For natural selection to cause evolution, it must select for or against one or more of the genotypes for a trait. In the simple case of a trait that is determined by a single gene with two alleles, there are five combinations of genotypes that nature can select:

Question

1. either homozygous (AA or aa but not both)
2. both homozygotes (AA and aa)
3. either homozygote and the heterozygote (AA and Aa or aa and Aa)
4. the heterozygote (Aa)
5. all alleles (AA, Aa, and aa).

Match the selected genotypes with the appropriate description.

Genotypes:
A. Selection for the homozygous dominant (AA)
B. Selection against all genotypes
C. Selection against the heterozygote
D. Selection for the heterozygote
E. Selection against the heterozygote and one homozygote

Descriptions:
1. The CCR5-delta 32 gene provided immunity to a deadly disease of bacterial origin, bubonic plague. People who are homozygous for the CCR5-delta 32 gene variant are completely immune, while heterozygotes only have partial immunity and homozygous recessive have none.
2. Sickle cell disease - homozygous dominant contract malaria; homozygous recessive die young from sickle cell; heterozygotes immune to malaria.
3. An example of disruptive selection because both extremes are favored to survive.
4. This type of selection will result in extinction of the species as all genotypes are at a selective disadvantage.
5. Huntington's disease is a single gene disorder caused by a malfunctioning gene on chromosome 4. It is an autosomal dominant disease, which means that only one copy of the gene needs to be defective.

Answer 1

Genotypes:
A. Selection for the homozygous dominant (AA)
B. Selection against all genotypes
C. Selection against the heterozygote
D. Selection for the heterozygote
E. Selection against the heterozygote and one homozygote

Descriptions:
1. A
2. E
3. D
4. B
5. C