Essay on Classical Genetics (2196 Words)

The Laws of Mendel:

Mendel’s Predecessors:

1. The hybrids obtained by mating of two different variety of a species. The hybrids of some characters are identical to those of one of the two parents while some others are intermediate between those of two parents. The concept of dominance is present in these observation.

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2. Characters of F₁ and F₂ progeny produced from reciprocal crosses are identical.

3. F₁ progeny are uniform in their characters. But F₂ generation show a large variation for different characteristics.

4. The appearance of the parental forms in F₂ phenotypes were intermediate in appearance.

5. Some phenotype in F₂ have entirely new character forms.

Reasons for their Failure:

1. They studied the plant as a whole.

2. As a result of the above, the plants could not be classified into few clearcut classes.

3. The data from different generations were not kept accurately.

4. Complete control on pollination in the F₁ was lacking.

5. In many studies the F₁ was an interspecific hybrid exhibiting partial to considerable sterility.

6. The number of plants studied in F₂ was relatively small.

7. Most of the characters studied by the earlier workers were quantitative in nature.

Pea-advantages as an experimental material:

1. Several characters had a contrasting forms.

2. Pea-self pollination crop.

3. Flowers are relatively large. So, emasculation and pollination is very easy.

4. Duration single season every year one generation can be grown.

5. There is no problem in germination.

Pairs of contrasting characters of pea:

Reasons for Mendel’s Success:

i. An accurate and incisive analysis of the reasons for failure of earlier workers.

ii. He accurately diagnosed the weakness of their experimental materials, techniques and approaches and avoided them in his own studies.

iii. Studied the inheritance of only one pair of contrasting characters at a time.

iv. His selected pea variety had clearly different forms of one or more characters.

v. He classified the plants of a population on the basis of contrasting characters, and kept an accurate record of the number of plants in each category for every generation.

vi. He carried out his experiments with great care and elaborateness.

vii. His knowledge of mathematics which helped him during the interpretation of his findings.

viii. He was able to formulate appropriate hypothesis.

Mendel was Lucky:

1. The characters choosen by him was qualitative inheritance.

2. The contrasting forms of each of seven characters were governed by a single gene.

3. The seven characters studied by him, the genes for two were located in one chromosome, while three others were present in another chromosome.

Reasons for the Neglect of Mendel’s Finding:

1. He used mathematical principles to explain a biological phenomenon this was not acceptable to biologists.

2. He studied contrasting pairs of characters exhibiting discontinuous variation many of his contemporaries (Darwin) pre occupied with characters exhibiting continuous variation.

3. The phenomenon of fertilization and the behaviour of chromosomes during cell divisions (mitosis and meiosis) were not known at that time.

4. Mendel did not publish his findings through further writing on the subject after his initial paper.

5. Mendel failed to demonstrate the validity of his conclusions in other species.

Highlights of the Mendel’s Paper:

1. Mendel concluded a complete fusion of male and female gametes, and equal contribution of the two parents to the development of characters of the hybrid (F₁).

2. He postulated the existence of genes, which are responsible for the development of various characters.

3. He clearly stated that genes were particulate.

4. He made clear distinction between the phenotype and genotype.

5. He gave the formula for determining the numbers of (i) different types of gametes produced by F₁ (ii) different genotypes in F₂ (iii) homozygous geno-types (iv) individuals in the perfect F₂ for segregation of n number of genes.

6. He introduced the concept of dominance and recessiveness.

7. He described laws of segregation and independent assortment.

8. He concluded that a large number of progeny would increase the precision of observation.

9. He stated that his explanations were based on two important assumptions (i) equal proportion of the different gametes produced by F₁ (ii) equal chance for each gamate thus produced to effect fertilization.

