BIOL 1111 Lecture Notes - Lecture 2: Heterozygote Advantage, Sickle-Cell Disease, Allele
13 Jun 2018
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Maintaining Genetic and Phenotypic Variation
1. Diploidy and Recessive Alleles
-most natural populations have high genetic and phenotypic variation
-maintains genetic variability; 3 diff possibilities for genotype, 2-3 for phenotype
-diploidy prevents harmful, recessive alleles from leaving populations
-harmful recessive alleles are carried in heterozygotes (masked alleles)
2. Balanced Polymorphisms
-two or more phenotypes in stable proportions
Heterozygote advantage
-heterozygotes have higher relative fitness
-heterozygotes selectively favoured even if homozygotes are selected against
-maintains high proportions of recessive alleles
Example: Sickle Cell Allele in areas with malaria
-sikle ell does’t for heogloi properly and the cell dies because it is oxygen deprived
-homozygous recessive disorder
-hoozygous doiat does’t hae a issue ith sikle ell disease ut oe ifeted y protists ith
alaria, o’t surie log
-heterozygous have some proper red blood cells but if they get infected, some of their cells are already
dying so that person has a higher chance at survival
Selection in Varying Environment
-varying environments favour different phenotypes
-camouflage from predators varies with background
Evolution of Adaptive Traits
Adaptive traits: products of selection that increase relative fitness
Adaptation (adaptive evolution): accumulation of adaptive traits over time
-hypotheses must be tested; current structure came from previous structures
-some traits arise by change and not selection
Constraints
-Usually compromises
-most environments have competing selective pressures
-environments are constantly changing (genetic variability comes into play)
-Adaptation lags environmental change
-each generation adapted to environment of their parents
-since you cannot anticipate what the environment will be, this plays a role in natural selection
as the environment changes
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