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Lecture 25

BIOLOGY 1M03 Lecture Notes - Lecture 25: Cystic Fibrosis, Effective Population Size, Vicia FabaPremium

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Jon Stone

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BIOLOGY 1M03 - Lecture 25 - Human Population Genetics (Continued)
Selective Sweep:
Effective population size is only the number giving birth to the new
Malaria-Protective Variation:
Hemoglobin variants include blood groups, such as Rh-negativity and Rh-
Positive have different tolerances against malaria.
The origin of these mutations is much older than the origin of malaria
so it didn’t evolve as a protection against malaria
Thalassemia is a mutation where two copies gives a disease, one mutation
(heterozygosity) gives protection against malaria.
Changes in metabolic enzymes can give rise to protection against malaria.
GP6D deficiency/favism (a digestive enzyme) is an X-linked gene.
Fava beans cannot be metabolized properly. Males suffer more from
this disease as a result. It arose 5-10K years ago. The question is,
why did this mutation arise not so late?
Agriculture arose around this time, and may be correlated with
this mutation.
Duffy antigens, a genetic system, protect against vivax malaria.
Cystic Fibrosis:
Cystic Fibrosis is most common in Europeans. There are about 4% carriers
(1/2500 diseased: 2pq=0.04, leading to q squared, which equals 0.0004)
This lung disease is lethal: involves mucus build-up. There are ¼ chances of
death before 30, but was much higher previously.
The mutated gene is cftr which changes chloride metabolism, which arose 50
Gene dating has only been known for ~20-25 years.
It was most likely not possible to have been a bottleneck to have
caused this mutation spread.
To keep this mutation at 4% in the population might be attributed to
heterozygosity advantage, possibly from cholera (intestinal disease).
The cholera toxin promotes big rush of fluid into lower intestine.
Severe dehydration is a result, leading to death.
The spread may have been due to the First European cholera
epidemic in 1817.
What cholera does to flush water out of the body is involved in the
cholera metabolism. By tweaking it, the carriers have a higher chance
of survival.
Possibly Typhoid?
It is an example of pleiotropy (multiple effects from one gene)
It happens too often to be explained by mutation selection balance
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