The biochemistry of genetics / by J.B.S. Haldane.

  • Haldane, J. B. S. (John Burdon Sanderson), 1892-1964.
Date:
[1954]
Catalogue details

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

Credit: The biochemistry of genetics / by J.B.S. Haldane. Source: Wellcome Collection.

    16/152 (page 12)
    14 THE BIOCHEMISTRY OF GENETICS When a double heterozygote segregates for two pairs of genes they usually segregate independently. Thus on crossing a homo2ygous coloured short-haired rabbit (CCLL) with a white long-haired (cell) we get CcLl (coloured short-haired) progeny. On crossing these to cell we get about equal numbers of CcLl, Cell, ccLl, and cell, showing that CL, CI, cL and cl gametes are formed in roughly equal numbers. Fat colour in rabbits is also genetically determined. Most rabbits have white fat. Others have yellow fat provided they are fed on green leaves or other food containing xanthophyll, but not if they are given a diet lacking xanthophyll, such as turnips. These rabbits lack a xanthophyll oxidase present in the normal animals, and provided the diet contains xanthophyll, yellow fat is a good recessive, due to a gene x. If we cross homozygous coloured white-fatted (CCXX) rabbits with white yellow-fatted (ccxx) the progeny are coloured white-fatted (CcXx) double heterozygotes. Crossed with ccxx they give about 86 CX: 14 Cx; 14 cX: 86 ex. CX 4-4- Such heterozygotes are called — or If we make up CcXx cx CX Cx 4~x rabbits from the cross CCxx x ccXX we get — or — double ^ cX c+ heterozygotes. Mated to ccxx they give about: 14 CX: 86 Cx: 86 cX: 14 cx. This kind of geometrical isomerism is called linkage. It has been shown in several animal and plant species to be due to the position of the genes on the chromosomes. Genes which are near together tend to be strongly linked. The simplest proof of this arises from the fact that a few genes are visible, some mutants being deficiencies of a small section of a chromosome, others being duplications. They are found where they should be found according to maps con¬ structed on the basis of hnkage data. Most genes are, however, invisible with a microscope, though the electron microscope may yet reveal some of them. Genes are generally reproduced accurately at a nuclear division, that is to say an A gives two A's, and an a two a's. If the accuracy were not very great, selection would be effective within pure lines. If it were always accurate a species could only try out combinations of pre-existing genes, and evolution could not have occurred. In fact genes do not always reproduce their like. The process of altera¬ tion is called mutation. It may be due to an alteration in a gene