Genetic control of meiosis, linkage and chromosomal mapping

Genetic control of meiosis, linkage and chromosomal mapping:-

Genetic control of meiosis:- All the events of meiosis are under some form of genetic control and mutations which are defective in functions specific to meiosis have been identified in a variety of eukaryotes. 

Meiotic mutations in Drosophila:- A majority of mutations in Drosophila affect only one sex. This is not surprising when one recalls that SC formation and crossing-over is restricted to the female.

Male mutations:- 

> Microtubules, as we have seen, are dimers composed of equimolecular amounts of two 50000 dalton subunits, α-and β-tubulin. 

> While there are other microtubule-associated proteins, the tubulins are the only ones that have been shown to play a role in chromosome movement since they are the principal functional components of the spindle microtubules. 

> A specific β2-tubulin subunit has been identified which is expressed only at male meiosis in D. melanogaster and so functions in the production of the meiotic, but not the mitotic, spindle as well as in the differentiation of the axoneme of the male sperm.

Linkage:- When two alleles present together on a chromosome are inherited together, it is called linkage.

Note:- Crossing over can be up to 50% only.

Linkage groups:- A linkage group is a linearly arranged group of linked genes which are normally inherited together except for crossing over.

Types of linkage:-

i. Complete linkage:- 

> The genes located on the same chromosome do not separate and are inherited together over the generations due to the absence of crossing over. 

> Complete linkage allows the combination of parental traits to be inherited as such. 

> It is rare but has been reported in male Drosophila and some other heterogametic organisms.

ii. Incomplete linkage:-

> Genes present in the same chromosome have a tendency to separate due to crossing over and hence produce recombinant progeny besides the parental type. 

> The number of recombinant individuals is usually less than the number expected in independent assortment.

Sex Linkage:- When genes found on the X-chromosome show linkage, it is called sex linkage.

Criss Cross Inheritance:- When the genes found on the X-chromosome are passed from father to daughter and from mother to son, it is called Criss Cross Inheritance.

i. Diagenic Gene:- A gene that is passed from father to daughter.

ii. Diandric Gene:- A gene that is passed from mother to son.

iii. Hologenic Gene:- A gene found on the X-chromosome that is passed from mother to daughter.

iv. Holandric Gene:- A gene found on the Y-chromosome that is passed from father to son.

Examples:-

1. Colour of Eyes in Drosophila:- T. H. Morgan conducted experiments on the colour of the eyes of Drosophila and found that the results are not the same upon reciprocal cross.

T. H. Morgan explained the reason for this and said that the gene for eye colour in males is linked to the X-chromosome.

2. Hemophilia:- It was discovered by Otto in 1803. It is also called royal disease because it started from Queen Victoria. Victoria's parents did not have the hemophilia gene. This gene was produced due to a mutation in Victoria and Victoria became a carrier of the disease. Victoria's husband Albert was also normal. This disease was observed in the offspring of Victoria. In this disease, the blood does not clot after the injury and there is continuous bleeding. The patient dies due to excessive blood loss. It is an X-linked recessive disease.

This disease is mostly seen in males. Females are mostly carriers.

3. Daltonism (Red Green Colour Blindness):- It was discovered by Horner. In this, the patient is unable to differentiate between red and green colour. It is also an X-linked recessive disease.

Chromosome Mapping:- The process of determining the sequence of genes present on a chromosome and the frequency of crossing over between them is called chromosome mapping.

Procedure:-

Ø  First of all, identify the parent types which are given in the highest number.

Ø  Now identify the Recombinant types.

Ø  Now sum up the numbers of all the individuals and find out the total Individuals.

Ø  Now find out the frequency.

Ø  Find the distance between two genes by the following formula:-

Ø Identification of Middle Gene:- Compare DCO with Parent type. The gene which shows exchange in the DCO progeny as compared to the parent type is the middle gene.

Ø  Now draw the chromosome map.