The Law of Segregation:

Inheritance of Seed Shape in Pea:

He crossed round with wrinkled. The seeds resulting from hybridization (F₁ seeds) were all round. The F₂ seeds (seeds produced by selfing of F₁ plants) were on an average 3 out of every 4 seeds were round and 1 out of every 4 seeds wrinkled. He allowed the F₂ plants to self pollinate. He found that all the plants from wrinkled -F₂ seeds produced only wrinkled F₃ seeds. But F₂ round seeds were of two types an average 1/3 only round F₃ seeds, 2/3 both round and wrinkled F₃ seeds in the ratio 3:1. He noted that the result from reciprocal crosses were identical.

The conclusion:

1. Both male and female parents make equal contribution to the development of characters in the progeny results.

2. F₁ → character of only one of the two parents is expressed (dominant character).

3. F₂ generation both dominant and recessive characters appear in 3:1 ratio.

4. The recessive character appears in F₂ unchanged and identical to that of the parent contributing this trait.

5. In F₂ 1/3 of the individual with the dominant character are pure. 2/3 of them are hybrid.

The Law of Segregation —

This law is explained by making the following assumptions.

1. A character is produced by a specific gene

2. Each gene has two alternative forms (Alleles)

3. The two alleles of a gene govern the development of contrasting forms of the character governed by the gene.

4. Each somatic cell of an organism has two copies of each gene.

Definition:

The two alleles of a gene present in the F, do not contaminate each other, they separate and pass into different gametes in their original form producing two different types of gametes in equal frequencies.

Essential features of this explanation of the 3:1 ratio in F₂

1. The presence of two copies of each gene in somatic cells.

2. Only one copy of each gene in gametes.

3. Lack of contamination or modification of each other by the two alleles of a gene during their stay together in the same cell of F₁ hybrids.

4. The separation of the two alleles of a gene and their transmission into separate gametes of F₁.

5. The production of two types of gametes with respect to the heterozygote gene by the F₁ in equal frequencies.

6. Random union between male and female gametes.

Law of Independent Assortment:

This law states that when two pairs of gene enter in F₁ combination, both of them have their independent dominant effect.

Main features —

i. It explains simultaneous inheritance of two plant characters.

ii. In F₁ when two gene controlling two different characters, come together, each gene exhibits independent dominant behaviour without affecting other gene.

iii. These gene pairs segregate during gamate formation independently.

iv. The alleles of one gene can freely combine with the alleles of another gene.

v. Each of two gene pairs when considered separately, exhibits typical 3:1 ratio in F₂ generation.

Example:

When plants of garden pea with yellow round seeds are crossed with plants having green wrinkled seeds. We get yellow round seeds in F₁

Thus yellow colour of seed exhibits dominance over green and round seeds shape over wrinkled independently. The F₁ produces – Yellow round (YR) yellow wrinkled (yr), green round (YR), and green wrinkled (yr). Selfing of F₁ gives rise to all above four types of individuals in 9:3:3:1 ratio

Independent assortment of two pairs of genes in garden pea.

Linkage:

All the genes on a chromosome are said to be linked to one another and belong to the linkage group. The phenomenon of inheritance of linked genes in same linkage group is called linkage.

Features:

i. Two or more genes linked

ii. It may involve either dominant or recessive genes

iii. Linkage between closely located gene

iv. Higher percentage of parent in F]

v. It leads to desirable or undesirable trait

vi. Strength of linkage depends on the distance between the linked genes.

vii. Maximum number of linkage groups in an organism is equal to its haploid chromosome number.

Phases of Linkage:

Coupling:

Linkage between two or more either dominant (AB) or recessive (ab) allele is refered as coupling.

Repulsion:

Linkage of dominant allele with recessive allele known as Repulsion.

Types of Linkage:

1. Based on crossing over —

(a) Complete linkage:

Linkage in which there is no crossing over

(b) Incomplete linkage:

Some frequency of crossing over between linked genes.

2. Based on genes involved:

(a) Coupling Linkage

(b) Repulsion Linkage

3. Based on Chromosome involved:

(a) Autosomal linkage: Linkage of genes which are located in other than sex chromosomes.

(b) X – chromosome linkage: Linkage of genes which are located in sex chromosomes.

