- Reciprocal cross
In
genetics , a reciprocal cross is a breeding experiment designed to test the role of parental sex on a given inheritance pattern [ [http://www.biochem.northwestern.edu/holmgren/Glossary/Definitions/Def-R/reciprocal_cross.html reciprocal cross definition ] ] . All parent organisms must betrue breeding to properly carry out such an experiment. In one cross, a male expressing the trait of interest will be crossed with a female not expressing the trait. In the other, a female expressing the trait of interest will be crossed with a male not expressing the trait.For example, suppose a biologist wished to identify whether a hypothetical allele Z, a variant of some gene A, is on the male or female sex chromosome. He might first cross a Z-trait female with an A-trait male and observe the offspring. Next, he would cross an A-trait female with a Z-trait male and observe the offspring. Via principles of dominant and recessive alleles, he could then (perhaps after cross-breeding the offspring as well) make an inference as to which sex chromosome contains the gene Z, if either in fact did.
Such types of experiments are unlikely to actually be carried out in the contemporary era of
DNA sequencing andmolecular genetics .Reciprocal cross in practice
Given that the trait of interest is either
autosomal or sex-linked and follows by eithercomplete dominance orincomplete dominance , a reciprocal cross following two generations will determine the mode of inheritance of the trait.White-eye mutation in "
Drosophila melanogaster "Sex linkage was first reported by Doncaster and Raynor in 1906 [Doncaster L and Raynor GH (1906). Breeding experiments with "Lepidoptera". "Proceedings of the Zoological Society London", 1:125–133.] who studied the white-eye mutation inDrosophila melanogaster . They found that a white-eyed male crossed with a red-eyed female produced only red-eyed offspring. However, when they crossed a red-eyed male with a white-eyed female, the male offspring had white eyes while the female offspring had red eyes. The reason was that the white eye allele is sex-linked (more specifically, on the X chromosome) and recessive.The analysis can be more easily shown with
Punnett square s:As shown in Table 1, the male offspring are white-eyed and the female offspring are red-eyed. The female offspring are carrying the mutant white-eye allele X(mut), but do not express it phenotypically because it is recessive. Although the males carry only one mutant allele like the females, the X-chromosome takes precedence over the Y and the recessive phenotype is shown.
As shown in Table 2, all offspring are Red-eyed. The males are free of the mutation. The females however, are
carrier s [ [http://www.hhmi.swmed.edu/Labs/rr/world/fly.html Fruit Fly Genetics ] ] .References
External links
* [http://math.hws.edu/javamath/ryan/Genetics1.html Simple Medelian Genetics in Drosophila]
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