READ: Macro Evolution

Genotype or Phenotype?

Natural selection acts on the phenotype - the traits or characteristics - of an individual, not on the underlying genotype. For many traits, the homozygous genotype, AA for example, has the same phenotype as the heterozygous Aa genotype. If both an AA and Aa individual have the same phenotype, the environment cannot distinguish between them. So natural selection cannot choose a homozygous individual over a heterozygous individual. If homozygous recessive aa individuals are selected against, that is they are not well adapted to their environment, acting on the phenotype allows the a allele to be maintained in the population through heterozygous Aa individuals.

Carriers

Because natural selection acts on the phenotype, if an allele is lethal in a homozygous individual, aa for example, it will not be lethal in a heterozygous Aa individual. These heterozygous Aa individuals will then act as carriers of the a allele. This allele is then maintained in the population's gene pool. The gene pool is the complete set of alleles within a population.

Tay-Sachs disease is an autosomal recessive genetic disorder. It is caused by a genetic defect in a single gene with one defective copy of that gene inherited from each parent, rr for example. Affected individuals usually die from complications of the disease in early childhood. Affected individuals must have unaffected parents, each being a carrier of the defective allele, so the parents are heterozygous Rr. This lethal allele is maintained in the gene pool through these unsuspecting heterozygous individuals; they do not show any symptoms of the disease, so most individuals do not get tested to see if they are carriers.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/