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Chapter 15

Chapter 15- Human Genetic Diveresity.docx

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Jon Stone

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Chapter 15: Human Genetic Diversity
Explaining Human Variation
-wide variation among humans
olanguage, fashions, customs, religion, technology, architecture, and other
aspects of behavior differ among societies
-peoples of the world vary in blood type and the incidence of many genetically
transmitted diseases
-scientists distinguish two sources of human variation: genetic and environmental
-genetic variation: differences between individuals that are caused by genes
they inherited from their parents
-environmental variation: differences between individuals caused by
environmental factors (i.e. climate, habitat, and competing species) on the
organisms’ phenotypes.
ofor humans, culture is an important source of environmental variation
-difficult to determine relative importance of genetic and environmental influences
for particular phenotypic traits
-genetic variation is governed by processes of organic evolution: mutation, drift,
recombination, and selection
-variation within groups: differences between individuals within a given group of
-variation among groups: differences between entire groups of people
Variation in Traits Influenced by Single Genes
-can be certain that variation arises from genetic differences between individuals
-example: sickle-cell anemia
oknow people with this disease are homozygous for a gene that codes for
one variant of hemoglobin
-can prove that traits are controlled by genes at a single genetic locus by showing
that their patterns of inheritance conform to Mendel’s principles
-if scientists suspect that a trait is controlled by genes at a single genetic locus,
they can test this by collecting data on occurrence of trait in families
-if inheritance shows close fit with Mendel’s principles, then we can be confident
that traits is affected by a single locus
-research on genetic basis of a language disorder called specific language
impairment (SLI)
otrouble learning to speak
orun in families, but genetic basis of condition is unclear in most areas
osome cases of SLI are caused by a dominant allele at a single genetic
obecause SLI is rare in population as a whole, the Hardy-Weinberg
equations tell us that almost all ALI sufferers will be heterozygotes

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oalthough pattern in is consistent with idea that SLI is caused by a single
dominant gene, it is possible that an environmental factor causes SLI to
run in families and that those observed pattern arose by chance
oin 1998, Wellcome Trust researchers found that families with SLI is closely
linked to a genetic marker (genes whose location in the genome is known)
on chromosome 7
so it seems likely that SLI is controlled by a gene closely linked to
this marker
osame allele of this gene FOXP2 has undergone strong directional
damage to this gene prevents normal development of some
psychological machinery necessary for language
Causes of Genetic Variation within Groups
-mutation can maintain deleterious genes in populations, but only at a low
-many diseases caused by recessive gene
-why haven’t such deleterious genes been eliminated by natural selection?
onatural selection steadily removes such genes, but they are constantly
being reintroduced by mutation
obecause mutation rates for deleterious genes are estimated to be a few
mutations per million gametes produced, mutation will introduce new
mutants to maintain a constant frequency of the gene
selection-mutation balance
-selection can maintain variation within populations if heterozygotes have higher
fitness than either of the two homozygotes
-when heterozygotes have a higher fitness than either homozygote has, natural
selection will maintain a balanced polymorphism, a steady state in which both
alleles persist in the population
oexample: sickle-cell anemia
individuals with genotype AS (A- regular hemoglobin; S- sickle-cell)
are immune to malaria
-scientists suspect that relatively high frequencies of genes that cause a number
of other genetic diseases may also be result of heterozygote advantage
-variation may exist because environments have recently changed and genes that
were previously beneficial have not yet been eliminated
-some genetic disease may be common because symptoms they create have not
always been deleterious
oexample: non-insulin-dependent diabetes (NIDD)
Neel (from University of Michigan) suggested that genes now
leading to NIDD were beneficial in the past because they caused a
rapid buildup of fat reserves during periods of plenty- fat reserves
that would help people survive periods of famine in harsh
Causes of Genetic Variation
-selection that favours different genes in different environments creates and
maintains variation among groups
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