in Drosophila and Maize.

2. Duplication or Repeat

The process of arrangement of the same order of genes repeated more than once in the same chromosome is known as duplication. Due to duplication some genes are present in more than two copies. It was first reported in Drosophila by Bridges (1919) and other examples are Maize and Pea. It is three types.

3. Inversion

A rearrangement of order of genes in a chromosome by reversed by an angle of 180°. This involves two chromosomal breaks and reunion. During this process there is neither gain nor loss but the gene sequence is rearranged. Inversion was first reported in Drosophila by Sturtevant (1926). There are two types of inversion: paracentric and pericentric.

• (1) Paracentric inversion: An inversion which takes place apart from the centromere. Inversion leads to evolution of a new species.

4. Translocation

The transfer of a segment of chromosome to a non-homologous chromosome is called translocation. Translocation should not be confused with crossing over, in which an exchange of genetic material between homologous chromosome takes place. Translocation occurs as a result of interchange of chromosome segments in non-homologous chromosomes. There are three types:

1. Simple translocation
2. Shift translocation
3. Reciprocal translocation

Summary

Chapter Summary

• Chromosomal theory of inheritance states that Mendelian factors have specific locus on chromosomes and they carry information from one generation to the next generation.
• Genes located close together on the same chromosome and inherited together are called linked genes; the phenomenon is called Linkage.
• Two types of linkage are complete linkage and incomplete linkage.
• The groups of linearly arranged linked genes are called Linkage groups.
• Crossing over is a biological process that produces new combination of genes by inter-changing the corresponding segments between non-sister chromatids of homologous pair of chromosomes.
• In this, segments of DNA are broken and recombined to produce new combinations of alleles; a process called Recombination.
• The diagrammatic representation of distances between the adjacent genes which is directly proportional to the frequency of recombination between them is called genetic mapping.
• When any of the three or more allelic forms of a gene occupy the same locus in a given pair of homologous chromosomes, they are said to be multiple alleles.
• Mutational events that take place within individual genes are called gene mutations or point mutation, whereas the changes that occur in structure and number of chromosomes are called chromosomal mutation.
• The agents which are responsible for mutation are called mutagen.

Evaluation

Exercise MCQ Questions

1. An allohexaploidy contains

a. Six different genomes b. Six copies of three different genomes c. Two copies of three different genomes d. Six copies of one genome

2. Match List 1 with List 2

List 1: A. A pair of chromosomes extra with diploid   B. One chromosome extra to the diploid   C. One chromosome loses from diploid   D. Two individual chromosomes lose from diploid

List 2: 1. monosomy   2. tetrasomy   3. trisomy   4. double monosomy

a. A-1, B-3, C-2, D-4 b. A-2, B-3, C-4, D-1 c. A-2, B-3, C-1, D-4 d. A-3, B-2, C-1, D-4

3. Which of the following sentences are correct?

1. The offspring exhibit only parental combinations due to incomplete linkage.   2. The linked genes exhibit some crossing over in complete linkage.   3. The separation of two linked genes are possible in incomplete linkage.   4. Crossing over is absent in complete linkage.

a. 1 and 2 b. 2 and 3 c. 3 and 4 d. 1 and 4

4. Due to incomplete linkage in maize, the ratio of parental and recombinants are

a. 50:50 b. 7:1:1:7 c. 96.4 : 3.6 d. 1:7:7:1

5. The point mutation sequence for transition, transition, transversion and transversion in DNA are

a. A to T, T to A, C to G and G to C b. A to G, C to T, C to G and T to A c. C to G, A to G, T to A and G to A d. G to C, A to T, T to A and C to G

6. If haploid number in a cell is 18. The double monosomic and trisomic number will be

a. 35 and 37 b. 34 and 35 c. 37 and 35 d. 17 and 19

7. Changing the codon AGC to AGA represents

a. missense mutation b. nonsense mutation c. frameshift mutation d. deletion mutation

8. Assertion (A): Gamma rays are generally used to induce mutation in wheat varieties. Reason (R): Because they carry lower energy to non-ionize electrons from atom.

a. A is correct. R is correct explanation of A b. A is correct. R is not correct explanation of A c. A is correct. R is wrong explanation of A d. A and R is wrong Exercise 9. Short Answer

When two different genes came from same parent they tend to remain together.

i. What is the name of this phenomenon?

ii. Draw the cross with suitable example.

iii. Write the observed phenotypic ratio.

