Localization. Recessivity And Dominance

We must now review some other fundamental facts and notions about mutations, again in a slightly dogmatic manner, without showing directly how they spring, one by one, from experimental evidence.

We should expect a definite observed mutation to be caused by a change in a definite region in one of the chromosomes.

I Ample discussion has been given to the question, whether natural selection be aided (if not superseded) by a marked inclination of mutations to take place in a useful or favourable direction. My personal view about this is of no moment; but it is necessary to state that the eventuality of ‘directed mutations’ has been disregarded in all the following.

Moreover, I cannot enter here on the interplay of ‘switch’ genes and ‘polygenes’, however important it be for the actual mechanism of selection and evolution.

Fig. g. Inheritance of a mutation. The straight lines across indicate the transfer ofa chromosome, the double ones that of the mutated chromosome.

The unaccounted-for chromosomes of the third generation come from the mates of the second generation, which are not included in the diagram.

They are supposed to be non-relatives, free of the mutation.

And so it is.

It is important to state that we know definitely that it is a change in one chromosome only, but not in the corresponding ’locus’ of the homologous chromosome.

Fig. 8 indicates this schematically, the cross denoting the mutated locus. The fact that only one chromosome is affected is revealed when the mutated individual (often called ‘mutant’) is crossed with a non-mutated one. For exactly half of the offspring exhibit the mutant character and half the normal one.

That is what is to be expected as a consequence of the separation of the two chromosomes on meiosis in the mutant- as shown, very schematically, in Fig. g. This is a ‘pedigree’, representing every individual (of three consecutive genera- tions) simply by the pair of chromosomes in question. Please realize that if the mutant had both its chromosomes affected, all the children would receive the same (mixed) inheritance, different from that of either parent.

But experimenting in this domain is not as simple as would appear from what has just been said. It is complicated by the second important fact, viz. that mutations are very often latent. What does that mean?

In the mutant the two ‘copies of the code-script’ are no

Fig. 10. Homozygous mutant, obtained in one-quarter of the descendants either from self-fertilization of a heterozygous mutant (see Fig. 8) or from crossing two of them.

longer identical; they present two different ‘readings’ or ‘versions’, at any rate in that one place. Perhaps it is well to point out at once that, while it might be tempting, it would nevertheless be entirely wrong to regard the original version as ‘orthodox’, and the mutant version as ‘heretic’. We have to regard them, in principle, as being of equal right - for the normal characters have also arisen from mutations.

What actually happens is that the ‘pattern’ of the individual, as a general rule, follows either the one or the other version, which may be the normal or the mutant one.

The version which is followed is called dominant, the other recessive; in other words, the mutation is called dominant or recessive, according to whether it is immediately effective in changing the pattern or not.

Recessive mutations are even more frequent than dominant ones and are very important, though at first they do not show up at all.

To affect the pattern, they have to be present in both chromosomes (see Fig. 10).

Such individuals can be produced when 2 equal recessive mutants happen to be crossed with each other or when a mutant is crossed with itself; this is possible in hermaphroditic plants and even happens spontaneously.

An easy reflection shows that in these cases about one-quarter of the offspring will be of this type and thus visibly exhibit the mutated pattern.

Introducing Some Technical Language

The ‘version of the code-script’ is the original one or a mutant one which represents the alele.

When the versions are different, as indicated in Fig. 8, the individual is called heterozygous, with respect to that locus.

When they are equal, as in the non-mutated individual or in the case of Fig. 10, they are called homozygous.

Thus a recessive allele influences the pattern only when homozygous.

Whereas a dominant allele produces the same pattern, whether homozygous or only heterozygous.

Colour is very often dominant over lack of colour (or white).

For example, a pea will flower white only when it has the ‘recessive allele responsible for white’ in both chromosomes in question.

When it is ‘homozygous for white’; it will then breed true, and all its descendants will be white. But one ‘red allele’ (the other being white; ‘heterozygous’) will make it flower red, and so will two red alleles (‘homozygous’).

The difference of the latter two cases will only show up in the offspring, when the heterozygous red will produce some white descendants, and the homozygous red will breed true.

Two individuals may be exactly alike in their outward appearance, yet differ in their inheritance, is so important that an exact differentiation is desirable.

The geneticist says they have the same phenotype, but different genotype. The contents of the preceding paragraphs could thus be summarized in the brief, but highly technical statement:

A recessive allele influences the phenotype only when the genotype is homozygous.

We shall use these technical expressions occasionally, but shall recall their meaning to the reader where necessary.