Lecture 18 : Hardy-Weinburg Equilibrium

Question 1

How does sexual reproduction influence genotypic and allelic frequencies?

Answer 1

The answer lies in the HARDY-WEINBERG LAW

1. named after the 2 scientists who discovered it in 1908.
2. Predicts how GENE FREQUENCIES WILL BE TRANSMITTED FROM GENERATION TO GENERATION given a specific set of assumptions.

Question 2

Explain the Hardy-Weinberg Law

Answer 2

“If a population is infinitely large, randomly mating and is NOT affected by the mutation, migration and natural selection, then:

1. the gene frequencies will not change over time, and
2. the genotypic frequencies will stabilize after one generation in the following proportions:

    p^2 for the AA genotype 
    2pq for the Aa genotype
    q^2 for the aa genotype 

When genotypes are in these expected proportions the population is said to be in Hardy-Weinberg Equilibrium.

Question 3

Hardy-Weinberg Equilibrium - a closer Examination.
What are it’s ASSUMPTION? (3)

Answer 3

Assumptions:

1. INFINITELY LARGE POPULATION
   - no such population actually exists
   - In practice,Random most large populations are in Hardy-Weinberg proportions and SIGNIFICANT DEVIATIONS ARISE ONLY WHEN POPULATIONS SIZES ARE VERY SMALL
2. RANDOM MATING
   - matings occur in proportion to their genotypic frequencies

• for example, if the genotypic frequencies in a population are:
  f(AA) = 0.6, f(Aa) = 0.3 and f(aa) = 0.1, then the frequency of a mating between two AA individuals is 0.6 x 0.6 = 0.36, whereas the frequency of a mating between two aa individuals is 0.1 x 0.1 = 0.01
• Random mating can occur at some loci but not at others

3. ALLELIC FREQUENCIES ARE NOT AFFECTED BY EVOLUTIONARY FORCES
   the principle forces are:
   1. Mutation - rate is so slow that it is unlikely to have short-term effects on a population
   **2. selection - likely to be more important
   **3. Migration -likely to be more important

Some loci in a population may be affected by these forces, and others may not. A population may be in Hardy-Weinberg equilibrium for one locus but not for others.

Question 4

What does the Hardy-Weinberg law tell us?

Answer 4

1. A POPULATION can NOT EVOLVE if it MEETS HW ASSUMPTIONS because EVOLUTION RESULTS FROM A CHANGE IN ALLELE FREQUENCIES.
   - This tells us that sexual reproduction alone will not cause evolution.
2. When a POPULATION is at HW EQUILIBRIUM, GENOTYPIC FREQUENCIES are DETERMINED By ALLELIC FREQUENCIES.
   - when allele frequencies are low, most of the copies of the rare alleles will be present in heterozygotes.
   - Genotypic frequency (y axis), Allelic frequency (p) a axis
3. A SINGLE GENERATION OF RANDOM MATING PRODUCES HW EQUILIBRIUM
   - if genotypes are in HW proportions, it means that evolutionary forces that disrupt HW have not acted since the last time random mating took place.

Question 5

What are the PRACTICAL uses of Hardy-Weinberg Law?

Answer 5

1. Provides a NULL MODEL for TESTING if a POPULATION is a RANDOMLY MATING or UNDER the INFLUENCE of EVOLUTIONARY FORCES
2. CALCULATION of ALLELIC FREQUENCIES when DOMINANCE is PRESENT

• look at the written example

3. GENETIC FINGERPRINTING and INDIVIDUAL IDENTIFICATION

• look at the written example

Question 6

X -linked alleles

Answer 6

For an X-linked locus with two alleles (X^A and X^a)
— expected FEMALE genotypes are:
p^2(XAXA) + 2pq (XAXa) + q^2 (XaXa)
— expected MALE genotypes are:
p (X^AY) + q (X^aY)

these are THE PROPORTIONS OF GENOTYPES AMONG MALES AND FEMALES RATHER THAN THE POPULATION.
to calculate the expected proportion of XAXA genotypes in the entire population need to multiply p^2 by the proportion of females in the population

For an X-linked recessive trait (e.g haemophilia) the frequency among males (q = 0.0001) will be much higher than in females (q^@ = 0.00000001)

Question 7

Testing for Hardy-Weinberg Equilibrium … explain the steps

Answer 7

3 steps:

1. Calculate allele frequencies from observed genotypes
2. Calculate the expected genotypic frequencies using HW law
3. Compare observed and expected genotypic frequencies using the chi-square test

LOOK AT THE WORKED EXAMPLE

Question 8

NON-RANDOM MATING: INBREEDING (4)

Answer 8

inbreeding
1. Most common TYPE OF NONRANDOM MATING

2. Positive ASSORTATIVE MATING for RELATEDNESS.
3. Differs from other types of assortative mating because it affects ALL genes, not just those that Determine the Trait for which the Mating Preference frequencies
4. INBREEDING DOES NOT CAUSE EVOLUTION - DOES NOT CHANGE ALLELE FREQUENCIES, but does ALTER GENOTYPIC FREQUENCIES

Question 9

Changes in genotypic frequencies with inbreeding

Answer 9

Self-fertilization
AA x AA –> AA only
aa x aa –> aa only
Aa x Aa –> 1/4 AA + 1/2 Aa + 1/4 aa (only half like the parent)

In each generation, the number of heterozygotes reduces by half until all genotypes in the population are homozygotes.

The increase in the frequency of homozygotes in a self-fertilisation population starting with p = q= 0.5

Question 10

Harmful effects of inbreeding

Answer 10

Offspring of inbreeding have an increased risk of homozygosity and are more likely to present with recessive traits that have been carried in the family - confirmed an increase in several genetic disorders due to inbreeding such as blindness, hearing loss, neonatal diabetes, limb malformations, disorders of sex development, schizophrenia and several others.

Question 11

Summary

Answer 11

1. Hardy-Weinberg states that allele frequencies DO NOT change as the results of sexual reproduction
2. Non random mating change the proportions of each genotypic class relative to that expected under random mating, but NOT allele frequencies
3. For evolution to occur the allele frequencies of that population must undergo change.