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Random Events


Research Randomizer: Instant Random Sampling and Random Assignment ... Hitch-hikers thumb. The same phenomenon can be seen in the Amish sects of Pennsylvania and Ohio. ... – PowerPoint PPT presentation

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Title: Random Events

Random Events
There is a random element involved in the
frequency of genes in one generation when
compared to the previous generation.
Genetic Drift a random change in the genetic
makeup of a population that may arise when the
population is restricted in size. Drift is
directionless. Compare to tossing a
coin. Research Randomizer Instant Random
Sampling and Random Assignment
In Florida, the greenhouse frog Eleutherodactylus
planirostris occurs in two pigment patterns,
mottled and striped. Why?
The theory of genetic drift was developed in the
1930s by Sewall Wright, and is sometimes referred
to as the Sewall Wright effect.
Consider a mathematical example. If two Aa frogs
have only two offspring, there is a 1/16 chance
of the a gene (or the A gene, for that matter)
gene being lost after the first generation.
Random sampling is more effective at creating
variations in small populations than in large
ones. Results from the Dobzhansky experiment
with Drosophila populations.
Founder effect unique gene frequencies arising
in small groups of individuals that migrate away
from a large population. The chance that a
founder population will be homozygous depends on
the number of founders and the gene frequencies.
One type of founder effect occurs when a new
population is established by a very few
individuals. The most extreme case would be that
in which the founder was a single pregnant
female. Much of the genetic diversity of the
parent population would be lost.
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The 300 inhabitants of Tristan da Cunha are
descended from 20 or so 19th Century immigrants.
Native Americans provide a possible example.
Most North American tribes lack the gene
governing type B blood. However, that gene is
widespread in Mongolia, their ancestral home.
The group that migrated across the Bering Strait
may have been small, and lacked this gene.
Native American populations also show a high
incidence of albinism. Can this also be
attributed to founder effects?
Another type of founder effect occurs when a
large, established population is reduced in size.
The remaining individuals may not be
representative of the genetic diversity that was
present in the original population. This is
referred to as a genetic bottleneck.
If a population passes through a bottleneck,
the amount of genetic diversity may be greatly
The drift to homozygosity. Over time, the
population must drift to become made up of the
descendants of only one gene from an ancestral
population. Any one gene has a 1/2N chance of
eventual fixation by random drift.
In small populations, Hardy-Weinberg equilibrium
is not reached until they reach homozygosity.
Homozygosity is the result in any small
population without mutation.
In human populations, religious isolates offer
one of the best opportunities to witness genetic
drift. The Dunkers, of eastern Pennsylvania are
descended from Old German Baptist Brethren who
came to the U.S. in the early 18th century. They
have remained genetically isolated by rigid
marriage customs. Bentley Glass compared certain
genetic traits of the Dunkers to the surrounding
population, and to their parent population in
His group looked at ABO blood groups MN blood
types Rh Blood types Four external features Ear
lobes Handedness Mid-digital hair Hitch-hikers
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The same phenomenon can be seen in the Amish
sects of Pennsylvania and Ohio.
Eight family names account for 80 of the Amish
families in Lancaster County, PA.
The high degree of consanguinuity has led to a
high frequency of recessive genetic disorders.
Among them are Ellis-van Creveld syndrome,
pyruvate kinase deficient hemolytic anemia,
Hemophilia B, and Troyer syndrome (a form of
muscular dystrophy).
Genetic drift is responsible for the introduction
of Rh disease into China. 70 years ago, all
Chinese women were Rh positive (RR).
Intermarriage between immigrant Americans has
introduced the Rh-negative (r) allele into the
Since the beginning of the slave trade some 250
years ago, there has been roughly a 20 Caucasian
contribution to the African American gene pool. A
specific blood group gene known as the Duffy
factor is virtually absent from West African
populations from which most of the original
slaves were derived. Its frequency in native West
Africans, African Americans, and American
Caucasians has been studied.
It appears that various African American
populations derive between 4 and 26 of their
genes from Caucasian ancestry, introduced since
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