Transcript Chapter 5

The Inheritance of Traits
 Offspring resemble their parents, but not exactly.
 Siblings resemble each other, but not exactly.
 How much is because of environment?
 How much is inherited?
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The Inheritance of Traits
 The human life cycle:
 Adults produce gametes in their gonads by meiosis.
 Sperm cells fertilize egg cells to form single-celled
zygotes.
 Repeated cell divisions form the embryo.
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 The human life cycle, cont.:
 The embryo grow to become a fetus.
 After birth, the individual continues to grow until
reaching adulthood.
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The Inheritance of Traits
 Genes are segments of DNA that code for proteins.
 Analogous to words in an instruction manual for
building a human
 We have many genes on each of our chromosomes.
 Chromosomes are analogous to pages in the
instruction manual.
 Each “page” contains thousands of “words”
 Different types of cells use different words, in different
orders
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The Inheritance of Traits - Producing Diversity in
Offspring
 Mistakes in copying DNA (mutations) produce
different versions of genes (alleles), with different
results.
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The Inheritance of Traits - Producing Diversity in
Offspring
 Parent cell has two complete copies of the manual:
23-page copy from mom and 23-page copy from dad
 23 pairs of homologous chromosomes
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 Segregation: in meiosis, one member of each
homologous pair goes into a gamete
 Gamete gets just one copy of each page of the
manual
 Independent assortment randomly determines
which member of a pair of chromosomes goes into
a gamete
 This is due to random alignment during metaphase I
 About 8 million different combinations of
chromosomes.
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Parent cells have 2 copies of each chromosome—that is, 2 full sets of
instruction manual pages, 1 from each parent.
Siblings share
50% of alleles
with each other,
on average
Sperm and egg cells each have only 1 full set—a random combination
of maternal and paternal instruction manual pages.
Possible sperm cell 1
Page 3
Blood-group
gene from dad
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Page 9
Eye-color genes
from mom
Possible sperm cell 2
Page 3
Blood-group
gene from mom
Page 9
Eye-color
genes from
dad
Figure 6.6
 Random fertilization
produces more diversity:
64 trillion possibilities!
 No two humans are
genetically identical, except
for monozygotic twins.
 Dizygotic twins are 50%
identical - just like siblings
born at different times.
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Mendelian Genetics: When the Role of Genes Is
Clear
 Gregor Mendel: first to accurately describe rules of
inheritance for simple traits
 His research involved controlled mating between pea
plants.
 His pattern of inheritance occurs primarily in traits
that are due to a single gene with a few alleles.
 Mendel’s principles also apply to many genetic
diseases in humans.
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 Phenotype: physical traits of an individual
 Genotype: description of the alleles for a particular
gene in an individual
 Homozygous (-ote): both alleles for a gene are
identical
 Heterozygous (-ote): the gene has two different
alleles
 Recessive: the phenotype of an allele is seen only
when homozygous
 Dominant: the phenotype is seen when
homozygous or heterozygous
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1 A pea flower normally self-pollinates.
3 Pollen from another flower is dabbed
on to stigma.
Paint brush
Stigma
Anthers
(contains
pollen)
Ovule
2 Pollen containing structures can be
removed to prevent self-fertilization.
Tweezers
The resulting seeds will
contain information on
flower color, seed shape
and color, and plant
height from both parents.
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Figure 9.5
Monohybrid Crosses
 A monohybrid
cross is a cross
between parent
plants that differ in
only one
characteristic.
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Mendelian Genetics: When the Role of Genes Is Clear
 Mendel developed four hypotheses from the
monohybrid cross:
 There are alternative forms of genes, called alleles.
 For each characteristic, an organism inherits two
alleles, one from each parent.
 Alleles can be dominant or recessive.
 Gametes carry only one allele for each inherited
characteristic.
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 Mendel’s law of segregation
 The two members of an allele pair segregate
(separate) from each other during the production of
gametes.
 An explanation of Mendel’s results, including a
Punnett square
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Figure 9.6b
 Cystic fibrosis: a recessive human genetic
disease
 Defect in chloride ion transport
 Causes recurrent lung infections, dramatically
shortened lifespans
 Heterozygotes (carriers) do not show the symptoms
 Most common recessive disease among Europeans
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 Huntington’s disease: a dominant human genetic
disease
 Progressive, incurable, always fatal
 Symptoms occur in middle age
 Mutant protein forms clumps inside nerve cell nuclei,
killing the cells
 Having a normal allele cannot compensate for this
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 Punnett square: graphic way to predict possible
outcomes of a cross
 Consider a cross between two cystic fibrosis
carriers
 “F” = normal allele; “f” = recessive disease allele
 The cross would be: F f x F f
 What offspring could result?
