The Double crossing over: Crossing over occurs

The other X chromosome of this female was of normal length, but the segment of the Y chromosome was translocated into its short arm, this chromosome had dominant gene car-i- (wild type allele of car, producing dull red eye color and the recessive gene B+wild type alleles of B, producing ovate eye shape). Stern taken test crosses of this female to a car B+ male. As expected the following four types of flies were recorded in test cross progeny 1) Red, normal (car-i- B+) 2) Red bar (car-i- B) 3) Carnation normal (car B+) 4) Carnation bar (car B) Two out of these four phenotypes viz.

, red, normal and carna­tion bar, are non crossover or non recombinant types. In contrast red bar and carnation normal are crossover or re­combinant type. Stern concluded that, during meiosis, there is ex­change of precisely homologous chromatid segments between ho­mologous chromosomes (crossing over) and crossing over is re­sponsible for the recombination between linked genes.

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Types of crossing over:

1. Single crossing over:

In this case there is only one chiasma i.e. frequency of crossing over is very less.

2. Double crossing over:

Crossing over occurs at two points in the same chromosome pair.

3. Multiple crossing over:

Crossing over’s occur at three or more points.


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