BIOL1003 Lecture Notes - Lecture 6: Sickle-Cell Disease, Genetic Drift, Genotype Frequency
BIOL1003: Module 2 - Genetics
Lecture 6 – Genetic Drift
• Hardy-Weinberg laws
• p and q and allelic frequencies
• p², q² and 2pq are genotype frequencies
• If its dominant-recessive, the q² will be the only one that displays the recessive phenotype
• therefore, the phenotype frequency may be different to the genotype frequency (if its
dominant- dominant then it will be the same)
• Observations form the study of the hardy-Weinberg system
o Populations in which the genotype frequencies are perturbed revert to hardy-
Weinberg equilibrium in the next generation
o The vast majority of the rare recessives in a populations "hide out" in the
heterozygotes
• eugenics - kill people with the rare disease won't get rid of it as it is recessive,
heterozygotes carry the trait
o Selection against rare recessive phenotypes is ineffective in removing the recessive
alleles from populations
o To show that a population is evolving, mutating, selectively mating, etc. - can prove
this is hardy-Weinberg is broken
o If there is evolution, there is a change in the frequency of the alleles
• Things that result in the changes of allelic frequencies
o Random selection event
• Drift
• Bottleneck, flounder effects
o Selection - can be apparent in a certain gene
• Directions
• Stabilising
• Diversifying
• Genetic drift
o Due to random changes in the populations, changes in allelic frequency in a
population is a result of random variation in reproductive success
o Results in loss of genetic variation
o More marked in small populations
o Happening in cheetahs, and happened in mammoths - lack of genetic variation,
cannot undergo natural selection
• Bottleneck affect
o A bottleneck occurs when a populations size is drastically reduced, then rises again
and is stabilised
o This causes a rise in certain genetic conditions and alleles
• Founder effects
o Changes is allelic frequencies
o On islands, you often have tiny versions of things that are big elsewhere (tiny
elephants on Cyprus), or giant versions of things that are small elsewhere
(Galapagos tortoise)
o Not due to natural selection
o Like bottle neck but in an area
• Stabilizing selection
o Sickle cell anemia is more common where malaria is common
o Sickle cell anemia has a heterozygote advantage
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