Genetics / D.J. Cove.

Cove, D. J.

Date:: 1971

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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Genetics / D.J. Cove. Source: Wellcome Collection.

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diploid organisms 15 The hypothesis which Mendel put forward to explain the results from his single character crosses, which have been summarised in figure 7, can be tested. If the scheme is correct, then the cross between the purple-flowered Fi hybrid ^^heterozygous and the true-breeding white-flowered parental strain ^homozygous should give equal numbers of — (purple-flowered) and — (white-flowered) progeny (see parents ±_ w JUL w by reduction division gametes + w w zygotes (in equal numbers upon random fusion) adult plants _+ w w w purple-flowered white-flowered Figure 8. (For explanation, see above.) figure 8 for breeding scheme). Mendel found that this was so. He obtained 84 purple-flowered and 81 white-flowered progeny, which is in close agreement to the predicted 1 : 1 ratio. This type of cross, where an Fi hybrid is crossed to one of the parent strains is called a back cross. The special case where the Fj hybrid is crossed to a strain homozygous for the recessive alleles involved (this need not necessarily be a parent strain) is called a test cross. The theory of dominance which proposes that the heterozygous strain resembles one of the two homozygous strains which gave rise to it, may at first sight seem to be proposing a rather unlikely situation. However it will be recalled that mutation commonly leads to the absence of an enzyme. It is probable that the w allele has arisen from the + allele by mutation, and that strains homozygous for the w allele are unable to make an enzyme necessary for the synthesis of