Genetics / D.J. Cove.

  • Cove, D. J.
Date:
1971
Catalogue details

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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    16 Patterns of inheritance П; purple flower pigment. The heterozygote, ^ will contain one wild-type allele, and so will be able to make some functional enzyme. It will therefore be able to synthesise purple pigment. The possession of one wild-type allele by a heterozygote does not necessarily ensure that the heterozygote will resemble the homozygous wild-type. There are other examples, some also involving flower pigment, where the heterozygote is intermediate between the two parental strains. Thus when a true-breeding red-flowered strain of Antirrhinum is crossed to a true-breeding white-flowered strain, the Fi hybrids are pink- flowered. The F2 from this Fi consists of red-flowered, pink-flowered and white-flowered plants in the ratio 1 : 2 : 1, and so it is possible here to observe directly the segregation which Mendel could only detect fully by further breeding experiments. In this case presumably the heterozygote with its single wild-type allele cannot make enough functional enzyme to catalyse the synthesis of the full amount of flower pigment, with the result that its flowers are pink not red. The breeding behaviour of Aspergillus indicated that its cells are haploid, with the exception of the diploid zygote which undergoes reduction division to restore the normal haploid state. In the pea we have deduced that its cells are diploid, with the exception of the gametes, which must therefore be produced by reduction division, and which fuse to form a zygote, so restoring the normal diploid state. These two apparently contrasting situations can be resolved by representing their life cycles as has been done in figure 9. In Aspergillus, the diploid phase is eliminated, leaving the zygote as the only diploid cell, whereas in the pea the haploid phase is much reduced, so that nearly all cells are diploid. The life cycle of all eukary- otic organisms conforms basically to this scheme. Man and most other animals have no haploid phase, the gametes being the only haploid cells. Most plants show to some extent both a diploid and a haploid phase, and in some cases, as for example is found in the ferns, С zygote > diploid phase —^ 1 by reduction division gametes < haploid phase Figure 9. (For explanation, see above.)