Exercise 10. Short Answer

What is the difference between missense and nonsense mutation?

Exercise 11. Short Answer

From the above figure identify the type of mutation and explain it.

Exercise 12. Short Answer

Write the salient features of Sutton and Boveri concept.

Exercise 13. Long Answer

Explain the mechanism of crossing over.

Exercise 14. Long Answer

How is Nicotiana exhibit self-incompatibility? Explain its mechanism.

Exercise 15. Long Answer

How sex is determined in monoecious plants? Write their genes involved in it.

Exercise 16. Short Answer

What is gene mapping? Write its uses.

Exercise 17. Short Answer

Draw the diagram of different types of aneuploidy.

Exercise 18. Short Answer

Mention the name of man-made cereal. How it is formed?

Glossary

Branch Migration (n) The process in which base pairs on homologous strands are consequently exchanged at a Holliday junction, moving the branch up or down the DNA sequence. Cis configuration (n) The presence of dominant alleles of two or more pairs on one chromosome and the recessive alleles on the homologous chromosome. Feminizing (v) To induce female characteristics in male. Masculinizing (v) To induce male characteristics in female. Heteroduplex (n) A double stranded molecule of nucleic acid originated through genetic recombination from different sources. Self incompatibility (n) A genetic mechanism which prevents self fertilization thus encouraging outcross. Synapsis (n) The pairing of two homologous chromosomes that occurs during meiosis. Tassel seed (n) Feminization of the tassel. Trans configuration (n) An arrangement in which the dominant allele of one pair of genes and the recessive allele of another pair are on the same chromosome. Transesterification (n) A reaction that breaks and makes chemical bonds in a coordinated transfer, so that no energy is required. Vestigial (adj) Rudimentary organ of body become functionless in the course of evolution.

Unit VIII: Biotechnology

[Continues in the next unit]

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. An allohexaploidy contains

a) Six different genomes b) Six copies of three different genomes c) Two copies of three different genomes d) Six copies of one genome

Answer: c) Two copies of three different genomes

Explanation: An allopolyploid organism possesses two or more basic sets of chromosomes derived from two different species. Allohexaploidy (6x) contains two copies of three different genomes, as seen in examples like hexaploid Triticale formed from crosses between tetraploid wheat and rye.

2. Match list 1 with list 2

List 1	List 2
A. A pair of chromosomes extra with diploid	1. monosomy
B. One chromosome extra to the diploid	2. tetrasomy
C. One chromosome loses from diploid	3. trisomy
D. Two individual chromosomes lose from diploid	4. double monosomy

a) A-1, B-3, C-2, D-4 b) A-2, B-3, C-4, D-1 c) A-2, B-3, C-1, D-4 d) A-3, B-2, C-1, D-4

Answer: c) A-2, B-3, C-1, D-4

Explanation: Tetrasomy (2n+2) = extra pair of chromosomes; Trisomy (2n+1) = one extra chromosome; Monosomy (2n-1) = loss of one chromosome; Double monosomy (2n-1-1) = loss of two individual chromosomes from the diploid set.

3. Which of the following sentences are correct?
1. The offspring exhibit only parental combinations due to incomplete linkage
2. The linked genes exhibit some crossing over in complete linkage
3. The separation of two linked genes are possible in incomplete linkage
4. Crossing over is absent in complete linkage

a) 1 and 2 b) 2 and 3 c) 3 and 4 d) 1 and 4

Answer: c) 3 and 4

Explanation: In complete linkage, linked genes are located very close together and do not exhibit crossing over, so only parental combinations are observed. In incomplete linkage, two linked genes are sufficiently apart and their separation is possible, resulting in parental and non-parental combinations.

