Mendelism, Second Edition by Reginald Crundall Punnett

By Reginald Crundall Punnett

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These facts led Mendel to the conception of pairs of unit-characters,* of which either can be carried to any one gamete to the exclusion of the other. A fundamental property of the gamete is that it can bear either one of such a pair of characters, though not both. But the heterozygote is formed by the union of two dissimilar gametes, and consequently the cells of the individual into which it grows must contain both characters. To reconcile these statements it must be supposed that a t some cell division in the formation of gametes a primitive germ-cell divides into two dissimilar portions.

This is easily tested by breeding them together. I t is found that from such matings one quarter of the offspring are coloured recessives, whilst the remainder are pure white, or white with a few ticks. The heterozygote resembles the dominant form much more closely than it does the recessive. Though we may speak of dominance in such a case, it is necessary to remember the dominance is not perfect. This, however, makes no difference to the essential feature of Mendel's discovery, which is of course the segregation in the gametes of the factors corresponding to the dominant and recessive characters.

He may, for example, possess tall green-seeded and dwarf yellow-seeded peas, and may wish to raise a strain of green dwarfs. He makes his cross-and nothing but tall yellows result. At first sight he would appear to be further than ever from his end, for the hybrids differ more from the plant at which he is aiming than did either of the original parents. Nevertheless, if he sow the seeds of these hybrids he may look forward with confidence to the appearance of the dwarf green. And owing to the recessive nature of both greenness and dwarfness, he can be certain that for further generations the dwarf greens thus produced will come true to type.

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