Changing Allele Frequencies

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Changing Allele Frequencies
BIO 2343 Oklahoma City Community College Dennis
Anderson
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Human populations are rarely in Hardy-Weinberg
equilibrium

• Factors that cause allele frequencies to change
  (not be in equilibrium)
• Nonrandom mating
• Migration
• Genetic drift
• Mutation
• Natural selection

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Nonrandom Mating

• Most people choose their mates based on
• Physical appearance
• Ethnic background
• Intelligence
• Shared interests
• One-third of marriages are between people born
  less than 10 miles apart

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Religious Cultural Influences

• Many people will only marry within their own
  religion or culture
• Consanguineous marriages increase risk of birth
  defects by 2.5 times

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Hypothetical example of the consequences of
consanguineous marriages.

• Brown family
• 1 in 2 people are carriers for 3 eyes (Ee)
• White family
• 1 in 4 people are carriers for 2 noses (Nn)
• John Brown (EeNN) marries Susan White (EENn)
• Normal offspring
• John Brown (EeNN) marries Julie Brown (EeNN)
• 25 chance for ee

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Hopi Indians

• Albinos stay in village with woman
• Cannot tolerate the sun
• Albinos have more opportunity to mate with
  females
• 1/200 Hopi Indians are albino
• 1/8 are carriers

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Migration

• When one group migrates to a different population
  it will introduce new alleles
• Europeans introduced new alleles to the African
  population
• Descendents of immigrants to America have a
  different gene frequency than their ancestors
• Frequency of blood alleles in southern Spain is
  about the same as the Arabian frequency
• Arabs ruled until 1492

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Genetic Drift

• Change in gene frequency when small a group of
  individuals leave or are separated from a larger
  population
• Founder Effect
• Bottleneck

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Founder Effect
Original Population 1 has allele A
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Founders

• 10 people leave to found a new population
• 1 of the founders has allele A
• 10 of new population will have allele A

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Ellis-van Creveld Syndrome

• Dwarfism
• Extra fingers
• Heart defects
• High frequency in Amish population of
  Pennsylvania
• A founder of the population had allele for the
  syndrome

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Bottleneck

• Population almost dies out
• Survivors genes are at a higher frequency in the
  descendants than the original population

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Cheetah Bottleneck

• 2 major bottlenecks
• 10,000 years ago
• 1800s
• Present cheetah are more alike genetically than
  inbred lab mice

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1780 Typhoon Creates Bottleneck

• Typhoon kill all but 9 males and 10 females
• One survivor had a gene for blindness
• Now 4-10 of the population has blindness

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Jewish Bottleneck

• Many Jews in eastern Europe were killed in the
  1600s
• Increased frequency of disease causing genes in
  their descendants
• Tay-Sachs disease
• Blindness, death by age 3 or 4
• Neimann-Pick disease
• Mental retardation, death by age 3
• Canavan disease
• Brain degeneration, death by 18 months

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Mutations

• Introduces new alleles into a population
• Most mutations are lethal
• Mutation for no heart would be lethal
• Some mutations are beneficial
• Block infection of HIV

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Beneficial Mutation

• Mutation for albinism beneficial for bears who
  live on the ice and snow
• Polar bears were once part of a population of
  brown black bears
• Now polar bears are a separate species

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Natural Selection

• Some individuals are more likely to survive and
  pass on their genes than others
• Nature selects against gene for black fur in the
  arctic
• Black fur does not enable bears in that
  environment to survive as well
• Nature selects against gene for white fur in
  Oklahoma
• White fur is not as advantageous in Oklahoma

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Tuberculosis

• Number 1 killer in 1900
• Antibiotics decreased cases dramatically
• 1980 very few cases
• Bacterium that causes TB is constantly mutating
• Mutant strains are now resistant to TB
• Mutant strains resistant to antibiotics are
  naturally selected to survive

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Evolution of Tuberculosis
Cases of TB
1900
2000
1980
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Balanced Polymorphism

• Sometimes harmful alleles remain in a population
• Heterozygotes are protected from another disease
• Sickle cell anemia carriers are protected from
  malaria

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Sickle Cell Anemia Frequency

• Sickle cell anemia is most common in parts of
  Africa with malaria
• Carriers who live in an environment with malaria
  have an advantage
• Immune to malaria

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Why is the frequency of sickle cell anemia lower
in the USA population of Blacks than African
populations from which they originated?

• There is no selective advantage for the s allele
  in an environment with no malaria
• The frequency of the s allele in the USA Black
  population has dropped significantly in the last
  300 years.

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Cystic Fibrosis

• Abnormal chloride channels in cell membranes
• Traps salt and water inside of cells
• Results in thick mucus outside of cells
• Mucus plugs up the pancreas and lungs

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Cystic Fibrosis and Cholera

• Carriers for CF are resistant to cholera
• Cholera opens normal chloride channels to cause
  fatal diarrhea
• Abnormal chloride channels in cystic fibrosis are
  not affected by the cholera bacterium
• Carriers from the past were naturally selected to
  survive
• Cystic fibrosis is very common

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Why is PKU Disease Most Common in Scotland
Ireland?

• Mold produces a poison, ochratoxin A
• Causes miscarriages
• Carriers for PKU do not have miscarriages
• During famines people ate moldy grain
• Carriers were naturally selected by nature to
  survive

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Why is Diabetes So Common?

• Body stores extra fat
• Beneficial trait for surviving famines
• Nature selected for diabetes gene

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The End