BIMM 110   LECTURE 30

HUMAN POPULATION GENETICS

 TEXTBOOK: Strachan and Read, Chapter4; (see also NOTES)                        

PROBLEMS      ANSWERS

SLIDES

A. Hardy-Weinberg Law

  Hardy-Weinberg      

 
- Gene pools and random combinations
note that in the following tables the formating for the exponent was lost ! exponents are indicated in red color

 B. Estimation of gene and genotype frequencies

		female gametes
		p(T)	q(t)
male gametes	p(T)	p2(TT)	pq(Tt)
	q(t)	pq(Tt)	q2(tt)

 

Gene Frequencies			Genotype frequencies
p(T)	q(t)		p2(TT)	2pq(Tt)	q2(tt)
0.5	0.5		0.25	0.50	0.25
0.9	0.1		0.81	0.18	0.01
0.99	0.01		0.98	0.02	0.0001
			Tasters: p2 + 2pq		Nontasters: q2

- Assumptions of the Hardy-Weinberg Law
                          large population
                          random mating; no inbreeding
                          no new (spontaneous) mutations
                          no selections, no fertility problems, no lethal mutations
                          applicable over relatively short periods with no evolution

- Genotype frequencies and phenotype frequencies:
- different equilibrium frequencies for females and males for X-linked genes

FREQUENCIES OF MATING TYPES AND OFFSPRING

			Offspring genotype
Mating types	Frequency		TT	Tt	tt
TT x TT	p4		p4
TT x Tt	2 x p2 x 2pq		2p3q	2p3q
TT x tt	2 x p2 x q2			2p2q2
Tt x Tt	2pq x 2pq		p2q2	2p2q2	p2q2
Tt x tt	2 x 2pq x q2			2pq3	2pq3
tt x tt	q4				q4

------------------------------------------------------------------------------------------------------------------

sum of total offspring: p2 x () 2pq x () q2 ()             (the factor in brackets is p2 + 2pq + q2)

i.e in the offspring the distribution of genotypes is exactly the same as in the parental population !

The alleles at ONE locus come to Hardy-Weinberg equilibrium in one round of random mating

 C. Deviations from the Hardy-Weinberg Law

- reasons for discrepancies between predicted and observed frequencies:
                           selective mating
                           extensive migration into a population (what is "the population")
                           high spontaneous mutation rate (e.g. Duchenne Muscular Dystrophy)

Linkage disequilibrium
- consideration of two linked loci, each with at least two alleles
- the allele frequencies for each allele at each locus are known in population
- gametic and zygotic disequilibrium
- comparison :      [P(ab,t)] with [P(a,t)] x [P(b,t)], where [P(ab,t)] is the observed frequency of booth alleles a and b in a gamete, and [P(a,t)] and [P(b,t)] are the individual allele frequencies for a and b at each locus. If the the two are not the same, we have linkage disequilibrium
- the rate of approach to linkage equilibrium depends on the recombination frequency between the two loci: for closely linked genes disequilibrium persists for a long time.
            

 D. Human evolution

- population origins
- modern human population groups

Over the period of human evolution allele frequencies were obviously not constant; none of the conditions for the applicability of the Hardy-Weinberg Law were valid over a long time span and for the human population as a whole.
 
 

- Cavalli-Sforza, L.L., and Bodmer, W.F. (1971)The Genetics of Human Populations , W.H. Freeman and Company, San Francisco.