In this article we will discuss about:- 1. Exception of Linkage of Chromosomes 2. Operation of Linkage from Morgan’s Data 3. Characteristics 4. Groups 5. Map.
- Exception of Linkage of Chromosomes
- Operation of Linkage from Morgan’s Data
- Linkage—its Characteristics
- Linkage Groups
- Linkage Map
1. Exception of Linkage of Chromosomes:
The seven pairs of characters in garden peas with which Mendel worked were located in separate chromosomes. But when genes for different characters stay together in the same chromosome a very confusing phenomenon occurs.
Genes in the same chromosome do not abide by the principles of independent assortment. Independent assortment does not hold good in cases where genes are located in the same chromosome. Thus, Mendel’s law of independent assortment is not universal but is limited to genes situated in different chromosomes.
The exception to this law was first noticed by Bateson and Punnet in England in 1906.
They bred a dihybrid strain of sweet peas in which one pair of characters was Purple (P) and Red (p) in the colour of the flowers and the other pair of characters Long (L) and Round (1) in the shape of pollen grains. The F1 dihybrids were all purple long when a cross was made between purple long and red round peas because purple and long and dominant characters.
But when these dihybrids having PL, PI, pL and pi were crossed together to form the F2 generation the four expected phenotypic kinds in the ratio of 9:3-3: 1 never turned up. What actually turned up was purple long. 4831 purple long, 390 purple round, 393 red long and 1338 red round were obtained out of a total of 6952.
To account for this deviation from the expected assortment, Bateson and Punnet put forward the view that when two dominant characters go in together from the same parent to form the dihybrid offsprings there is a tendency for the same pair of characters to enter the same gamete and to be transmitted together.
This phenomenon, they called coupling occurs when a dominant and a recessive factor enters the F1 dihybrid from each parent instead of two dominant factors from one parent and two recessive factors from the other. There occurs always a tendency for two dominant factors entering separately and likewise for two recessive factors to avoid or repel each other. This tendency of unlike pairs (one dominant and one recessive) to stay together avoiding union with their own dominant or recessive sort is termed repulsion.
Similar situation encountered by Morgan in Drosophila in the year 1910 resulted in his putting forward a satisfactory explanation of coupling and repulsion. Morgan advocated that coupling and repulsion are but two aspects of a single phenomenon called linkage. He supposed that this tendency of linked genes to remain in their original combination was due to their residence in the same chromosome.
Thus certain genes located on the same chromosome tend to remain linked together while passing from one generation to another. This tendency of certain genes to stick together in the way in which they entered the cross has been termed linkage and such genes are called linked genes.
2. Operation of Linkage from Morgan’s Data:
A wild type Drosophila having gray body (G) and long wings (L) is- crossed with a fly having two recessive mutations of black body (g) and vestigial wing (1). The members of F1 generation were all like wild types, that is, gray bodied and long winged as gray body colour and long wings are dominant over black body and vestigial wings (Fig. 2.12).
Now a male fly from this, dihybrid is back-crossed to a double recessive black vestigial female. Had there been independent assortments, the kinds of flies would have been formed in the following way as shown in Table Genetics—6.
But from actual experiments only two classes of offsprings namely, gray long and black vestigial like two grandparents were obtained (Fig. 2.12). This shows that gray body and long wings entering the dihybrid cross from one parent stay linked together.
3. Linkage—its Characteristics:
(a) Linkage is an exception to Mendel’s principle of independent assortment.
(b) Linked genes are housed in the same pair of chromosomes.
(c) Lesser the distance between the linked genes stronger is the linkage bond. Genes lying farther apart show less linkage.
(d) Linked genes tend to be transmitted together from one generation to the other.
(e) Separation of linked genes occurs very seldom.
4. Linkage Groups:
If a certain gene X is linked to two other genes Y and Z, then it is true that Y and Z are linked. In plants or animals where several genes exist, crosses are arranged to ascertain the existence of linkage of pairs or of groups of several pairs of genes. The genes known to exist in a species is thus divided into linkage groups.
5. Linkage Map:
Constancy of percentages of crossing over and fixed location of genes on chromosomes offer an opportunity to measure the comparative linear distance between any two genes in question. The units for measuring such a distance between two genes is called a ‘Morgan’.
One unit or Morgan is actually the percentage of crossing over. If two genes show 10 per cent of crossing over it is assumed that the genes for these two characters are 10 Morgans apart along the length of the chromosome on which they are housed.
From linkage and crossing over studies it has become possible to determine the specific location of genes on chromosomes with reference to one another. This plotting of specific location of genes on the chromosomes is known as linkage map, genetic map or chromosome map. This has been done thoroughly for hundreds of genes in Drosophila and many other organisms.
Method for construction:
Let US assume that three genes K, L and M are located in a chromosome along its length and we want to know the order of the genes on the chromosome and the relative distance between them. If the genes K and L upon breeding back to the recessive show 10 per cent of crossing over with k and 1, we can say that they are 10 Morgans apart.
If the genes L and M upon breeding back to the recessive show 30 per cent of crossing over with 1 and m, then it is clear that L and M are 30 Morgans apart.
Thus it becomes obvious that the relative distance between K and M will be 40 units (10+30) in case K, L and M are serially arranged. In case K, L, M are not serially arranged along the length of the chromosome the relative distance between K and M will be (30—10) or 20 units and the order of the genes will be M, K and L.
The work of gene localization is an amazing work. It ranks equal to the accomplishment of the mathematicians and astronomers who have been able to measure the vast distances that separate the stars. The wise application of genetic map stands in a good way to discover a good many facts about the orderly mechanism of heredity (Fig. 2.22).