Reciprocal cross

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In genetics, a reciprocal cross is a breeding experiment designed to test the role of parental sex on a given inheritance pattern. [1] All parent organisms must be true 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. It is the cross that could be made either way or independent of the sex of the parents. 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. They might first cross a Z-trait female with an A-trait male and observe the offspring. Next, they would cross an A-trait female with a Z-trait male and observe the offspring. Via principles of dominant and recessive alleles, they 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.

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Reciprocal cross in practice

Given that the trait of interest is either autosomal or sex-linked and follows by either complete dominance or incomplete 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 [2] who studied the inheritance of a colour mutation in a moth, Abraxas grossulariata. Thomas Hunt Morgan later showed that a new white-eye mutation in Drosophila melanogaster was also sex-linked. He 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 squares:

Table 1. Wild-type Male x Mutant Female ( X(wt)Y x X(mut)X(mut))
X (mut)X (mut)
X (wt)X (mut) X (wt)

Red eye Female

X (mut) X (wt)

Red eye Female

YX (mut) Y

White eye Male

X (mut) Y

White eye Male

Table 2. Mutant Male x Wild-type Female ( X(mut)Y x X(wt)X(wt) )
X (wt)X (wt)
X (mut)X (mut) X (wt)

Red eye Female

X (mut) X (wt)

Red eye Female

YX (wt) Y

Red eye Male

X (wt) Y

Red eye Male

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 carriers. [3]

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References

  1. ↑ reciprocal cross definition
  2. ↑ Doncaster L and Raynor GH (1906). Breeding experiments with Lepidoptera. Proceedings of the Zoological Society of London, 1:125–133.
  3. ↑ Fruit Fly Genetics
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