The foundations of genetics / by F.A.E. Crew.

  • Francis Crew
Date:
1966
Catalogue details

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

Credit: The foundations of genetics / by F.A.E. Crew. Source: Wellcome Collection.

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    RAMIFICATIONS OF GENETICS 143 linkage groups, to establish the linkage relationships of genes must be very difficult save in the case of those of the sex-linked characters. Thus far the six linkage groups have been described, that of the eleven X-borne genes being the largest. Altogether about 80 pairs of Mendelian characters have been identified, some of them being of considerable economic importance. Several lethal genes have been detected ; they so affect the processes of development that the resulting characterization is incompatible with continued life and the affected individuals die usually before hatching. There is also an interesting multiple allelomorphic series of genes which affect the rate of feathering. A breed consists of a group of animals which have a few striking characters—the trademarks of the breed, e.g. plumage colour, combform—each based upon a single gene, and a number of quantitative characters such as body size or egg-yield, based upon multiple genes, in common. Their creation is made possible by the occurrence of mutation. The state of the existing knowledge of the genetics of an animal of economic importance or of one that is prized by the fancier is determined by many factors, the more important of which have already been mentioned in connection with the fowl. Far more is known of the genetics of the rabbit, or the hamster, for example, small, quickly maturing and reproducing, giving large numbers of offspring, easily and cheaply kept and with many mutant genes, than of the genetics of the horse, the pig, the sheep or of cattle. The fancier tends to cherish any variant that appears in his stock for the reason that it might prove to be the origin of a new variety; the commercial breeder tends to get rid of any variant for the reason that what he seeks is high-grade uniformity. Thus it is that the animals that are the favourite of the fancier and that can be used as experimental material in scientific laboratories have contributed more to genetical knowledge than have the animals of the farm. BIOCHEMICAL GENETICS In 1908 Garrod, an English physician, gave a series of lectures entitled Inborn Errors of Metabolism to a London medical society