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Genes and Variation
EQ: How is the gene pool affected by selection pressure?
Genotype and Phenotype
Genotype: particular combination of alleles
Phenotype: physical, physiological, and behavioral characteristics
Allele Frequency
Number of times an allele occurs in a gene pool, as a percentage
of the total occurrence of all alleles
In 50 alleles:
In 100 alleles:
40
20 alleles are B (black)
30 alleles are b (brown)
are B (black)
60
are b (brown)
Alleles in a Population
Evolution involves any change in the frequency of alleles in a
population over time.
12
4
9
Single-Gene Traits
Phenotypic ratios are determined by the frequency of alleles and
by whether the alleles are dominant or recessive.
77%
23%
Overview: Genes and Variation
Evolution
frequency
occurs due to the change of the relative
of alleles in a(n)
The alleles in a(n)
organism’s
organism
lifetime
frequency
organism
population
over time.
do not change during the
.
population
evolution
lifetime
Evolution Versus Genetic Equilibrium
If a population is not evolving, the
population is in genetic equilibrium.
•Allele frequencies do not change
Hardy–Weinberg principle
Population should remain constant
if all 5 conditions are met:
1.Large population
2.Random mating
3.No natural selection
4.No immigration/emigration
5.No mutations
Hardy–Weinberg Principle
and
In words, this is stated:
(frequency of AA) + (frequency of Aa) + (frequency of aa) = 100%
and
(frequency of A) + (frequency of a) = 100%
If p = 0.40 and q = 0.60:
36%
16%
48%
Probability of genotype Aa:
aa:
AA:
Hardy–Weinberg Principle
And
Try this: Population of quackers
Dominant: B – dark feathers
Recessive: b – light feathers
In a population of 50 ducks, 10 were light and 40 were dark.
(Frequency of homozygous recessive organisms - bb)= q2 = ____
(Frequency of recessive allele - b) q = _______
(Frequency of dominant allele - B) p = _______
(Frequency of homozygous dominant organisms - BB) p2 = ______
(Frequency of heterozygous organisms - Bb) 2pq =________