In terrapins, sharp beaks are an autosomal dominant condition with a penetrance of 87%. If two heterozygous sharp-beaked terrapins mate, how many of their 100 offspring would you expect to have sharp beaks

65

Explanation:

An autosomal dominant condition is one where one copy of the allele is sufficient to cause the condition. Penetrance refers to the degree to which that mutation causes the phenotype

Lets say the allele for sharp beak is S and for normal beak is s. If 2 heterozygous sharp-beaked terrapins mate, the cross is the following:

Ss x Ss

         S       s  

   S   SS     Ss

   s    Ss     ss

The possible genotypes are SS, Ss and ss.  50% of their offspring will be heterozygous, and 25% homozygous each. Therefore, if the mutation was fully penetrant, it would be 75 out of 100 offspring would be affected.

However, the mutation is only 87% penetrant, which means only 87% of those 75 offspring would have the condition.

87% of 75 = 65.25

Therefore, we would expect 65 of their offspring to have sharp beaks

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