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Chapter 7: Mendelian Inheritance
Family resemblance: how traits are inherited
Lectures by Mark Manteuffel, St. Louis Community College
7.1 Family resemblance: your mother
and father contribute equally to your
genetic makeup.
Selective Breeding: Observing Heredity
Traits that are
determined by the
instructions a person
carries at one gene are
called single-gene
traits.
More than 9,000
single-gene traits have
been identified in
humans.
7.3 Mendel learned about heredity
by conducting experiments.
It’s easy to control fertilization in
plants.
True-Breeding
True-breeding = offspring always have the
same trait as their parents. Traits were
easily observed.
Homologous chromosomes have alternative alleles.
Homologous chromosomes may bear either
the same alleles or different ones at a given
locus.
• Homozygous for that gene…
– When an organism has identical alleles for a
gene. (Ex. PP or pp)
• Heterozygous for that gene…
– When an organism has different alleles for a
gene. (Ex. Pp)
A
dominant trait
masks the effect
of a recessive
trait.
 Dominant traits
determine the
phenotype.
Three Ideas Mendel Used for Explaining
This Pattern of Inheritance
1) Each parent puts into every sperm or egg it
makes a single set of instructions for building
the trait.
2) Offspring thus find themselves with two copies
of the instructions for any trait (called alleles).
3) The actual trait produced by an individual
depends on the two copies of the gene that
they inherit from their parents.
• homozygous and heterozygous
7.4 The
Principle of
Segregation:
you’ve got two
copies of each
gene but put
only one copy
in each sperm
or egg.
Mendel’s Test: a
monohybrid
cross…
7.5 Observing an individual’s phenotype
is not sufficient for determining its
genotype.
• Genotype
– An organism’s genetic makeup – designated by alleles
(alternative forms of genes on homologous
chromosomes)
• Phenotype
– An organism’s physical traits – appearances, may also
manifest in behaviors.
• Assign letters to represent alleles…
– Capital letters = dominant traits – determine
phenotype
– Lower-case letters = recessive traits – may be hidden
It is not always possible to determine an individual’s
genotype from its phenotype.
7.7 A testcross enables
us to figure
out which
alleles an
individual
carries.
You would like to produce white alligators via a
mating program.
 The problem is that you cannot be certain of the
genotype of your alligators.

They might be homozygous dominant, MM, or they
might be heterozygous, Mm.
 In either case their phenotype is normal coloration.
 How can you figure out which of these two
possibilities is the actual genotype?

In a test-cross, an
individual with a
dominant phenotype
and an unknown
genotype is mated
with a homozygous
recessive individual.
The phenotypes of
the offspring reveal
the unknown
genotype.
7.8 Using pedigrees to
decipher and predict the
inheritance patterns of genes.
Pedigree: a type of family tree
Analyzing Which Individuals Manifest
the Trait and Which Do Not
Probability
has
central role in
for
7.6
Chance
isaimportant
ingenetics
genetics.
two reasons:
1.
The first is a consequence of segregation.
 In segregation, each gamete receives only one of
the two copies of each gene.
 It is impossible to know which allele goes into which
gamete.
2.
The second reason is that fertilization, too, is a chance
event.
 All of an individual’s sperm or eggs are different.
 Any of these gametes may be the gamete
involved in fertilization.
Punnett
Squares
are very
useful in
predicting
probability.
7.9 Incomplete dominance and
codominance: the effects of both alleles
in a genotype can show up in the
phenotype.
Incomplete dominance — a heterozygote
displays a characteristic somewhere between
the characteristics (a blended version of the
trait) of the two homozygotes.
Codominance — a heterozygote displays
characteristics of both homozygotes equally.
Incomplete dominance, in which the
heterozygote appears to be intermediate between
the two homozygotes.
• Hypercholesterolemia
– Is a human trait that is incompletely dominant
LDL (carries
cholesterol)
LDL receptor
(mops up LDL)
Cell
Normal
HH
Mild disease
Hh
Heterozygous
Severe disease
hh
Figure 9.19
Codominance, in which the heterozygote
displays characteristics of both homozygotes.
Both phenotypes are expressed
equally in the heterozygote.
7.10 What’s your blood type?
Some genes may have more
than two alleles.
Multiple allelism, in which a single gene
has more than two alleles. Each
individual still carries only two alleles.
Inheritance of the ABO Blood
Groups

A, B, and O alleles

The A and B alleles are both completely
dominant to O.

The A and B alleles are codominant to each
other.

Individuals can be one of four different blood
types: A, B, AB, and O.
Why are people with type O blood considered
“universal donors”? Why are those with type AB
considered “universal acceptors”?
7.11 Multi-gene Traits
How are continuously varying traits
such as height influenced by genes?
• Polygenic inheritance is
the additive effect of
two or more genes on a
single phenotype.
aabbcc
(very light)
AABBCC
(very dark)
AaBbCc
Eggs
Sperm
• This hypothetical
example shows a range
of possible skin colors
in the F2 Generation
based on just 3 alleles.
Figure 9.22
7.12 Pleiotropy: How can one gene
influence multiple traits?
What is the benefit of “almost” having sickle cell
disease?
7.13 Why are more men
than women color-blind?
Sex-linked traits differ in their
patterns of expression in males
and females.
If a man is color-blind, did he inherit
this condition from his mother, his
father, or both parents?
Hemophilia is another
sex-linked (X-linked)
trait. Women are
often carriers.
7.14 Environmental effects:
identical twins are not
identical.
Genotypes are not like blueprints that
specify phenotypes.
Many phenotypes are a product of the
genotype in combination with the
environment.
Could you
create a
temporarily
spotted
Siamese
cat with an
ice pack?
The Role of Environment
• Many human characteristics result from a
combination of heredity and environment.
– Eye color appears to be entirely genetic.
– The height of an individual is partially genetic,
but can also be influenced by health and diet
during childhood and adolescence.
– Often characteristics such as susceptibility to
heart disease, cancer, alcoholism, and
schizophrenia are influenced by both genes
and environment.
The inheritance pattern of one trait doesn’t
usually influence the inheritance of any
other trait.
7.15 Most traits are passed on as
independent features: Mendel’s law of
independent assortment.
Mendel tested this using a
dihybrid cross.
7.16 Red Hair and Freckles
Genes on the same
chromosome are
sometimes
inherited together.
The alleles for two genes
may be inherited and
expressed together when
they are close together on the
same chromosome.