Detection of Linkage:

1. Test Cross

2. Individual heterozygous at two loci in self pollinated linkage and pleiotropy.

The only way to distinguish between linkage and pleiotropy is to find out a cross over product between linked characters.

Significance of linkage in plant breeding:

1. Effect on selection

2. Effect on Genetic variance

3. Effect on Genetic Correlation

Thomas H. Morgan (Nobel Lauerate) was first biologist to relate linkage to segregation of homologus chromosomes. He also found crossing over between homologus chromosomes during meiosis.

Table 4. Differences between crossing over and linkage:

Crossing over:

1. Separation of linked genes

2. Non-sister chromatids of homologous chromosomes involves.

3. Frequency of crossing over never exceed 50%.

4. It increases variability.

5. Provides equal frequency of parental and recombinant types in test crosses progeny.

Linkage:

1. Keeps the genes together

2. Individual chromosome involves

3. It can never be more than haploid chromosome number

4. It reduces variability

5. Provides higher frequency of parental types than recombinant types in test cross progeny.

Crossing Over:

Interchange of parts between non sister chromatids of homologus chromosomes during meiotic prophase.

Main features —

1. Take place during meiotic prophase.

2. It occurs between non sister chromatids

3. It takes place at four strand stage

4. It yields two recombinant types or crossover types and two parental types or non cross over types.

5. It leads to exchange of equal genes.

6. The frequency of recombinants can be worked out from the test cross progeny.

Chiasma and Crossing Over:

The point of exchange of segments between non sister chromatids of homologous chromosomes during meiotic prophase is called chiasma. Here only crossing over takes place.

Chiasma Terminalization:

The movement of a chiasma away from the centromere and towards the end of tetrads is called terminalization. Chiasma terminalization occurs between diplotene and metaphase I.

Three theories to explain the mechanism of chiasma terminalization.

1. Electrostatic hypothesis

2. Coiling hypothesis

3. Elastic chromosome repulsion.

Relationship between Crossing Over and Chiasma Formation:

Two theories explain these relationship —

1. Classical Theory:

This theory states that first chiasma is formed then crossing over takes place. The chiasma is formed at diplotene stage of meiosis and crossing over occurs between diplotene and anaphase.

2. Chiasma type Theory:

Here first crossing over occurs and then chiasma is formed.

Molecular Mechanism of Crossing Over:

1. Copy Choice Theory:

The entire recombinant section arises from the newly synthesised section. The non sister chromatids when come in close contact they copy some section of each other resulting in recombination.

2. Breakage and Reunion Theory:

The crossing over takes place due to breakage and reunion of non sister chromatids. The two segments of parental chromosomes which are present in recombinants arise from physical breaks in the parental chromosomes with subsequent exchanges of broken segments.

Interference (Muller):

Tendency of one crossover to reduce the chance of another crossover in its adjacent region.

Coefficient of interference (%) = 1 – Coefficient of Coincidence x 100.

Coincidence (Muller):

It explained the strength or degree of interference.

Coefficient of coincidence (%)

=Observed double crossover/ Expected double crossover

Chromosome Mapping:

A line diagram which depicts various genes present on a chromosome and recombination frequency between them. Such maps are also known as genetic maps or linkage maps. The mapping of chromosomes is done with the help of three point test cross. A three point test cross is a cross of trihybrid with its homozygous recessive parent.

Types of Crossing Over:

Single Crossing Over:

Formation of a single chiasma between non sister chromatids of homologous chromosomes.

Double Crossing Over:

Formation of two chiasma between non sister chromatids of homologous chromosomes.

Multiple Crossing Over:

Formation of more than two cross overs between non sister chromatids of homologous chromosomes.

Factors affecting crossing over are —

i. Distance

ii. Temperature

iii. Sex

iv. Nutrition

v. Chemicals

vi. Structural charges

vii. Centromere effect

Significance of crossing over —

i. Creation of variability

ii. Locating genes

iii. Linkage Maps