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Changing Allele Frequencies
Allelic Frequencies Change When
There Is:
Non-random mating
Gene flow/Migration
Genetic drift
Natural selection
Non-random Mating
We marry people similar to ourselves 80% of the time
1/3 of all marriages occur between people who were
born <10 miles apart
Certain individuals contribute more to the next
generation than others
– Prize bull semen
– Chinese immigrant to South African with rare dominant mutation
that causes teeth to fall out by age 20 had 7 wives
– Albinism in Hopi Indians
– Genocide by rape in Darfur
– Consanguinuity (a.k.a. inbreeding)
– Endogamy =marriage within a community
Non-random Mating
Gene Flow/Migration
Individuals may join the migration
Individuals may mate within other populations
along the way
Immigrants introduce alleles and emigrants
remove them
A cline exists when neighboring populations
have differing allelic frequencies
– Geographical barriers (e.g. mountains)
– Language barriers
Gene Flow/Migration
Genetic Drift
Chance sampling of alleles from the whole
Due to:
– Founder effect
– Population bottleneck
Founder Effect
Small group leaves the population to start
a new settlement
New colony may have different allele
frequencies than the original population
Ex: Small religious sect community in
– Founded by 2 individuals in the 1930’s
– 50% of all fumarate deficiency
Mental retardation, seizures, coma
Population Bottleneck
Many members of a population die and only a
few are left to re-populate
Much more restricted gene pool than original
Ex: Pingalapese people of the East Caroline
Islands in Micronesia
– Typhoon wiped out all but 9 males and 10 females
– Autosomal recessive achromatopsia very prevalent
Color-blindnesss, nearsightedness, and cataracts
Genetic Drift
Major and continual source of genetic
One allele changes into another
Must occur in gamete in order to affect
future generations and allele frequencies
Genetic Load
Mutations that lead to lethal traits are often
eliminated from the gene pool, however,
some mutant alleles can persist in
Genetic load refers to the collection of
these deleterious alleles in the population
– Each of us has 5-10 recessive lethal alleles
Natural Selection
Guided by changes in the environment
Individuals with certain phenotypes are
more likely to survive and have
reproductive success
Ex: Antibiotic resistance
– Antibiotics select for those bacteria which are
Natural Selection
Forces that Alter Allele Frequencies