A husband and wife both have cleft chins, and when you look at their four children, three also have cleft chins; however, one does not. What can you deduce from this observation, if you already know the cleft phenotype is due to two alleles of the same gene?

(A) A cleft chin is an example of a recessive trait.
(B) A cleft chin is due to a dominant allele.
(C) This pattern of inheritance in the children must result from both parents being homozygous for the same allele.
*D) All four children are heterozygous for the two different cleft chin alleles

A husband and wife both having the phenotype for cleft chin and producing an offspring without a cleft chin, indicates heterozygousity in their genotype.

With the heterozygous genotype however, they both expressed the phenotype for cleft chin, indicating that the allele for cleft chin is dominant over the allele coding for normal chin.

Since both parents are heterozygous for cleft lip, the sequence of genotype in their offspring will be:

25% chance of having homozygous cleft chin
50% chance of having heterozygous cleft chin
25% chance of having homozygous normal chin

Because of the dominance of the cleft chin allele, both homozygous and heterozygous forms of the cleft chin allele, will be expressed in the offspring and only the offspring homozygous for normal chin will not express this allele.