Cytological Basis of Crossing Over

The hypothesis of crossing over was proposed by Morgan to explain some abnormal results in F2 generation from a dihybrid cross. It remained a working hypothesis for about 20 years in the absence of any concrete evidence.

Normally, the homologous chromosomes of a pair are morphologically alike and indistinguishable. But C.Stern (1931) in Drosophila and H.B Creighton and B.MeClintock in maize found the homologous of a pair of homologous chromosomes had a ‘marker’ by which it could be distinguished easily from its counterpart.

Stern’s Experimental on Drosophila:

Curt Stern obtained a strain of Drosophila, which possessed a heteromorphic pair of X-chromosomes. The two X-chromosomes differed in following features:

1. On X-chromosome had a portion of Y-chromosome attached to it, so that it appeared L-shaped. This X-chromosome was used as a Cytological marker for demonstrating crossing over.

2. Other X-chromosome was broken into two unequal sengements. One segment behaved as an X-chromosome and carried sex-linked genes, while the other segment was translocated to a member of the fourth pair of chromosomes.

Creighton and MeClintock Experiment in Corn:

H.B Creighton and B.MeClintock (1931) obtained a corn plant in which ninth chromosome had a darkly stained knob at one and a translocated segment of eighth chromosome at the other end. This plant was heterozygous for color of aleurone and the type of carbohydrate in endorsperm of seeds. These genes can be represented.

These two pair of genes were present on 9th chromosome and were arranged in repulsive phase (C wx/c Wx). When this plant was test crossed with a double recessive homozygous plant (C wx/c Wx), there were two types of noncross over gametes and six types of cross over gametes. They had the following arrangement:

The study of chromosomes of the offspring with different phenotypes clearly indicates that in all the six crossover types genetic recombination has occurred on the basis of the following observations:

1. Presence of knob on chromosome with the phenotype: colourseeds (C) and nonwaxy endosperm (Wx) indicating crossing over between C and Wx loci.

2. Presence of knob on a chromosome carrying genes for coloured and nonwaxy character (C Wx) indicating crossing over between C and wx loci.

3. In parents genes C and wx, for characters colored and waxy endosperm are present on the same chromosome but in double cross over, these are present on different chromosomes (Cwx→ cwx or cWx→CWx) indicating double crossing over between knob and c locus and between c and wx loci.