Updated: 12/20/2019

Population Genetics

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  • Overview
    • Forces responsible for genetic variation
      • mutation
        • de novo mutation rates constant among populations
          • intrinsic error rate in DNA polymerase
      • founder effect
        • if one member of a small community carries a triat, as the population expands there will be a higher frequency of that trait in the new community than there is in the general population
        • Ex.) Pennsylvania Amish and Ellis-van Creveld syndrome
      • genetic drift
        • a dramatic change in allele frequency based on chance
          • small populations are more vulnerable to genetic drift
      • natural selection
        • ↑ in allelic frequency that ↑ species fitness
        • ↓ in allelic frequency that ↓ species fitness
        • some genes ↑ species fitness as heterozygote but ↓ species fitness as a homozygote
          • ex.) sickle cell trait lowers malarial infections, while sickle cell anemia is detrimental
      • bottleneck
        • Even when fitness is equal for all phenotypes, a population bottleneck can result in disrupted allelic frequencies or loss of a genotype all together by chance
      • gene flow
        • transfer of alleles from one population to another
    • Hardy-Weinberg equilibrium
      • states that genotype and allele frequencies remain constant through generations
      • disease prevalence equation
        • p2+ 2pq + q2 = 1
          • where p = frequency of allele A
          • where q = frequency of allele B
          • p2 = frequency of homozygous individuals for allele A
          • q2 = frequency of homozygous individuals for allele B
          • 2pq = frequency of heterozygotes
      • requirements for validity
        • large population
        • random mating
      • the genotypic frequencies of the population will remain stable from generation to generation
      • assumptions
        • no mutation
        • no selection for any of the genotypes at the locus
        • no migration
      • other notes
        • prevalence of an X-linked recessive disease in males = q
          • prevalence of an X-linked recessive disease in females = q2
        • possible to assume in most cases that p = 1 as the wild-type allele is approximately 1

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(M1.BC.14.27) A 21-year-old female presents to the clinic requesting prenatal counseling. She was born with a disease that has led her to have recurrent upper respiratory infections throughout her life requiring antibiotic prophylaxis and chest physiotherapy as well as pancreatic enzyme replacement therapy. She marries a man (without this disease phenotype) from a population where the prevalence of this disease is 1/100. What is the chance that their child will have the disease of interest?

QID: 107015

9/100

32%

(29/92)

1/10

21%

(19/92)

18/100

22%

(20/92)

81/100

10%

(9/92)

9/10

5%

(5/92)

M 1 D

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(M1.BC.14.27) Red-green color blindness, an X-linked recessive disorder, has an incidence of 1/200 in males in a certain population. What is the probability of a phenotypically normal male and female having a child with red-green color blindness?

QID: 107024

1/200

19%

(51/272)

199/200

5%

(14/272)

1/100

15%

(40/272)

1/400

49%

(133/272)

99/100

7%

(19/272)

M 1 E

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