Population genetics Flashcards
What is population genetics?
quantitative study of distribution of traits in a population and how they change over time
What is HW for gene with 2 alleles A and a in population?
p(sq) + 2pq + q(sq) = 1
AA + 2(a)(A) + aa = 1
What are the assumptions of HW?
- random mating
- allele frequency same in males and females
- no selection
- no migration into population
- no new mutation
- population large enough that allele frequency doesn’t change over populations
How does HW change for multiple alleles? For example, B-hemoglobin has 3 alleles (A, S, C)
(p + q + r)(p + q + r) A = p, S = q, C = r
p(sq) + pq + pr + qp + q(sq) + qr + rp + rq + r(sq)
A(sq) + 2AS + S(sq) + 2AC + C(sq) + 2SR
How can Hardy Weinberg apply to disease like PKU and CF with hundreds of mutations implicated in disease?
treat as 2 alleles because variants produce the same phenotype
How do you use Hardy Weinberg for X-linked recessive disease?
Males = p + q = 1 Females = p(sq) + 2pq + q(sq) = 1
What adjustments to HW do you make for rare autosomal dominant diseases?
Like with Familial hypercholesterolemia, q is basically 1 so use 1 = 2p + p2, where 2p = Hh.
What are deviations from random mating?
3 inter-related forms: stratification, assortative mating, inbreeding
What is stratification?
population subgroups that remain genetically separate - example - Ashkenazi jews
What is assortative mating?
Mating choice based on some trait, positive or negative. Example for neg - deaf marrying deaf, achondroplasia marrying achondroplasia
What is Inbreeding
marrying of closely related individuals like Amish
What are ethnic diseases?
Caused by stratification. Cystic Fibrosis among caucasians, Sickle cell anemia among African Americans, Tay Sachs among Ashkenazi
Does inbreeding increase frequency of allele?
No, just frequency of homozygotes
What increases the risk of a rare recessive disease?
Related parents
What mutations are favored by selection?
Cystic fibrosis mutation confers resistance to typhoid, sickle cell anemia confers resistance to malaria
What patterns of inheritance deviate from Hardy Weinberg?
autosomal dominant and x-linked recessive. If mutation produces genetic lethal, alleles are removed from each generation
What is fitness?
probability of transmitting one’s genes to the next generation relative to to the population’s average
What reduces fitness to zero?
a mutant allele that is lethal or causing sterility. For example, Cystic Fibrosis reduces male fitness to near zero.
What are examples of diseases that deviate from HW and have a fitness near 0.
Dominant mutations - osteogenesis imperfecta, Rett syndrome
X-linked recessive - duchenne muscular dystrophy. All either cause sterility or lethality
What is genetic drift?
Allele frequency can fluctuate from generation to generation by chance.
What is gene flow?
migration, can cause allele to enter new populations. PKU allele is far more common in Irish, but has entered other parts of Europe though far less in Asia
What is founder effect?
when small population separates from a larger population, allele frequencies may be different in that smaller group.
What is heterozygote advantage?
Those heterozygous for sickle cell anemia are more likely to survive malaria