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Animation: Mendel’s Experiments
Click “Go to Animation” / Click “Play”
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Quantitative Genetics: When Genes and
Environment Interact
 Quantitative traits show continuous variation:
 Large range of phenotypes
 E.g., height, weight, intelligence
 Variation due to both genetic and environmental
differences
 Heritability: proportion of the variation within a
population due to genetic differences among
individuals
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Mean: sum up all the phenotypic values and divide
by the number of individuals; same as the average.
(a) Normal distribution of student height in one
college class
5 ft, 10 in (1.78 m )
Number of men
Mean
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Bell-shaped
curve
Variability
Height (ft, in)
Figure 6.16a
Quantitative Genetics - Why Traits Are
Quantitative
 Quantitative traits, with continuous variation, are
polygenic traits.
 Result of several genes
 Each with more than one allele
 Interaction of multiple genes with multiple alleles
results in many phenotypes.
 Example: human eye color
 Heritability: proportion of the variation within a
population due to genetic differences among
individuals
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Quantitative Genetics - Calculating Heritability in
Human Populations
 Have to use correlation to measure heritability in
humans
 Scientists seek “natural experiments,” situations in
which either the overlap in genes or environment is
removed
 Twins are often used
 Monozygotic twins share all their genes and their
environment
 Dizygotic twins share environment, but only half their
genes
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Usually influenced
by both genes and
environment
Monozygotic
twins, genetically
identical, but
different
environments
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Figure 6.17
Genes, Environment, and the Individual The Use and Misuse of Heritability
 Differences between groups may be environmental,
despite a high heritability
 A heritability value pertains just to the population in
which it was measured, and to the environment of
that population
 Imagine a laboratory population of mice of varying
weights
 Divide this population into 2 genetically identical
groups
 Give one group a rich diet, the other a poor diet
 The “rich diet” mice will be bigger than the “poor diet”
mice.
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Genes, Environment, and the Individual The Use and Misuse of Heritability
 Allow the mice in
each group to
breed, maintaining
their diets.
 Measure the weight
of adult offspring;
correlation with
parents shows
high heritability.
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Genes, Environment, and the Individual The Use and Misuse of Heritability
 Instead of body weight in mice, consider IQ in
humans.
 Affluent group: higher IQs
 Impoverished group: lower IQs
 Conclude that the difference is probably due to
genetics?
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 Heritability does not tell us about individual
differences.
 Heritability is based on variances in populations.
 High heritability value for a trait does not
automatically mean that most of the difference
between two individuals is genetic.
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Genes, Environment, and the Individual How Do Genes Matter?
 Genes have a strong influence on even complex
traits.
 But, independent assortment of multiple genes with
multiple alleles produces a large number of
phenotypes.
 Environment can also have big effects.
 For quantitative traits, it is difficult to predict the
phenotype of children from the phenotypes of the
parents.
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Quantitative Genetics - Measuring Heritability in
Animals
Artificial selection:
 Only the cow giving the most milk was allowed
to breed
 The next generation has a higher mean milk
production
 Milk production has a high heritability
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Artificial selection:
2.6
Average = 2.6
gallons of milk
Selective breeding
per day
of most productive
cow with a bull
3.2
Average = 3.2
gallons of milk
per day
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Figure 6.19
Genes are segments of DNA that code for ________.
A. proteins
B. centromeres
C. carbohydrates
D. karyotypes
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Prokaryotes typically contain _______.
A. single, circular chromosomes
B. multiple, circular chromosomes
C. multiple, linear chromosomes
D. no chromosomes
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Which of these events does not contribute to unique
combinations of alleles.
A. Mutations
B. Independent assortment
C. Random fertilization
D. Cell cycle checkpoints
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True or False: Monozygotic twins occur when two
separate eggs fuse with different sperm.
True.
False.
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A pea plant has one recessive allele for wrinkled seeds
and one dominant allele for smooth seeds. What
will the pea plant look like?
A. Wrinkle
B. Smooth
C. Half wrinkled, half smooth
D. Not enough information to tell
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Two heterozygotes mate. What are the odds that their
offspring will be homozygous recessive?
A. 100%
B. 75%
C. 50%
D. 25%
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Does nature or nurture play a bigger role in
determining who we are?
A. Nature
B. Nurture
C. They both play a large role
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The Punnett square shown here illustrates the outcome
of a cross between a man who carries a single copy of
the dominant Huntington’s disease allele and an
unaffected woman. What are the odds that
Huntington’s disease will not be passed to this
offspring?
A. 100%
B. 75%
C. 50%
D. 25%
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This Punnett square illustrates the likelihood that a
woman who carries the cystic fibrosis allele would
have a child with cystic fibrosis if the sperm donor
were also a carrier. What are the odds that this
offspring will have cystic fibrosis?
A. 100%
B. 75%
C. 50%
D. 25%
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