environment, acting on the phenotype allows theaallele to be maintained in the population
through heterozygousAaindividuals.
Carriers
Because natural selection acts on the phenotype, if an allele is lethal in a homozygous
individual, aafor example, it will not be lethal in a heterozygous Aaindividual. These
heterozygous Aaindividuals 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,
rrfor 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 heterozygousRr. 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.
Figure 7.25: Tay-Sachs disease is inherited in the autosomal recessive pattern. Each parent
is an unaffected carrier of the lethal allele. ( 37 )
Hardy-Weinberg Equilibrium
The Hardy-Weinberg model (sometimes called a law) states that a population will remain
atgenetic equilibrium- with constant (unchanging) allele and genotype frequencies and