Linkage, crossing over and chromosomal mapping
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.
Crossing Over:- The process in which genes are exchanged between the non-sister chromatids of homologous chromosomes is called crossing over.
· This is a process occurring only in the Prophase - I of meiosis.
Mechanism:- It has 5 main steps –
1. Synapsis
2. Duplication
3. Crossing Over
4. Chiasmata Formation
5. Terminalization
1. Synapsis:-
Ø It occurs in the zygotene sub-stage of prophase - I.
Ø In this step the homologous chromosomes are arranged near each other at a distance of 1000A°. This process is called Synapsis.
Ø 3 types of coupling occurs in the formation of synapsis.
Ø Synaptonemal complex:- After formation of synapsis, it forms a fibrous complex in the space of 1000A°. It was discovered by M. J. Moses in the Spermatocyte of Crayfish.
2. Duplication:-
Ø It occurs in the Pachytene sub-stage of prophase - I.
Ø In this step, each homologous chromosome splits to form 2 sister chromatids. There is no splitting of the centromere.
3. Crossing Over:-
Ø It occurs in the Pachytene sub-stage of prophase - I.
Ø In this step, crossing over takes place between the internal non-sister chromatids. Recombinase enzyme takes part in this process.
Ø The internal non-sister chromatids are broken into segments. Now these segments are exchanged.
4. Chiasmata Formation:-
Ø It occurs in the diplotene sub-stage of prophase - I.
Ø After the exchange of segments between non-sister chromatids, they are joined. As a result, an X-shaped structure appears in the diplotene sub-stage, which is called Chiasma.
Ø The number of Chiasmata depends on the length of the homologous chromosomes. Usually their number does not exceed 4.
Ø There are 2 types of Chiasma depending on the position:-
i. Terminal Chiasmata:- Present at the top of the chromosome.
ii. Intercalary Chiasmata:- It is present in the middle of the chromosome.
5. Terminalization:-
Ø It occurs in the Diakinesis sub-stage of Prophase - I.
Ø After the process of crossing over is complete, the chromatids move away from each other and the chiasmata get disappeared. This process is called Terminalization.
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.