Every trait is encoded by a particular gene and each gene has various alternative forms. These alternative forms of a given gene are known as alleles.
The alleles that encode the shape of the thumb are of two types - one is the S allele which encodes a straight thumb and is dominant in nature whereas the second allele is the h allele which encodes for the bent thumb and is recessive in nature.
Thus, the two children have different thumbs as they have received different alleles from their parents.
