3.8 Linkage and Crossing Over :

Linkage :

It is a known fact that several genes are present on the chromosome. As chromosomes are carriers of heredity, these genes have tendency to be inherited together. Such genes are called linked genes. This tendency of two or more genes present on the same chromosomes that are inherited together is known as linkage.

Linkage was discovered in plants by Bateson and Punnett and in animals by T. H. Morgan.

Linkage is of two kinds - complete and incomplete linkage:

I. Complete linkage :

The linked genes which are closely located on the chromosome do not separate (no crossing over) and inherit together. They are called completely linked (strongly linked) genes and the phenomenon of their inheritance is called complete linkage. Thus the parental traits are inherited in offsprings.

e.g. X chromosome of Drosophila males- show complete linkage.

II. Incomplete linkage :

The linked genes which are distantly located on the same chromosome and have chances of separation by crossing over, are known as incompletely linked (weakly linked) genes. The phenomenon of their inheritance, is called incomplete linkage. Thus, new traits occur in offsprings.

e.g. In Zea mays - colour and shape of grain show incomplete linkage.

Linkage Groups :

All the linked genes in a particular chromosome, constitute a linkage group. The number of linkage groups of a particular species corresponds to its haploid number of chromosomes.

e.g. Drosophila melanogaster has 4 linkage groups that correspond to the 4 pairs of chromosomes.

Garden pea has 7 linkage groups and 7 pairs of chromosomes.

Sex-linkage :

The transmission (inheritance) of X - linked and Y-linked genes from parents to offspring is called sex-linked inheritance.

Sex-linked inheritance is of three types viz.

X-linked, Y-linked and XY-linked.

Sex linkage is of two kinds :

a. Complete sex linkage :

It is exhibited by genes located on non-homologous regions of X and Y chromosomes. They inherit together because crossing over does not occur in this region.

Examples of X-linked traits are haemophilia, red-green colour blindness, myopia (near sightedness) and for Y-linked are hypertrichosis, Ichthyosis, etc.

b. Incomplete sex linkage : It is exhibited by genes located on homologous regions of X and Y chromosomes. They do not inherit together because crossing over occurs in this region.

Examples of X-Y linked traits are total colour blindness, nephritis, retinitis pigmentosa, etc.

Crossing Over :

Crossing over is a process that produces new combinations (recombination) of genes by interchanging and exchanging of corresponding segments between non-sister chromatids of homologous chromosomes. It occurs during pachytene of prophase I of meiosis. The term crossing over was coined by Morgan. The mechanism of crossing over consists four sequential steps such as synapsis, tetrad formation, crossing over and terminalization. This was studied in the chapter on cell division in class XI. The phenomenon of crossing over is universal and it is necessary for the natural selection, because it increases the chances of variation.