Endurance of life : the implications of genetics for human life / Macfarlane Burnet.

  • Burnet, F. M. (Frank Macfarlane), Sir, 1899-1985.
Date:
1978
Catalogue details

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

Credit: Endurance of life : the implications of genetics for human life / Macfarlane Burnet. Source: Wellcome Collection.

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    12 Endurance of Life Some elementary genetics To go further in understanding the changes in the gene pool by random prédation and accident as modified by genetic variation in the probability of survival, it is necessary to move more directly into genetic concepts. Genetics is concerned primarily v^ith the process by which the information in the maternal reproductive cells is, as it were, spht in half, one half being discarded and the other half being carried and supported in the ovum. A process quite similar in prin¬ ciple results in the sperm also carrying a half complement of chromosomes and information. With fertilization, the full number of chromosomes (the diploid number) is reconstituted. What is im¬ portant to grasp is the part chance plays in these processes, and for this the analogy of the two packs of playing cards can be useful. We can think of an organism that has a diploid number of eight chromosomes, four pairs, in each of which one comes from the male, one from the female, parent. We can take each suit as representing a chromosome and we start off with two complete packs separated into four double suits. In the two sets of spades the two aces will corre¬ spond but they will be recognizably different in the way the ace is depicted. There will be similar minor differences between the two deuces, and in general for every other pair of cards in the pack. The first procedure, corresponding more or less to the reduction division, is to shuffle the two spade suits and then go through them, putting the first of each pair to appear on the left hand side and the other on the right, when in due course it emerges. Again, we have two full suits of spades, but by a process technically called crossing-over they are no longer the same as the original male- and female-derived chromosomes. The actual process of crossing-over is not quite of this type, but for the present purpose the shuffle is sufficiently close in principle. Eventually, we have the four suits in sets of two. From each we take one set at random and discard the other. Those we retain represent the single set of chromosomes (the haploid number) for ovum or sperm. Every ovum will end up with a somewhat different combination of genes from any other, and the same will hold for the sperms. At fertilization, one or a small number of ova will be fertilized very much at random by the winners of a race in which there are some hundreds of millions of competing spermato¬ zoa. In terms of our packs of cards, the set of four suits carried by the ovum is now matched with the corresponding suits carried by the successful sperm, giving the new diploid combination of two sets of each suit which is different from that of either parent and also from that which would have resulted from fertilization by any other sperm.