Stern's experiment in Drosophila
Drosophila (fruitfly) stocks carrying translocations were utilised by C. Stern in order to produce a female Drosophila having a part of Y-chromosome attached to one of the two X-chromosomes. The other X-chromosome of this female fly was also marked (identifiable due to distinct morphology) and consisted of two approximately equal fragments, each carrying its centromere. These two X-chromosomes in the female fly could be distinguished not only from each other, but also from normal X-chromosome under the microscope.
In the above female fly, one of the two fragments of an X-chromosome carried mutant alleles for carnation eye (car is recessive showing light eye colour) and barred eye (B is dominant showing narrower eyes). The other X-chromosome, having a part of Y attached, carried normal alleles of these two genes, so that the female heterozygote for both these genes (car B/+ +) had barred eyes (but normal eye colour, since car is recessive to +). Such females were crossed with male flies having recessive alleles for both these genes (car, +). In such a situation, this makes a simple testcross. If no crossing over takes place between the two genes in question, two types of gametes i.e., car B and ++ will be produced from the female flies. Crossing over will give two additional types ot gametes i.e., car + and + B (Figure 10.16). Due to fertilization of two types of non-crossover and the other two types of crossover gametes by male gametes carrying X-chromosome (car +), four kinds of female flies will be produced. Another four kinds of male flies will be produced due to fertilization by Y carrying male gametes (Fig. 10.16).
The flies which are classified as crossovers on the basis of phenotype i.e., carnation (with normal eye shape) and barred (with normal eye colour) were studied cytologically. It was found that carnation flies did not have any fragmented X-chromosome, but rather had normal X-chromosome. On the other hand, barred flies had a fragmented X-chromosome with a segment of Y-chromosome attached to one of the two fragments of X-chromosome. Such cytological observations suggested that genetic crossing over was accompanied with an actual exchange of chromosome segments.
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