GENE MUTATION 



in equal numbers. Hence random fertilization leads to an Fo generation 

 consisting of 25 per cent red flowering ( RR ) , 50 per cent pink flowering 

 (Rr), and 25 per cent white flowering plants (rr). 



Such pairs of alleles exist because the original gene has mutated, that 

 is, it has undergone a reproducible change which results in a modified 

 character. There is nothing, however, to restrict the number of alternative 

 forms of a gene to two, and actually large series of multiple alleles are 

 known. Thus the white eye gene of the fruit fly, Drosophila, is represented 

 by at least 14 alleles, and the self-sterility genes of many plants are rep- 

 resented by large numbers of alleles, up to around 200 in some cases. 

 Only two members of any such series can be represented in any individual 

 (although any number can be present in a population), and these are 

 inherited in the usual Mendelian fashion, as described above. 



If a cross is made between organisms differing in two pairs of genes, 

 then each segregates as though the other were not there. This is shown 

 by the fact that the ratio obtained is nine dominant for both genes, to 

 three dominant for the first but recessive for the second, to three recessive 

 for the first but dominant for the second, to one recessive for both 

 (9:3:3:1). But this is simply the algebraic expansion of the binomial 

 (3 + 1)^ 



An example is shown in Figure 72. In the guinea pig, short hair (S) 

 is dominant over long (s). If a black, short variety (BBSS) is crossed to 

 a brown, long variety, the Fi are all dihybrids (BbSs), and they are black 

 and short because of dominance. When gametes are formed, one gene of 

 each pair must be included in each gamete, but these include all possible 

 combinations in equal numbers. In this example, gametes are formed of 

 types BS, Bs, bS, and bs. Random fertilization yields an F2 consisting of 

 9B?S?, 3B?ss, 3bbS?, and 1 bbss (the question mark after a dominant gene 

 indicates that the second member of the pair might be either dominant 

 or recessive); or, restating it, 9 black, short; 3 black, long; 3 brown, short; 

 and 1 brown, long. 



If three pairs of genes, all on different chromosomes, differ in a cross, 

 then the ratio of phenotypes (appearances) obtained is the expansion of 

 (3 + l)\ or 27:9:9:9:3:3:3:1. The same principle is indefinitely appli- 

 cable, and is called the principle of Independent Assortment. But if a 

 cross involves gene differences for two or more pairs of genes located on 

 the same pair of chromosomes, then one would expect that the number of 

 recombinations of the genes would be like that of a monohybrid cross. 

 That is, if a cross were made between two homozygous individuals, AABB 

 and aahh, then one of the chromosomes of the offspring should contain 

 the two dominant genes, AB, its homologue should contain the two reces- 

 sive genes, ah, and these two combinations should be maintained indefi- 

 nitely. This is the phenomenon of linkage, and it is a simple consequence 

 of the fact that the genes are more numerous than the chromosomes. But 

 linkage is not absolute, for blocks of material may be exchanged between 

 homologous chromosomes. Thus, in the example discussed above, the 

 combination Ah and aB could be formed, though only in a minority of 

 the gametes. This is called crossing over. 



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