Coiling of shell in snail (Limnaea peregra)
In snails, coiling of shell can be dextral (coiling to the right) or sinistral (coiling to the left). This direction of coiling is genetically controlled, the dextral-coiling depending upon dominant allele D and sinistral coiling depending upon recessive-allele d, so that dextral is DD and sinistral is dd.
The phenotype in progeny obtained from reciprocal crosses (♀ DD x ♂ dd : ♀ dd x ♂ DD)is determined by the genotype and not by the phenotype of female parent. In reciprocal crosses shown in Figures 18.3 and 18.4, it is evident that the genotype Dd (F1 can be dextral (Fig. 18.3) as well as sinistral (Fig. 18.4) depending upon the genotype of female parent. Similarly, dd can be dextral if genotype of female parent carries dominant allele (Dd). It should be carefully noted that phenotype of female parent does not have any effect on phenotype of progeny. It is the genotype of female parent which is really decisive. This is an example of delayed effect of genotype.
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