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Population Genetics
Hardy-Weinberg Equilibrium
Background Info for Fishy Frequencies Lab
• Phenotype
Key Terms
– Physical appearance (expression of genotype)
• Allele
– A form of a particular gene
• Dominant allele
– The allele that is always expressed when present in the
genotype
– Represented by CAPITAL letters
• Recessive allele
– The allele that is only expressed if the dominant allele
is absent in the genotype
– Represented by lowercase letters
Alleles
B  Black is dominant
b  Red is recessive
What are the phenotypes of these genotypes?
BB
Black
Homozygous
Bb
Black
Heterozygous
bb
Red
Homozygous
Frequencies
1
=
0.3 =
0.5 =
100%
30%
50%
Your turn!
0.16 =
16%
0.75 =
75%
0.02 =
2%
Allele Frequencies Card Game
KEEP YOUR CARDS HIDDEN!
1. Start out homozygous
All black (BB) or all red (bb)
Recall: Sex = random gene shuffling
1. In a moment, you will “mate” with one of your
classmates by RANDOMLY trading a card.
KEEP YOUR CARDS HIDDEN!
2. Mingle around the room. Say hello to people you meet
but only “mate” with the 4th person you pass. Once
you have “mated,” have a seat.
Allele Frequencies Card Game
Total population: 30
Generation # Black
# Red
phenotype phenotype
1
(After mating)
Sum of
Frequencies
20
10
20/30 =
.7
.3
1
28
2
28/30=
.93
.07
1
(All
homozygous )
2
Frequency Frequency
of Black
of red
phenotype phenotype
Hardy-Weinberg theorem
 Counting Individuals



frequency of homozygous dominant: p x p = p2
frequency of homozygous recessive: q x q = q2
frequency of heterozygotes: (p x q) + (q x p) = 2pq
 frequencies of all individuals must add to 1 (100%), so:
p2 + 2pq + q2 = 1
BB
Bb
bb
B: Brown is dominant
What are the possible genotypes for brown fish?
BB or Bb
b: Gold is recessive
What are the possible genotypes for gold fish?
bb
frequency of dominant allele (B) = p
frequency of recessive allele (b) = q
