Transcript notes
Lecture 43 Prof Duncan Shaw 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” 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 (2x300 + 500)/2000 = 0.55 • Allele frequency of a is (500 + 2x200)/2000 = 0.45 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 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” 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 = 1/1700 = 0.00059 q = 0.024 p = 1 - q = 0.976 2pq = 0.047 • About 1 in 21 Caucasians is a carrier for CF 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 = 0.0025 • About 1/400 colour-blind females 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 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 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