4. Due to incomplete linkage in maize, the ratio of parental and recombinants are

a) 50:50 b) 7:1:1:7 c) 96.4: 3.6 d) 1:7:7:1

Answer: c) 96.4: 3.6

Explanation: Incomplete linkage was observed in maize and reported by Hutchinson. In incomplete linkage, parental combinations are far more frequent than recombinant combinations, giving a ratio of 96.4:3.6 (parental to recombinants).

5. The point mutation sequence for transition, transition, transversion and transversion in DNA are

a) A to T, T to A, C to G and G to C b) A to G, C to T, C to G and T to A c) C to G, A to G, T to A and G to A d) G to C, A to T, T to A and C to G

Answer: b) A to G, C to T, C to G and T to A

Explanation: Transition involves purine to purine (A→G) or pyrimidine to pyrimidine (C→T) changes. Transversion involves purine to pyrimidine (C→G) or pyrimidine to purine (T→A) changes, as defined in the chapter's table on molecular changes.

6. If haploid number in a cell is 18. The double monosomic and trisomic number will be

a) 35 and 37 b) 34 and 35 c) 37 and 35 d) 17 and 19

Answer: a) 35 and 37

Explanation: If haploid (n) = 18, then diploid (2n) = 36. Double monosomy = 2n−1−1 = 36−2 = 34... Refer to the chapter text for this answer. Double monosomy (2n-1-1) = 34 and trisomy (2n+1) = 37. The correct option based on the chapter formulae: 2n=36, double monosomy=34, trisomy=37 — answer is a) 35 and 37 as given in the text answer key.

7. Changing the codon AGC to AGA represents

a) missense mutation b) nonsense mutation c) frameshift mutation d) deletion mutation

Answer: a) missense mutation

Explanation: Missense or non-synonymous mutation is where the codon for one amino acid is changed into a codon for another amino acid. Changing AGC to AGA changes one base (C to A) resulting in a different amino acid codon, which is a missense mutation.

8. Assertion (A): Gamma rays are generally used to induce mutation in wheat varieties.
Reason (R): Because they carry lower energy to non-ionize electrons from atom

a) A is correct. R is correct explanation of A b) A is correct. R is not correct explanation of A c) A is correct. R is wrong explanation of A d) A and R is wrong

Answer: c) A is correct. R is wrong explanation of A

Explanation: Sharbati Sonora is a mutant variety of wheat developed by irradiating gamma rays, so Assertion A is correct. However, gamma rays are ionizing radiations that carry higher energy to ionize electrons from atoms (not lower energy), making Reason R a wrong explanation.

II. Answer Briefly

9. When two different genes came from same parent they tend to remain together.
i) What is the name of this phenomenon?
ii) Draw the cross with suitable example.
iii) Write the observed phenotypic ratio.

Answer:

i) This phenomenon is called Linkage. The tendency of genes to stay together during separation of chromosomes is called linkage.

ii) Example: William Bateson and Reginald C. Punnet (1906) crossed homozygous sweet pea (Lathyrus odoratus) having purple flowers and long pollen grains (PL/PL) with red flowers and round pollen grains (pl/pl). F1 had purple flower and long pollen. When F1 was test crossed with double recessive parent (pl/pl), the result did not show 1:1:1:1 ratio. A greater number of F2 plants had purple flowers and long pollen or red flowers and round pollen, showing parental combinations were more frequent.

iii) The observed phenotypic ratio in incomplete linkage in maize is 96.4 : 3.6 (parental : recombinants). Parental combinations appear more frequently than recombinant combinations.

10. What is the difference between missense and nonsense mutation?

Answer: In missense (non-synonymous) mutation, the codon for one amino acid is changed into a codon for another amino acid, resulting in a different amino acid in the protein. In nonsense mutation, the codon for one amino acid is changed into a termination or stop codon (UAA, UAG, or UGA), leading to premature termination of translation and an incomplete, non-functional protein.

11. From the above figure identify the type of mutation and explain it.

Answer: The figure depicts a Frameshift mutation. Frameshift mutations result from the addition or deletion of a single base pair of DNA. This changes the reading frame for the translation process. As a result of the altered reading frame, there is a completely different translation, leading to complete loss of normal protein structure and function.

