Bio Review Notes #27
NON-MENDELIAN PHENOMENA
Performance Objectives:
Genes may have more than two alternative states (multiple alleles), or they may show varying degrees of dominance. One gene may influence many phenotypic traits. Genes may also interact with each other and with the environment to produce phenotypes.

Multiple alleles are genes that have more than 2 alternative states. For example, in the A-B-O blood group system, alleles A and B are both dominant to o, and they are codominant (both are expressed) when they occur together.
BLOOD TYPE Antigens Antibodies     Can give blood to: Can receive blood from: Possible genotypes:
  A   A   anti-B   A or AB   A or O   AA, Ao
  B   B   anti-A   B or AB   B or O   BB, Bo
  AB   A and B   neither   AB only   any type   AB
  O   neither  anti-A, anti-B    any type   O only   oo

Incomplete dominance (sometimes incorrectly called "blending"): heterozygotes have an intermediate phenotype.     Example: in snapdragons, RR flowers are red, Rr flowers are pink, and rr flowers are white.

Dominance can exist in varying degrees:
  • None: heterozygotes are intermediate between homozygotes
  • Partial: heterozygotes are closer to one homozygous phenotype
  • Complete: heterozygotes are identical to one homozygous type
  • Overdominance ("hybrid vigor"): heterozygotes are more extreme than the nearest homozygous phenotype.

Pleiotropy: condition in which one gene affects many traits

Gene-gene interactions (polygeny): several genes combine to make a phenotype.   Size is often controlled by additive polygenes (each gene adds a small amount; the effects add up to make a bell curve).

Epistasis: one gene "masks" or suppresses the phenotypic effects of a totally different gene (not just a different allele).

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