Ahusband and wife have two sons. one boy has a straight thumb, while the other has a bent thumb. what causes this difference

The answer is; alleles

Most genes have various forms called alleles. In diploid organisms, most alleles are two located at the same loci in homologous chromosomes.  One allele is usually dominant over the other, meaning it makes the effects of the recessive allele. In some occasions, the recessive allele has disadvantageous traits and manifest when the individual is homozygous recessive.

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