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Chapter 5
Are You Only as Smart as Your Genes?
Mendelian and Quantitative Genetics
Fourth Edition
BIOLOGY
Science for Life | with Physiology
Colleen Belk • Virginia Borden Maier
© 2013 Pearson Education, Inc.
Copyright © 2009 Pearson Education, Inc.
PowerPoint Lecture prepared by
Jill Feinstein
Richland Community College
1 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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1 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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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.
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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
 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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1 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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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
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1 The Inheritance of Traits - Producing Diversity 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
 This is due to random alignment during metaphase I
 About 8 million different combinations of
chromosomes.
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1 The Inheritance of Traits - Producing Diversity in
Offspring
 Due to independent assortment, the instructions in
one sperm cell is an unique combination of pages.
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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.
 Dizygotic twins are 50%
identical just like siblings born
at different times.
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2 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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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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2 Mendelian Genetics - 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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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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2 Mendelian Genetics - 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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2 Mendelian Genetics - Using Punnett Squares to
Predict Offspring Genotypes
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Animation: Mendel’s Experiments
Click “Go to Animation” / Click “Play”
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2 Mendelian Genetics - Using Punnett Squares to
Predict Offspring Genotypes
 Dihybrid crosses are crosses that involve two traits.
 The first step in a dihybrid is to determine the
possible gametes.
 Yellow (Y) is dominant to green (y) and Round (R) is
dominant to wrinkled (r).
 If you cross YyRr x YyRr,
 Possible gametes for parent 1 are YR, Yr, yR, yr
 Possible gametes for parent 2 are YR, Yr, yR, yr
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2 Mendelian Genetics - Using Punnett Squares to
Predict Offspring Genotypes
 The results of the cross results in a 9:3:3:1
phenotypic ratio.
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3 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
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3 Quantitative Genetics: When Genes and
Environment Interact
 Mean: sum up all the phenotypic values and divide
by the number of individuals; same as the average.
 Variance: a measure of how much variability there
is in the population
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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
 Heritability: proportion of the variation within a
population due to genetic differences among
individuals
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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
 Monozygotic twins share all their genes and their
environment
 Dizygotic twins share environment, but only half their
genes
 Heritability of IQ from such twin studies estimated to be
about 0.52
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3 Quantitative Genetics - Calculating Heritability
in Human Populations
 Twins share a more similar environment than most
humans
 Similar treatment of twins might explain why their
IQs are so similar
 Monozygotic twins raised apart share all genes but
are treated like everyone else
 Estimates of IQ heritability for such twins is 0.72
 Drawback: limited number of such twins to study
 Table 7.2 discusses IQ heritability
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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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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.
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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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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.
© 2013 Pearson Education, Inc.
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.
 High heritability value for a trait does not
automatically mean that most of the difference
between two individuals is genetic.
© 2013 Pearson Education, Inc.
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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Genes are segments of DNA that code
for ________.

proteins

centromeres

carbohydrates

karyotypes
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Genes are segments of DNA that code
for ________.

proteins

centromeres

carbohydrates

karyotypes
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Genes are comparable to________.

words in an instruction manual

pages in an instruction manual

copy of pages in an instruction manual

appendix in an instruction manual
© 2013 Pearson Education, Inc.
Genes are comparable to________.

words in an instruction manual

pages in an instruction manual

copy of pages in an instruction manual

appendix in an instruction manual
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Which of these events does not contribute to
unique combinations of alleles?

mutations

independent assortment

random fertilization

cell cycle checkpoints
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Which of these events does not contribute to
unique combinations of alleles?

mutations

independent assortment

random fertilization

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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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?

wrinkle

smooth

half wrinkled, half smooth

not enough information to tell
© 2013 Pearson Education, Inc.
A pea plant has one recessive allele for wrinkled
seeds and one dominant allele
for smooth seeds. What will the pea plant look
like?

wrinkle

smooth

half wrinkled, half smooth

not enough information to tell
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Two heterozygotes mate. What are the odds that
their offspring will be homozygous recessive?

100%

75%

50%

25%
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Two heterozygotes mate. What are the odds that
their offspring will be homozygous recessive?

100%

75%

50%

25%
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Which trait is a quantitative trait that does not
show continuous variation?

height

skin color

presence or absence of a widow’s peak

intelligence
© 2013 Pearson Education, Inc.
Which trait is a quantitative trait that does not
show continuous variation?

height

skin color

presence or absence of a widow’s peak

intelligence
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Does nature or nurture play a bigger role in
determining who we are?

nature

nurture

they both play a large role
© 2013 Pearson Education, Inc.
Does nature or nurture play a bigger role in
determining who we are?

nature

nurture

they both play a large role
© 2013 Pearson Education, Inc.
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?

100%

75%

50%

25%
© 2013 Pearson Education, Inc.
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?

100%

75%

50%

25%
© 2013 Pearson Education, Inc.
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?

100%

75%

50%

25%
© 2013 Pearson Education, Inc.
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?

100%

75%

50%

25%
© 2013 Pearson Education, Inc.