College of Cat Genetics: Part XI
by Patricia Turner
Study Unit 11
The different kinds of gametes produced by an F1 hybrid increases as the number of gene pairs for which it is heterozygous increases and the number of combinations of gametes resulting which F1 hybrids are mated together is, of course, the product of multiplying the different kinds of gamete possibilities produced by each. Hybrid is a word used to describe heterozygotes. Thus cats heterozygous for two gene pairs as described with with d/D.r/r+ will produce the homozygous double recessive d/dr/r in one out of 16 combinations or once in a perfect cat population. The cat population of 16 is the smallest able to accommodate all the possible combinations with two gene pairs and is thus the perfect cat population for such combinations. Within this some genotypes will be produced more than once as has been demonstrated in the Punnetts Square diagrams. Obviously the number of genotypes produced increases with the number of gene pairs involved in the matings. Differences of 20 gene pairs would result in about three and a half billion possible genotypes. The table at the end of the next Study Unit gives an idea of the effects of segregation and independent assortment for up to five gene differences. It is unlikely that cat breeders would have the need or indeed the opportunity to study the inheritance of more than that.
As the number of gene pairs increase the relative proportion of homozygous genotypes decreases and a mating between cats heterozygous for a single gene difference as shown in the example DD x dd will produce three genotypes DD; Dd; dd. Two of these are homozygous. When two gene pairs are involved as in D/D.r+/r+ x d/d.rr/r then 9 genotypes are produced, four of which are homozygous. The ratio of homozygous genotypes to that of heterozygous genotypes is seen to decrease with every additional gene pair even when it is not one relating to obvious things such as coat type, pattern or colour.
The idea that continuous variation in a character actually occurs by a number of small discontinuous variations determined by Mendelian Laws was not generally accepted for a very long time. Nor was it realized that there is a correspondence between chromosomes and Mendelian factors. Mendel's work was experimental and statistical but about the same time and unknown to him another approach was being made to the problems surrounding the unravelling of systems of inheritance by observation of the cell under the microscope.
Mendel's First Principle stated that in any pair of alleles studied only one could be carried in each gamete. This follows closely the situation seen in meiosis where only one of a pair of chromosomes can be represented in any gamete. His Second Principle stated that either one of any pair of alleles could be recombined with either one of any other pair of alleles. This suggests that the distribution and allocation of the genes in the gametes is a random matter where all possibilities have an equal chance as so their recombination at fertilization. This is closely paralleled in meiosis. Finally, given the possibility of a wide variety of gametes the nature of the pairs which eventually fuse is also dictated by chance. At fertilization the zygote receives its pairs of genes at the same time as the diploid number is restored to 38.
Mendel's success was largely due to the lucky choice of simple characters and to confining his studies to their presence or absence. He knew nothing about the physical explanation of his results and his basic contribution was his discovery of the unit nature of inheritance. His work explains that genes can be traced from one generation to another without any dilution or blending taking place.
Mendel used the word element to denote the determinent of characters; later, Carl Correns, who was responsible for summarizing Mendel's conclusions in the form of the two principles, used the word factor. Nowadays, the word suggested by the Danish geneticist, Johannsen, is used: gene.
In general, the F2mendelian ratios of 3:1 in single gene crosses and 9:3:3:1 in two gene crosses, etc., are found as expected. It has been explained that these ratios derive from the segregation of two different alleles at any one locus, one being dominant and the other recessive. However, research by later investigators has shown that phenotypes sometimes appear in ratios that do not accord with those for simple dominance or the presence of only two alleles at any one locus. Ratios of genotypes remain constant but ratios of phenotype may be modified by a number of causes such as incomplete dominance, modifiers, linkage, epistasis and hypostasis, etc.
Any particular gene pair in the cat obviously represents only two alleles at a time and it has been explained how this condition arises from the presence of homologous chromosomes containing one allele of the gene pair each. In all the experiments carried out by Mendel, there were only two alternatives at each locus but, in fact, there are often a tremendous number of possibilities at various loci. The grouping of all the different possible alternatives proved to be situated at the same locus is known as a series or multiple allelic system. Such series are found in many species of mammal although the allele that is the mutant in some species is sometimes seen as the normal allele in others. Although there is now a revived interest in the genetics of the domestic cat, there remains a number of gaps in our knowledge. Where, at present, only two alleles have been reported, it is quite possible that there may be others as yet unidentified. The only allelic series formally reported and accepted in the cat are those of the albino and tabby series but there may well be another allele, similar to cordovan in the mouse, at the brown locus and giving rise to the dark chocolate phenotypes seen in some Siamese and the almost sepia phenotype in some Chestnut Browns. There may also be further alleles at the agouti locus. The albino series is currently reported as including four definite alleles with a fifth (albino) being regarded as provisional until formal publication of breeding data proving the mutant is possible. The tabby series is reported as including three alleles, although there may be more. While it is realized that there are many more than four phenotypes in the albino series and three in the tabby series, their existence does not necessarily infer the presence of further mutants. The additional phenotypes can be due to a number of other causes. Allelic series are always listed in order of dominance.