Recessive alleles

In humans and many other diploid organisms, provided there is no mutation, we have two alleles of each gene on autosomal chromosomes. Recessive alleles are only expressed in an orangisms phenotype if their genotype is homozygous recessive (i.e. both recessive alleles are present). If an organism has a heterozygous genotype, containing one recessive allele, the phenotype of the other allele (the dominant allele) is expressed, and the organism is known as a carrier of the recessive trait. As a result, two carrier parents with seemingly normal phenotypes can have a child that has the recessive trait. There is only a 25% chance of this happening however, as the child must inherit the recessive allele from both the mother and the father.

King, W, S et al (2007). Essentials of Genetics. 8th ed. Pearson. p36-37.