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) 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? Tested Concept

QID: 107024
1

1/200

19%

(47/247)

2

199/200

5%

(13/247)

3

1/100

15%

(38/247)

4

1/400

48%

(119/247)

5

99/100

8%

(19/247)

M 1 D

Select Answer to see Preferred Response

(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? Tested Concept

QID: 107015
1

9/100

44%

(14/32)

2

1/10

16%

(5/32)

3

18/100

19%

(6/32)

4

81/100

6%

(2/32)

5

9/10

9%

(3/32)

M 1 D

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