Genetics for medical students / by E.B. Ford.

  • Ford, E. B. (Edmund Briscoe), 1901-1988.
Date:
[1942]
Catalogue details

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

Credit: Genetics for medical students / by E.B. Ford. Source: Wellcome Collection.

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    28 GENETICS FOR MEDICAL STUDENTS values provide a relative measure of distance. In our last instance, gene В lies four times as far from A as it does from C. This last proposition will be strictly true for short distances only, say under 8 per cent, for with a considerable length of chromosome the possibility of multiple crossing-over arises. When crossing-over occurs twice between two genes, the interchange produced by one is restored by the other. Thus the resulting double cross-over individuals are scored in the non-cross-over classes, so that the cross-over value is assessed too low; but triple crossing-over raises it, and so on. How¬ ever, the effect of multiple crossing-over is not so considerable as might be anticipated, since the occurrence of ©ne cross-over interferes with that of another in its neighbourhood: for crossing-over is determined in the chromosomes, and inter¬ twining threads have a modal length of twist. Thus, for short distances, the cross-over value is not obscured by multiple crossing-over. The tendency for one cross-over to prevent the occurrence of another in its neighbourhood is called Interference. It is measured by Coincidence. This is calculated as the actual amount of double crossing-over between any genes, divided ' by the amount expected. The expectation is the simple probability that two cross-overs should occur simultaneously. Thus, with a cross-over value of 20 per cent (obtained by adding together short lengths), the expectation, in the absence of interference, is that the apparent cross-over value should be about 16-5 per cent. Owing to interference it is found that the observed cross-over value falls short of 20 per cent to a less extent than this. It will now be evident that chromosome maps can be constructed from the data supplied by crossing-over. For the genes can be assigned to groups representing those situated on the same chromosome, while within these they can be placed in their correct order and at their relative distances apart. A map of part of a human chromosome obtained in this way is given on p. 51. Apart from those Hnked genes associated with sex, to be
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