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.

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    18 THE BIOCHEMISTRY OF GENETICS lethal in the normal environment. These are called auxotrophe, and are lethal because a particular synthetic capacity has been lost, but live normally, or nearly so, when supplied with the missing metabolite. I deal with such genes in Chapter III. One point about polyploidy is of biochemical importance. We might expect that a cell with three, four, or more sets of chromo¬ somes would differ greatly in its physiology from one with only two, as all kinds of biochemical processes would be proceeding at more than the normal rate. This is not the case. The differences are usually slight. Sometimes a tetraploid cell is about twice the volume of a diploid, so we can suppose that cytoplasmic as well as nuclear processes are going on at twice the normal rate. But the cell size is not invariably proportional to the chromosome number. On the other hand, if one chromosome is present in triplicate or only once, i.e. the nucleus can be represented as ABCD... ABCD... ABCD... ABCD...or BCD... instead of A В С D . . . A the organism is often inviable, and almost always abnormal. An exception to this rule is given by the sex-determining X chromosome. Thus the whole set of chromosomes is responsible for a quantitatively balanced system of reactions, and the X chromosome for a similarly balanced subset. I have not referred to the special genetical methods and criteria employed in human genetics. An account of some of them is given by Haldane (1948) and Harris (1953). For example, a rare condition found among the children of normal parents can only be due to an autosomal recessive gene if it is commoner among the children of married pairs of first cousins than in other families. In conclusion, I shall define a few words. If two or more genes, like A and a, X and x, and so on, have the property that only one can get into a gamete, they are said to be allelomorphs (in America allels). If there are more than two they are called multiple allelo¬ morphs. They are said to occur at the same locus on a chromosome. A gene located on an X or sex-determining chromosome, of which women and cocks have two and men and hens one, is called sex- linked. One located elsewhere is called autosomal.