The Classical Physicist's Approach to the Subject

The Harmful Effect Of Close-Breeding

Recessive mutations, as long as they are only heterozygous, are of course no working-ground for natural selection.

If they are detrimental, as mutations very often are, they will never- theless not be eliminated, because they are latent.

Hence quite a host of unfavourable mutations may accumulate and do no immediate damage.

But they are transmitted to half of the offspring.

In Fig. 9 it is assumed that a male individual (me) carries such a recessive detrimental mutation heterozygously, so that it does not show up.

Assume that my wife is free of it. Then half of our children (second line) will also carry it - again heterozygously.

If all of them are again mated with non-mutated partners (omitted from the diagram, to avoid confusion), a quarter of our grandchildren, on the average, will be affected in the same way.

No danger of the evil ever becoming manifest arises, unless equally affected individuals are crossed with each other, when, as an easy reflection shows, one-quarter of their children, being homozygous, would manifest the damage.

Next to self-fertilization (only possible in hermaphrodite plants) the greatest danger would be a marriage between my son and daughter.

Each of them standing an even chance of being latently affected or not, one-quarter of these incestuous unions would be dangerous inasmuch as 1/4 of its children would manifest the damage.

The danger factor for an incestuously bred child is thus I: 16. In the same way the danger factor works out to be I :64 for the offspring of a union between two (‘clean-bred’) grand- children of mine who are first cousins.

These do not seem to be overwhelming odds.

The second case is usually tolerated.

But these are the consequences of only one possible latent injury in one partner of the ancestral couple (‘me and my wife’).

Actually both of them are quite likely to harbour more than one latent deficiency of this kind.

If you know that you harbour a definite one, you have to reckon with 1 out of 8 of your first cousins sharing it!

Experiments with plants and animals indicate that in addition to comparatively rare deficiencies of a serious kind, there seem to be a host of minor ones whose chances combine to deteriorate the offspring of close-breeding as a whole.

Since we are no longer inclined to eliminate failures in the harsh way the Lacedemonians used to adopt in the Taygetos mountain, we have to take a particularly serious view about these t~hings in the case of man, where natural selection of the fittest is largely retrenched, nay, turned to the contrary.

The anti-selective effect of the modern mass slaugh- ter of the healthy youth of all nations is hardly outweighed by the consideration that in more primitive conditions war may have had a positive value in letting the fittest tribe survive.

General And Historical Remarks: The early history of genetics

The recessive allele, when heterozygous, is completely overpowered by the dominant and produces no visible effect at all, is amazing.

I thought at least to be mentioned that there are exceptions to this behaviour. When homozygous white snapdragon is crossed with, equally homo- zygous, crimson snapdragon, all the immediate descendants are intermediate in colour, i.e. they are pink (not crimson, as might be expected).

A much more important case of two alleles exhibiting their influence simultaneously occurs in blood-groups - but we cannot enter into that here. I should not be astonished if at long last recessivity should turn out to be capable of degrees and to depend on the sensitivity of the tests we apply to examine the ‘phenotype’.

Gregor Mendel (1822-84) is the source of the law of inheritance.

Mendel knew nothing about mutations and chromosomes.

He made experiments on the garden pea, of which he reared different varieties, crossing them and watching their offspring in the 1st, 2nd, 3rd, … , generation.

He experimented with mutants which he found ready-made in nature.

The results he published as early as 1866 in the Proceedings of the Naturforschender Verein in Brunn.

The Necessity Of Mutation Being A Rare Event

If a spontaneous mutation is a small step in the development of the species, we get the impression that some change is ’tried out’ in rather a haphazard fashion at the risk of its being injurious, in which case it is automatically eliminated.

This brings out one very important point. In order to be suitable material for the work of natural selection, mutations must be rare events, as they actually are.

If they were so frequent that there was a considerable chance of, say, a dozen of different mutations occurring in the same individ- ual, the injurious ones would, as a rule, predominate over the advantageous ones and the species, instead of being improved by selection, would remain unimproved, or would perish. The comparative conservatism which results from the high degree of permanence of the genes is essential. An analogy might be sought in the working of a large manufacturing plant in a factory. For developing better methods, innovations, even if as yet unproved, must be tried out. But in order to ascertain whether the innovations improve or decrease the output, it is essential that they should be introduced one at a time, while all the other parts of the mechanism are kept constant.

MUTATIONS INDUCED BY X-RAYS

The percentage of mutations in the offspring, the so-called mutation rate, can be increased to a high multiple of the small natural mutation rate by irradiating the parents with X-rays or y-rays. The mutations produced in this way differ in no way (except by being more numerous) from those occurring spontaneously, and one has the impression that every ’natural’ mutation can also be induced by X-rays.

In Drosophila, many special mutations recur spontaneously again and again in the vast cultures;

they have been located in the chromosome, as described on pp. 26-9, and have been given special names. There have been found even what are called ‘multiple alleles’, that is to say, two or more different ‘ver- sions’ and ‘readings’ - in addition to the normal, non-mutated one - of the same place in the chromosome code; that means not only two, but three or more alternatives in that particular ’locus’, any two of which are to each other in the relation ‘dominant-recessive’ when they occur simultaneously in their corresponding loci of the two homologous chromosomes.

The experiments on X-ray-produced mutations give the impression that every particular ’transition’, say from the normal individual to a particular mutant, or conversely, has its individual ‘X-ray coefficient’, indicating the percentage of the offspring which turns out to have mutated in that particular way, when a unit dosage of X-ray has been applied to the parents, before the offspring was engendered.