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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    diploid organisms 13 The first conclusion which can be drawn is that the hybrid progeny from the first cross (these are generally called the Fj generation) although apparently resembling the purple-ñowered parent strain cannot be identical to it, for whereas the parent strain upon self- pollination gives rise only to progeny like itself, this is not true of the hybrids. These, although they have purple flowers, must also contain information for white flowers in some latent form, because they regularly give rise to white-flowered progeny. The situation is clarified if the second generation progeny (usually called the Fj generation) are investigated further. Mendel self pollinated the members of the F2 generation and found that whereas the quarter which produced white flowers were true breeding just like the white-flowered parent strain, the purple-flowered progeny were of two types. One third of the purple-flowered F2 progeny (i.e. ^ of the total progeny) were true breeding, but the remaining two-thirds (i.e. ^ of the total progeny) behaved just like the purple-flowered Fi generation, giving rise to purple-flowered and white-flowered progeny in the ratio of three to one. The simplest way to explain these results is to postulate that each pea plant contains two pieces of information for any one character, i.e. is diploid. In a true-breeding strain these two pieces of information are similar. During gamete formation, the information content is halved, so that the gametes are haploid. Gamete production must then involve a reduction division. When gametes from two true- breeding strains fuse to form a zygote this will be diploid again, and will contain an equal amount of information from each parent. The same will be true of the hybrid which develops from this zygote. It must next be proposed that although this hybrid contains information both for purple flowers and white flowers it produces purple flowers. When this hybrid comes to produce gametes by reduction division, two types will be produced in equal numbers. It is easier to see the result of random fusion of these gametes among themselves, if symbols are again adopted. With a cultivated species like the pea, it is a little difficult to know what is v/ild type, but the purple-flowered allele will rather arbitrarily be designated +, and the white-flowered w. The two parental strains are diploid and may therefore be represented yj; - for the purple-flowered strain, and — for the white-flowered strain. The breeding scheme can then be represented as in figure 7. It is convenient at this juncture to introduce a few more technical