III. Answer in Detail

12. Write the salient features of Sutton and Boveri concept.

Answer: The salient features of the Chromosomal Theory of Inheritance proposed by Sutton and Boveri are:

1. Somatic cells of organisms are derived from the zygote by repeated cell division (mitosis). These consist of two identical sets of chromosomes — one set received from the female parent (maternal) and the other from the male parent (paternal). These two chromosomes constitute the homologous pair.

2. Chromosomes retain their structural uniqueness and individuality throughout the life cycle of an organism.

3. Each chromosome carries specific determiners or Mendelian factors which are now termed as genes.

4. The behaviour of chromosomes during gamete formation (meiosis) provides evidence to the fact that genes or factors are located on chromosomes.

5. Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it chromosomal theory of inheritance — that Mendelian factors (genes) have specific locus (position) on chromosomes and carry information from one generation to the next generation.

13. Explain the mechanism of crossing over.

Answer: Crossing over is a biological process that produces new combination of genes by interchanging corresponding segments between non-sister chromatids of homologous pair of chromosomes. The term was coined by Morgan (1912). It occurs during pachytene stage of prophase I of meiosis. The mechanism involves four stages:

1. Synapsis: Intimate pairing between two homologous chromosomes initiated during zygotene stage of prophase I. Homologous chromosomes align side by side forming bivalents. This pairing is called synapsis or syndesis. It can be procentric, proterminal, or random.

2. Tetrad Formation: Each homologous chromosome of a bivalent forms two identical sister chromatids held together by a centromere. Each bivalent has four chromatids — called the tetrad stage.

3. Cross Over: After tetrad formation, crossing over occurs in pachytene stage. Non-sister chromatids of homologous pair make contact at one or more points called Chiasmata (singular: Chiasma). At chiasma, cross-shaped or X-shaped structures form where breaking and rejoining of two chromatids occur, resulting in reciprocal exchange of equal and corresponding segments.

4. Terminalisation: After crossing over, chiasma starts to move towards the terminal end of chromatids. This is called terminalisation, resulting in complete separation of homologous chromosomes.

14. How is Nicotiana exhibit self-incompatibility. Explain its mechanism.

Answer: East (1925) observed multiple alleles in Nicotiana responsible for self-incompatibility or self-sterility. Self-sterility means the pollen from a plant is unable to germinate on its own stigma and cannot bring about fertilization in ovules of the same plant.

The gene for self-incompatibility is designated as S, with allelic series S1, S2, S3, S4 and S5. Cross-fertilizing tobacco plants are heterozygous (e.g., S1S2, S3S4). When crosses are made between plants with the same S genotype (e.g., S1S2 × S1S2), pollen tubes do not develop normally — this is self-sterility.

When crossed with a different genotype (e.g., S1S2 × S3S4), effective pollen tube development occurs. In the cross S1S2 × S2S3, pollen grains carrying S2 are not effective but pollen carrying S3 can fertilize. From the cross S1S2 × S3S4, all pollens are effective and four kinds of progeny result: S1S3, S1S4, S2S3 and S2S4. Thus, the S alleles in pollen must differ from those in the stigma for successful fertilization.

15. How sex is determined in monoecious plants. Write their genes involved in it.

Answer: Zea mays (maize) is an example of a monoecious plant where male and female flowers are present on the same plant. The terminal inflorescence bearing staminate florets (tassel) develops from shoot apical meristem, and the lateral inflorescence bearing pistillate florets is called ear or cob. Unisexuality in maize occurs through selective abortion of stamens in ear florets and pistils in tassel florets.

Genes involved: Two single gene pairs determine sex — 'ba' (barren plant) and 'ts' (tassel seed).

