Cytological proof of crossing over

 The first cytological evidence in support of genetic crossing over was provided by Curt Stern in 1931 on the basis of his experiments conducted with Drosophila. He used cytological markers in his studies. He selected a female fly in which one X-chromosome was broken into two segments. Out of these two segments, one behaved as X-chromosome. The other X-chromosome had a small portion of Y-chromosome attached to its one end. Thus, both the X chromosomes in the female had distinct morphology and could be easily identified under a microscope. In female fly, the broken X-chromosome had one mutant allele (carnation) for eye color and another dominant allele (B) for bar eye shape. The other X-chromosome with an attached portion of the Y chromosome had alleles for normal eye color (red eye) and normal eye shape (oval eye). Thus, the phenotype of females was barred.

A cross of such females was made with carnation male (car+). As a result of crossing over female flies produce four types of gametes, viz., two parental types or non-crossover types (car B and ++) and two recombinant types of crossover types (car+ and B+). The male flies produce only two types of gametes (car + and Y) because crossing over does not take place in

Drosophila male. A random union of two types of male gametes with four types of female gametes will produce males and females in an equal number means there will be four females and four males. Stern examined the chromosomes of recombinant types, viz., red bar, and carnation normal under the microscope. He observed that in carnation normal females both the X-chromosomes were of equal length. In red barflies, one X-chromosome was normal and the other was fragmented. The fragmented X-chromosome also had attached part of the Y-chromosome. Such chromosome combination in the red bar is possible only through the exchange of segments between non-sister chromatids of homologous chromosomes. This has proved that genetic crossing over is the result of cytological crossing over. Similar proof of cytological crossing over was provided by Creighton and McClintock in