1. Lecture 43
Prof Duncan Shaw

2. Alleles & Fitness
“Fitness” means the relative ability of organisms to survive and pass on genes
Alleles can affect fitness:
In most cases not at all (neutral)
Sometimes to decrease it (deleterious)
Very rarely to increase it (advantageous)
The frequency of alleles in the whole population affects the health of the population, so it’s important to know about “population genetics”

3. Calculating Allele Frequencies
We have a gene with alleles A and a
Count number of individuals in population with each genotype:
300 AA
500 Aa
200 aa
Allele frequency of A is (2x )/2000 = 0.55
Allele frequency of a is ( x200)/2000 = 0.45

4. Allele & Genotype Frequencies
In previous example, we had 300 AA, 500 Aa, and 200 aa individuals
Genotype frequencies are therefore:
0.3 AA
0.5 Aa
0.2 aa
Allele frequencies and genotype frequencies are related, but not the same thing

5. The Hardy-Weinberg Law
Frequencies of alleles ‘A’ and ‘a’ are p and q, respectively (so p + q = 1)
Calculation involving a Punnet square shows that genotype frequencies will be:
AA p2
Aa 2pq
aa q2
Also, p2 + 2pq + q2 = 1
These frequencies stay the same over time, if population is large, randomly mating, and alleles have same fitness
Such a population is in “equilibrium”

6. Recessive alleles & carrier frequencies
For a rare allele, heterozygotes (called “carriers” if the allele is recessive) are much more frequent than homozygotes: 2pq >>q2
Cystic fibrosis is caused by a recessive allele and affects 1/1700 Caucasian newborns:
q2 = 1/ =
 q =
 p = q =
 2pq =
About 1 in 21 Caucasians is a carrier for CF

8. X - linked genes
Males only have 1 copy of each gene on the X chromosome (“hemizygous”), from mother
Therefore, for X-linked genes in males, genotype frequency is the same as allele frequency
For rare X-linked recessive alleles, more males than females will be affected
Example: X linked colour blindness affects 1/20 males
q = 0.05
q2 =
About 1/400 colour-blind females

9. Population Evolution
Changes in the gene pool resulting a species adapting to its environment
Dependent on genetic variation
Driven by natural selection - differences in fitness make better adapted individuals more likely to pass on their genes
Can be described in terms of allele frequencies in the population

10. Factors that change allele frequencies
Mutation - formation of new alleles, leading to new capabilities of organism
Migration - movement of individuals between populations
Natural selection - different abilities of organisms to survive and reproduce
Genetic drift - in small populations, random changes in allele frequency

11. Heterozygote advantage
Sickle-cell anaemia is caused by a recessive allele - homozygotes have reduced fitness (without modern medical care), so it should disappear
But heterozygotes have increased fitness - don’t have anaemia, but are more resistant to malaria than individuals without allele
Distribution of sickle-cell anaemia in the world is similar to that of the malaria parasite, Falciparum