The answer is recessive alleles. Generally, a recessive allele is the form of a gene that codes for a non-functional protein. When an individual inherits recessive allele for a gene from both parents, then they are more likely to suffer a disorder that is attributed to lack of the respective functional protein. Often the heterozygous individual will not bear the disorder even though they carry the recessive allele. This is because the dominant allele is able to mask the recessive allele by producing a functional protein.
