Hardy-Weinberg Principle

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Hardy-Weinberg Principle. Hardy-Weinberg - original proportions of genotypes in a population will remain constant from generation to generation Sexual reproduction (meiosis and fertilization) alone will not change allelic (genotypic) proportions. Hardy-Weinberg Equilibrium. - PowerPoint PPT Presentation

Transcript of Hardy-Weinberg Principle

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bb = white
B_ = black
population size is very large
random mating
no mutation
no selection occurring
(p+q)2 = p2 + 2pq + q2
p2 = individuals homozygous for first allele
2pq = individuals heterozygous for alleles
q2 = individuals homozygous for second allele
16 cats white = 16bb then (q2 = 0.16)
This we know we can see and count!!!!!
If p + q = 1 then we can calculate p from q2
Q = square root of q2 = q √.16 q=0.4
p + q = 1 then p = .6 (.6 +.4 = 1)
P2 = .36
All we need now are those that are heterozygous (2pq) (2 x .6 x .4)=0.48
.36 + .48 + .16
Mutation rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles.
ultimate source of genetic variation
Gene flow
tend to homogenize allele frequencies
Nonrandom mating
assortative mating - phenotypically similar individuals mate
Genetic drift – statistical accidents.
important in small populations
Selection – Only agent that produces adaptive
evolutionary change
variation must exist among individuals
variation must result in differences in numbers of viable offspring produced
variation must be genetically inherited
Selection pressures:
avoiding predators
Measuring Fitness
Fitness is defined by evolutionary biologists as the number of surviving offspring left in the next generation.
relative measure
Interactions Among Evolutionary Forces
Levels of variation retained in a population may be determined by the relative strength of different evolutionary processes.
Gene flow versus natural selection
Frequency-dependent selection
Negative frequency-dependent selection favors rare phenotypes.
Positive frequency-dependent selection eliminates variation.
Oscillating selection
Sickle cell anemia
Homozygotes exhibit severe anemia, have abnormal blood cells, and usually die before reproductive age.
Heterozygotes are less susceptible to malaria.
Stabilizing selection
Selection on Color in Guppies
Selection on Color in Guppies
High predation environment - Males exhibit drab coloration and tend to be relatively small and reproduce at a younger age.
Low predation environment - Males display bright coloration, a larger number of spots, and tend to be more successful at defending territories.
Evolution requires genetic variation
Intense selection may remove variation from a population at a rate greater than mutation can replenish.
thoroughbred horses
epistatic interactions