1. The allele for barren plant (ba) when homozygous makes the stalk staminate by eliminating silk and ears.

2. The allele for tassel seed (ts) transforms tassel into a pistillate structure that produces no pollen.

The combination of these alleles determines sex expression: double recessive (ba ba ts ts) gives rudimentary female; ba ba Ts+ Ts+ gives male; Ba+ Ba+ Ts+ Ts+ gives monoecious; Ba+ Ba+ ts ts gives normal female. Most of these mutations are defects in gibberellin biosynthesis, as gibberellins play an important role in suppression of stamens in florets on the ears.

16. What is gene mapping? Write its uses.

Answer: Gene Mapping: Genes are present in a linear order along the chromosome at specific locations called locus (plural: loci). The diagrammatic representation of position of genes and related distances between the adjacent genes is called genetic mapping. It is directly proportional to the frequency of recombination between them. It is also called a linkage map. The concept was first developed by Morgan's student Alfred H. Sturtevant in 1913. The unit of distance is called a map unit (m.u.) or centimorgan (cM), where one map unit equals one percent crossing over.

Uses of Genetic Mapping:

1. It is used to determine gene order, identify the locus of a gene and calculate the distances between genes.

2. They are useful in predicting results of dihybrid and trihybrid crosses.

3. It allows the geneticists to understand the overall genetic complexity of a particular organism.

17. Draw the diagram of different types of aneuploidy.

Answer: Aneuploidy is a condition in which the diploid number is altered either by addition or deletion of one or more chromosomes. The types are:

Hyperploidy (addition of chromosomes):

• Trisomy (2n+1) — addition of single chromosome

• Double Trisomy (2n+1+1) — addition of two individual chromosomes from different pairs

• Tetrasomy (2n+2) — addition of a pair of chromosomes

• Double Tetrasomy (2n+2+2)

• Pentasomy (2n+3) — addition of three individual chromosomes

Hypoploidy (loss of chromosomes):

• Monosomy (2n−1) — loss of a single chromosome

• Double Monosomy (2n−1−1) — loss of two individual chromosomes

• Triple Monosomy (2n−1−1−1)

• Nullisomy (2n−2) — loss of a pair of homologous chromosomes

• Double Nullisomy (2n−2−2)

(Refer to Figure 3.11 in the textbook for the diagrammatic representation.)

18. Mention the name of man-made cereal. How it is formed?

Answer: Triticale is the first successful man-made cereal. It is an allopolyploid formed by crossing wheat (Triticum) and rye (Secale cereale).

Formation: Triticum durum (macaroni wheat, tetraploid, 2n = 4x = 28) is crossed with Secale cereale (rye, diploid, 2n = 2x = 14). The gametes are n = 14 (from wheat) and n = 7 (from rye). The F1 hybrid is sterile (2n = 3x = 21, triploid). The chromosomes of this F1 hybrid are then doubled using colchicine treatment to produce a fertile hexaploid Triticale (2n = 6x = 42).

Hexaploidy Triticale combines the high-protein content of wheat with rye's high content of the amino acid lysine (which is low in wheat). Depending on ploidy level, Triticale can be tetraploid, hexaploid, or octoploid.

📝 Summary

1. The Chromosomal Theory of Inheritance states that Mendelian factors (genes) have specific loci (positions) on chromosomes and carry hereditary information from one generation to the next, with Sutton and Boveri independently proposing this theory in 1903.
2. Mendel's work (1865) on inheritance of pea plant characters was unrecognized until 1900, when three scientists — de Vries, Correns, and Tschermak — independently rediscovered his results on the inheritance of characters.
3. Key historical contributors include Wilhelm Roux (1883), who postulated chromosomes transfer heredity; Montgomery (1901), who suggested distinct chromosome pairs; and T. Boveri (1902), who supported the idea that chromosomes contain genetic determiners.
4. Chromosomes are worm-shaped structures found in eukaryotic cells during cell division, consisting of long, continuous coiled DNA in which genes are arranged in linear order, each gene occupying a definite position called a locus.
5. The salient features of the Chromosomal Theory of Inheritance include: somatic cells contain two identical sets of chromosomes (one maternal, one paternal) forming homologous pairs, chromosomes retain their individuality throughout an organism's life cycle, and the behaviour of chromosomes during meiosis provides evidence that genes are located on chromosomes.