Describing an allele that is only expressed when homozygous. Recessive alleles are in most cases considered to be the mutant form of dominant alleles. They are often rarer in the population and usually lower the fitness of the individual. Their effect is often due to their failure to produce a normal functional protein. They are masked (i.e. recessive) in a heterozygote because the normal (*dominant
) allele will produce a normal protein and hence a normal phenotype.