EVOLUTION QUESTION - 1989 L. PETERSON/AP BIOLOGY
Do the following with reference to the Hardy-Weinberg model.
A. Indicate the conditions under which allelic frequencies (p and q)
remain constant from one generation to the next.
B. Calculate, showing all work, the frequencies of the alleles and the
frequencies of the genotypes in a population of 100,000 rabbits,
of which 25,000 are white and 75,000 are agouti.
(In rabbits the white color is due to a recessive allele, w, and agouti
is due to a dominant allele, W.)
C. If the homozygous dominant condition were to become lethal, what
would happen to the allelic and genotypic frequencies in the rabbit
population after two generations?
STANDARDS:
A. CONDITIONS FOR HARDY-WEINBERG:
H-W applies if:
large population size (1 pt) no genetic drift or founder effect
random mating (1 pt) no mating preference or inbreeding
no mutation (1 pt)
no selection (1 pt) all genotypes have equal chance to reproduce
no migration (1 pt) no differential migration;
no gene flow among populations;
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5 pts Max 3
B. PROBLEM
formula (1 pt) p2 + 2pq + q 2 = 1
relationship to genotypes WW Ww ww or W = p
(1 pt) w = q
definition of all terms of equation
calculation to frequency 25,000/100,000 = frequency ww = q2
(1 pt) = 0.25 or 1/4 or 25%
allele frequencies (2 pts) q = .25 = .5 = frequency of w
(1 pt if no explanation)
formula (1 pt) since p + q = 1
p = 1 - q = .5
frequency of W
genotype frequencies p2 = (.5)2 = .25 - WW
(3 pts) 2pq = 2(.5) (.5) = .5 = Ww
q2 = (.5)2 = .25 = ww
or
1 pt for frequencies with no explanation
or
W w
____.5__.5___
W .5 .25 .25
____________
w .5 .25 .25
(in context)
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9 pts Max 6
C. APPLICATIONS (WW genotypes die)
genotype frequency changes p2 decreases (does not disappear)
(1 pt) or
Ww decreases &/or ww increases
or
2 pq decreases &/or q2 increases
or
heterozygotes decrease &/or homozygotes increase
allele frequency changes p decreases
(but is not eliminated because of heterozygotes)
(1 pt)
q increases
Bonus: Some discussion e.g.
selection (1 pt) death of homozygotes due to selection
(decreased fitness)
fitness = 0
s = 1
A rare student may know that in 2 generations p is halved i.e. p = .25, q = .75
If n = # of generations = 2
pn - po /(1 + npo) = .5/(1+2(.5)-.25
p2 = .06; 2 pq = .38; q2 = .56
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4 pts Max 2