Question

A heterozygote could have a variety of phenotypes depending on whether a mutation is recessive , dominant or haploinsufficient. gene A is necessary to form a tail in mice. If the mouse is a heterozygote (A / a) , then what would you predict the phenotype to be if the mutation was recessive , dominant or haploinsufficient

Answer 1

Ans) The gene A is required to form a tail in mice which suggests that dominant allele is responsible for the generation of tail in mice. If the mouse with tail has the genotype Aa, 1) The dominant mutation will result in the genotype aa the result of which is tailless mouse (without tail). If the mouse would have been homozygous dominant then even if one of the alleles is suppressed,then the genotype would have been Aa and still there had been a tail in mouse.But when the heterozygous genotype is there,even when a single allele is suppressed it will have the genotype "aa" and thus no tail formation.

(Here the wild type gene is AA which is required for the formation of tail and the mutant is aa that is lacking tail. If the case is reversed i.e AA genotype lacks tail and aa is wild type and is required for tail formation then the dominant mutation of Aa to AA will result in the expression of mutant phenotype i.e lacking tail.Even in the genotype aa, the dominant mutation of aa to Aa will result in no tail phenotype)

2) When the recessive mutation occurs i.e both of the alleles are suppressed to express the phenotype.When recessive mutation occurs i.e "aa" genotype,the phenotype of the mouse will be tailless (lacking tail).For the recessive mutation both of the alleles should suppressed.For example if the genotype is AA and if only the 1 allele is suppressed i.e Aa this is known as haploinsufficient but in this case Aa will produce tail because 1 dominant allele is still present.