# Genetic Variation, Genotypes, and Disease

1 Define the term "genetic variation."

If a gene or locus has two alleles (A and a) in a population, what are all the possible genotypes?

If the frequency of (A) allele = p, and the frequency of (a) allele is q, what is the frequency of all possible genotypes in this population?

2 What is the Hardy-Weinberg equilibrium?

In a population, a locus A has two alleles (A) and (a). The frequency f of (A) is f (A) = 0.6; what is the f (a)?

Using these frequencies, calculate the frequencies of all possible genotypes in a population in Hardy-Weinberg equilibrium.

3 In a population (Z), the frequencies of genotypes of a two allele locus (B and b) are f(BB) = 0.3, f(bb) = 0.6, f (Bb) = 0.1. Calculate the frequencies of both alleles.

Using the allele frequencies calculated in a., calculate the frequencies of all possible genotypes at this locus in a population after one generation of random mating.

Is the population (Z) in part a above in Hardy-Weinberg equilibrium? The population size is 1000.

4 Describe how you would use the Hardy-Weinberg equilibrium to calculate genotype frequencies of a locus with three alleles.

If the f(D) of an X-linked gene in a population = 0.8, what is the f(d) of the other allele at this locus?

What is the frequency of females' homozygous for d allele (XdXd)?

What is the frequency of males' hemizygous for the d allele (XdY)?

5 Explain why more males present with X-linked recessive diseases. Show your reasoning.

Why must the five assumptions/criteria apply to a population before we can say it is in Hardy-Weinberg equilibrium?

Take any one of these assumptions and explain in detail how it could disrupt the Hardy-Weinberg equilibrium for a particular gene locus.

#### Solution Preview

1. a. Define the term "genetic variation."

Genetic variation refers to the range of phenotypes caused by different genotypes. This variation comes from mutations in the genetic code, gene flow, nonrandom mating, natural selection, and assortment that occurs during meiosis.

Web reference:

http://evolution.berkeley.edu/evosite/evo101/IIICGeneticvariation.shtml

b. If a gene or locus has two alleles (A and a) in a population, what are all the possible genotypes?

AA, Aa, aa

c. If the frequency of (A) allele = p, and the frequency of (a) allele is q, what is the frequency of all possible genotypes in this population?

AA = p squared (p^2)

Aa = 2pq

aa = q^2

2. a. What is the Hardy-Weinberg equilibrium?

This equation allows to predict the frequency of genotypes and phenotypes within a population that conforms to the assumptions of the equation, which include no mutation, gene flow, genetic drift, nonrandom mating, or natural selection. Hardy-Weinberg states that the sum of allele frequency is equal to 1 (p+q=1) and the sum of the genotype frequencies also equals 1 (p^2+ 2pq + q^2 = 1). Additionally, these frequencies do not change in subsequent generations as long as the assumptions of the equilibrium are met.

Web reference:

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/Hardy_Weinberg.html

http://www.tiem.utk.edu/~gross/bioed/bealsmodules/hardy-weinberg.html

b. In a population, a locus A has two alleles (A) and (a). The frequency f of (A) is f (A) = 0.6; what is the f (a)?

Remember p+q=1 Therefore, if A=0.6 then a =1-0.6=0.4

c. Using these frequencies, calculate the frequencies of all possible genotypes in a population in Hardy-Weinberg equilibrium.

AA = p^2 = (0.6)^2 = 0.36

Aa = 2pq = 2(0.6)(0.4) = 2(0.24) = 0.48

q^2 = (0.4)^2 = 0.16

Check that p^2+ 2pq + q^2 = 1, 0.36+0.48+0.16= 1

3. a. In a population (Z), the frequencies of genotypes of a two ...