# NATS 1940 Lecture Notes - Lecture 1: Genotype Frequency, Allele Frequency, Genetic Drift

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October 5, 2017

CHAPTER 9 POPULATION GENETICS

Review questions

1. A population consists of

25 people with the genotype AA

40 people with the genotype Aa

35 people with the genotype aa

What are the frequencies of genotypes AA, Aa, and aa, respectively?

a) 25, 40 and 35

b) 12.5, 20 and 17.5

c) 0.25, 0.40 and 0.35

d) 0.125, 0.20 and 0.175

e) 0.50, 0.80 and 0.70

answer:

25/100 =0.25

2. What is the frequency of the allele A? Of the allele a?

25 people with the genotype AA

40 people with the genotype Aa

35 people with the genotype aa

a) 0.25 and 0.35, respectively

b) 0.25 and 0.75, respectively

c) 0.65 and 0.35, respectively

d) 0.55 and 0.45, respectively

e) 0.45 and 0.55, respectively

answer:

total number of alleles: 200 (100x2)

frequency of A=p= ((25x2) + 40)/ 200 = 0.45

frequency of a = q= (40+(35x2))/200 = 0.55

check: p + q=0.45+0.55=1

3. What are the frequencies of the dominant and recessive traits (i.e. phenotypes),

respectively?

25 people with the genotype AA

40 people with the genotype Aa

35 people with the genotype aa

a) 0.65 and 0.35

b) 0.25 and 0.75

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c) 0.25 and 0.35

d) 0.40 and 0.35

e) 0.35 and 0.65

answer

total number of phenotypes=100

frequency of dominant = (25+40)/100 = 0.65

frequency of recessive= 35/100= 0.35

check p + q=0.65+0.35=1

Genotype frequencies can be predicted from allele frequencies (p and q)

• Recall: Mendel’s model of heredity states that gametes (sperm or egg) carry one of the

two alleles for each gene

• Alleles segregate at random when gametes are formed (i.e., it is entirely random as to

which of two alleles a gamete receives for any given gene)

• What are the odds that a sperm “picked at random”, from a random male, will carry a

dominant allele?

The Rule of Multiplication

• the probability of two or more independent events occurring together is equal to the

product of their individual probabilities

• Example: Tossing a coin.

o Odds of “heads” = 0.5 (or 50%)

o Toss a coin twice. What are the odds of getting heads both times?

Odds of getting heads on both the first toss and the second toss = 0.5 * 0.5 = 0.25 (or 25%)

25%, 1 in 4 chance

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