Download Chapter 6: Cancer - Mendelian and Quantitative Genetics

Chapter 6
 Are You Only As Smart As Your Genes?
 Mendelian and Quantitative Genetics
Copyright © 2010 Pearson Education, Inc.
6.1 The Inheritance of Traits
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?
 Nature Versus Nurture
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6.1 The Inheritance of Traits
The human life cycle:
 Adults produce gametes in their gonads by
meiosis.
 Sperm cells fertilize egg cells to form singlecelled zygotes.
 Repeated cell divisions form the embryo.
Egg
Mother’s egg and father’s
sperm each contain half of
the information to “build a
human.”
This single cell contains
all the information on
“how to build a human.”
Meiosis
Fertilization
Zygote
Mitosis
and
differentiation
Sperm
Adult
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Gametes
Single-celled embryo
Body axis
establishment,
tissue
differentiation,
organ system
formation
Multicellular embryo
Figure 6.1
6.1 The Inheritance of Traits
The human life cycle, cont.:
 The embryo grow to become a fetus.
 After birth, the individual continues to grow
until reaching adulthood.
Birth
Mitosis
and
differentiation
Fetus
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Mitosis
and
differentiation
Baby
Mitosis
and
differentiation
Child
Adult
Figure 6.1
6.1 The Inheritance of Traits
 Genes are segments of DNA that code for
proteins.
 Analogous to words in an instruction manual
for building a human
build
Genes
expressed in
strong heart muscle
muscle cell
build grow long dark blood brown strong hair for eyes heart small red muscle
build
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dark
brown
eyes
Genes expressed
in eye cell
Figure 6.3
6.1 The Inheritance of Traits
 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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6.1 The Inheritance of Traits - Producing
Diversity in Offspring
 Mistakes in copying DNA (mutations)
produce different versions of genes
(alleles), with different results.
Mutation
Mutation
Normal allele:
grey
strong
nerve
Mutant allele:
gray
string
nzrve
(a) The mutant allele has the same
meaning (mutant allele function
the same as the original allele).
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(b) The mutant allele has a different
meaning (mutant allele functions
differently than the original allele).
Mutation
(c) The mutant allele has no
meaning (mutant allele is
no longer functional).
Figure 6.4
6.1 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
Egg
Sperm
+
The 23 pages of each instruction manual
are roughly equivalent to the 23
chromosomes in each egg and sperm.
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Zygote
=
The zygote has 46
pages, equivalent to 46
chromosomes.
Figure 6.5
6.1 The Inheritance of Traits - Producing
Diversity in Offspring
Meiosis creates variation in offspring
 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
 Due to random alignment during
metaphase I
 About 8 million different combinations of
chromosomes.
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6.1 The Inheritance of Traits - Producing
Diversity in Offspring
 Siblings share
50% of alleles
with each other,
on average
Parent cells have 2 copies of each chromosome—that is, 2 full sets of
instruction manual pages, 1 from each parent.
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
6.1 The Inheritance of Traits - Producing
Diversity in Offspring
 Random
fertilization
produces more
diversity: 64
trillion
possibilities!
 No two humans
are genetically
identical, except
for monozygotic
twins.
(a) Dizygotic (fraternal) twins
Egg
Sperm
Egg
(b) Monozygotic (identical) twins
Sperm
Egg
Zygote
Zygote
Zygote
Embryo
Embryo
Embryo
Sperm
Embryo
splits
Two
embryos
50% identical (no more
similar than siblings born at
different times)
100% genetically identical
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Figure 6.7
6.2 Mendelian Genetics: When the Role of
Genes Is Clear
Gregor Mendel: first to
accurately describe
rules of inheritance for
simple traits
 Controlled mating
between pea plants
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6.2 Mendelian Genetics: When the Role of
Genes Is Clear
Gregor Mendel
 Studied traits due to a
single gene with a few
alleles
 Discovered that both
parents contribute
equally to offspring
(genetically)
 Mendel’s principles
also apply to many
genetic diseases in
humans
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6.2 Mendelian Genetics: When the Role of
Genes Is Clear
 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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6.2 Mendelian Genetics - Genetic Diseases in
Humans
Genetic Diseases in Humans
 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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6.2 Mendelian Genetics - Genetic Diseases in
Humans
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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6.2 Mendelian Genetics - Using Punnett
Squares to Predict Offspring Genotypes
Using Punnett Squares to Predict
Offspring Genotypes
 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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6.2 Mendelian Genetics - Using Punnett
Squares to Predict Offspring Genotypes
Possible types of eggs
Ff
Female
carrier Ff
Possible types of sperm
Sperm
sample
Ff
F
FF
Ff
f
Ff
ff
25% chance that a child will
not have cystic fibrosis
50% chance that a child will
be an unaffected carrier of
the cystic fibrosis allele
25% chance that a child will
have cystic fibrosis
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Figure 6.13
6.3 Quantitative Genetics: When Genes and
Environment Interact
Qualitative traits are on or off traits
- such as wrinkled or smooth
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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6.3 Quantitative Genetics: When Genes and
Environment Interact
Distribution of Phenotypes in Population
 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
6.3 Quantitative Genetics: When Genes and
Environment Interact
 Variance: a measure of how much
variability there is in the population
 The amount an individual varies from the
mean, on average
Number of
14-year-old boys
Number of
jockeys
(b) Variance describes the variability around the
mean.
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Low variance
Mean = 114 lbs
(51.7 kg)
High variance
Weight (lbs)
Figure 6.16b
6.3 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
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6.3 Quantitative Genetics
Why Traits Are Quantitative
 Usually
influenced by
both genes and
environment
 Monozygotic
twins, genetically
identical, but
different
environments
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Figure 6.17
6.3 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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6.3 Quantitative Genetics –
 When artificial
selection is
impossible,
correlations between
relatives estimates
heritability.
Blue tit chick immune response
Measuring Heritability in Animals
Points represent parent-offspring
pairs with matching immunity levels.
Weak Average Strong
On average,
parents and
offspring had
same level of
immunity.
Blue tit parent immune response
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Figure 6.20
6.3 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
 Dizygotic twins share environment, but only half their
genes
 Heritability of IQ from such twin studies estimated to
be about 0.52
 Similar treatment of twins might explain why their
IQs are so similar
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6.3 Quantitative Genetics –
Calculating Heritability in Human Populations
Another approach:
 Monozygotic twins raised apart share all
genes
 Estimates of IQ heritability for such twins is
0.72
 Drawback: limited number of such twins to
study
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6.4 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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6.4 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
Average weight of the mice in the
rich- diet environment is twice the
average weight of the population in
the poor- diet environment.
However, there is no genetic
difference between the two groups.
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1 Start with a population of
mice that are variable in size.
2 Randomly divide mice
into two groups. Feed
half a poor diet and the
other half a rich diet.
3 Allow the mice in both
groups to breed.
Measure the weight of
adult offspring.
Figure 6.22
6.4 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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6.4 Genes, Environment, and the Individual –
The Use and Misuse of Heritability
 A highly heritable trait can still respond to
environmental change.
 Maze-learning ability is highly heritable in
rats.
 Bright rats have bright offspring
 Dull rats have dull offspring
 Still, no rats learned well in a restricted
environment.
 All rats learned better in an enriched
environment
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6.4 Genes, Environment, and the Individual –
The Use and Misuse of Heritability
 Heritability does not tell us about individual
differences
 Heritability is based on variances in
populations, not individuals
 High heritability value for a trait does not
automatically mean that most of the
difference between two individuals is
genetic.
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6.4 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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