Question

Linkage and crossing-over of genes are alternatives of each other. Justify with the help of an example.

Answer 1

Linkage and crossing over of genes are alternatives of each other. Linkage is the tendency of the genes present in the same chromosome to stay together during hereditary transmission, while crossing over is the mutual exchange of segments of non-sister chromatids of homologous chromosomes during the process of meiosis. An example of linkage: A double dominant female Drosophila homozygous for red eyes and normal wings is crossed with a double recessive male Drosophila homozygous for purple eyes and vestigial wings. The F₁ flies are heterozygous red-eyed and normal winged. It is then test crossed with a double recessive male having purple eyes and vestigial wings. The F₂ generation comprises two types of flies—red eyed, normal winged and purple eyed, vestigial winged in the ratio of 1:1. No recombinant types are formed because linkage is complete and no crossing over occurs.

An example of crossing over: A double dominant sweet pea plant homozygous for blue flowers and long pollen grains is crossed with a double recessive plant homozygous for red flowers and round pollen grains. The F₁ flies are heterozygous blue flowered and long pollen grains. It is then crossed with a double recessive plant having red flowers and round pollen grains. The F₂ generation of plants has a new combination of traits— blue flowers and round pollen grains and red flowers and long pollen grains. This is due to crossing over, i.e. separation of genes for flower colour and form of pollen grains during gamete formation.