Linkage and Crossing Over of Genes

Linkage

Linkage is the phenomenon of certain genes staying together and their inheritance from generation to generation without any change or separation due to their presence on the same chromosome.

Genetic linkage describes the phenomenon in which two genes that are located close to each other on a chromosome are often inherited together from generation to generation without any separation.

Linkage of Genes

T.H. Morgan(1910) proved and defined linkage on the basis of his breeding experiments on fruit fly, Drosophila melanogaster.

T.H. Morgan (1910) proved and defined linkage on the basis of his breeding experiments on fruit fly, Drosophila melanogaster.

Genes are arranged in a linear fashion on the chromosome. Some genes situated in close proximity are always inherited together i.e., become linked.

The strength of the linkage between two genes is inversely proportional to the distance between the two i.e., two linked genes show a higher frequency of crossing over if the distance between them is higher and, they show lower frequency if the distance is small.

All those genes which are located in a single chromosome, constitute a linkage group. The number of linkage groups in a species corresponds to its haploid number of chromosomes.

Example:

• 23 linkage groups in Man (23 pairs of chromosomes)
• 7 linkage groups in Pea (7 pairs of chromosomes)
• 10 linkage groups in Maize (10 pairs of chromosomes)

Types of Linkage

1. Complete Linkage

A linkage or grouping of genes on a chromosome that is not altered and is inherited as such from generation to generation without any cross-over, it is called complete linkage.

If the linkage is complete, there should be parental combinations only and no recombination. Complete linkage is rare but Morgan (1919) reported a complete linkage in Drosophila.

• In Drosophila, genes of grey body and long wings are dominant over the black body and vestigial (short) wings.
  • If pure breeding grey bodied long-winged Drosophila (GL/ GL) flies are crossed with black-bodied vestigial winged flies (gl/gl), the F2 shows a 3 : 1 ratio of parental phenotypes (3 grey body long-winged and one black body vestigial winged).
  • This is explained by assuming that genes of body colour and wing length are found on the same chromosome and are completely linked.

2. Incomplete Linkage

The phenomenon of an occasional crossing over between two homologous chromosomes so that one or more alleles present in a linkage group are replaced by other alleles. Produces both parental and recombinant individuals.

• Morgan and his students have found that linked genes show varied recombinations, some being more tightly linked than others.
  1. In Drosophila, the crossing of yellow bodied (Y) and white-eyed (W) female with brown bodied (Y+) red-eyed (W+) male-produced F1 to be brown bodied red-eyed.
     • On intercrossing of F1 progeny, Morgan observed that the two genes did not segregate independently of each other and, therefore, the F2 ratio deviated significantly from the expected 9: 3: 3: 1 ratio.
     • He found 98.7% to be parental and only 1.3% recombinants.
  2. In the second cross in Drosophila between white-eyed and miniature winged (wwmm) female with wild type or red-eyed normal winged (w+w+m+m+) males.
     • All the F1 were found to be of a wild type, i.e., red-eyed and normal winged. An F1 female fly was then test crossed with the white-eyed and miniature winged male.
     • 62.8% of the progeny was of parental types while 37.2% were recombinants.

Crossing Over

Crossing over of Genes:

Crossing over is the recombination of genes due to the exchange of genetic material between two synapsed homologous chromosomes during meiosis. Which results in new allelic combinations in the daughter cells.

It is the mutual exchange of segments of genetic material between non-sister chromatids of two homologous chromosomes, so as to produce recombination of genes during the production of gametes.

• The non-sister chromatids in which the exchange of segments has occurred are called recombinants or cross-overs.
• It brings a new combination of genes that are different from parents thus introduces variations. The variations are helpful in the struggle for existence and adaptability to changes in the environment. Useful recombinations are picked up by breeders for the development of improved varieties.
• The frequency of crossing over between any two linked genes is proportional to the chromosomal distance between them, crossing over frequencies are used to construct genetic, or linkage maps of genes on chromosomes.
• Gene mapping describes the methods used to identify the locus of a gene and the distances between genes.
• Mutations, temperature changes, and radiation all affect crossing over frequency.