Heredity
Chapter 12, part 2
Beyond Mendel’s Laws
• Not all traits are controlled by single genes with
dominant and recessive alleles. Other patterns
of heredity involve:
• Incomplete dominance
• Sex-linked traits
• Multiple alleles
• Multiple genes (most traits involve this)
• Chromosomal abnormalities
Incomplete Dominance
• In incomplete dominance and codominance, both alleles are expressed
in the phenotype.
• incomplete dominance: two traits
appear to blend in the heterozygotes.
• co-dominance: both traits appear in
the heterozygotes.
These snapdragons
have two alleles
controlling flower
color: R (red) and R’
(white).
Heterozygotes (RR’)
have pink flowers.
Unlike the case in
complete
dominance, the
phenotypes show us
which plants are
heterozygous.
mother
C1 C2
C1
father
C1 C2
eggs
C2
C1
sperm
In humans, a gene
affecting hair
texture (curly,
wavy, straight)
shows incomplete
dominance.
C1C1
C1C2
C1C2
C2C2
C2
The golden palomino
horse is a cross
between a white and
a brown horse. This
is another example
of incomplete
dominance: the
colors appear to
blend in the horse’s
hairs.
The red roan horse has both white and redbrown hairs, while the blue roan has both
white and gray hairs. The coat colors of
both parents are expressed in the hairs.
This is co-dominance.
At the gene level, incomplete
dominance and codominance are the same: in
both cases, both alleles are
expressed in the
heterozygote. The only
difference is at the
phenotype level.
Solving single-gene (monohybrid) crosses with
incomplete dominance.
One hair color in cattle is
controlled by a gene that
produces red (R) or white
(R’) hairs. Heterozygotes
(RR’) are roan.
a. What color would the
offspring of a red bull and a
white cow be?
b. What are the phenotypic
ratios of a cross between a
white cow and a roan bull?
Solving single-gene (monohybrid) crosses with
incomplete dominance.
One hair color in cattle is
controlled by a gene that
produces red (R) or white
(R’) hairs. Heterozygotes
(RR’) are roan.
a. What color would the
offspring of a red bull and a
white cow be?
b. What are the phenotypic
ratios of a cross between a
white cow and a roan bull?
Sex-Linked Traits
• In humans, women have two X chromosomes
while men have an X and a Y.
• Traits that are carried on the sex
chromosomes will show different genotypic
and phenotypic rations in men and women.
• The X chromosome has many genes, while
the Y has only a few, so there are many more
X-linked traits than Y-linked traits.
Women pass their X
chromosomes to their
children. Men can contribute
either an X or a Y.
Which parent determines
the sex of the child?
Mother or father?
Can men be carriers of a
recessive X-linked trait?
If a boy has an X-linked trait,
which parent did he inherit
the trait from?
female parent
eggs
X1
male parent
Xm
Y
X1 Xm
sperm
When determining the
outcome of a cross
that involves an Xlinked trait, we have to
take into account how
the two sex
chromosomes are
distributed in the
offspring.
Xm
Y
X2
X2 Xm
female offspring
X1
Y
X2
Y
This diagram
illustrates a cross for
an X-linked trait in
fruit flies. Red eyes
are dominant, white
are recessive.
R
female parent
R
XR Xr
r
XR Xr
R
r
eggs
XR
R
Xr
R
r
R
sperm
male parent
XR Y
XR Y
XR
Y
XR XR
female offspring
R
all the F2
females
have red eyes
Xr XR
r
half the F2
males have
red eyes, half
have white eyes
Red-green color blindness is an X-linked trait.
Charts such as these are used to diagnose red-green
color blindness. They look very different for those with
normal vision and those with RG-color blindness.
Normal
RG color-blind
A
29
70
B
45
nothing
C
abstract
5
D
26
nothing
Solving X-linked crosses.
Red-green color-blindness is
X-linked. If a man and
woman with normal vision
have a color-blind son:
a. What are the genotypes
of the parents?
b. What are the odds of
having another color-blind
son?
c. What are the odds of
having a color-blind
daughter?
Solving X-linked crosses.
Red-green color-blindness is
X-linked. If a man and
woman with normal vision
have a color-blind son:
a. What are the genotypes
of the parents?
b. What are the odds of
having another color-blind
son?
c. What are the odds of
having a color-blind
daughter?
Multiple Alleles
• Human blood type (A, B, AB, and O) is
determined by a gene that has three
alleles.
• A and B are co-dominant
• O is recessive to both
• Though there are three alleles, each
person still has only two copies of the
ABO gene.
A and B alleles produce A and B proteins on the
surface of red blood cells. The O allele produces
neither of these proteins.
Multiple Genes
• Most human traits are the result of multiple
genes.
• In some cases (such as skin and hair color),
there are multiple copies of the same gene
(such as the melanin gene).
• In many others, there are many different
genes controlling a trait, and the environment
may affect how a trait is expressed.
(Example: human height)
Human skin color is
controlled by at least
three melaninproducing genes,
which are
incompletely
dominant.
sperm
This massive Punnet
square shows a cross
between two people
who are heterozygous
for all three genes.
eggs
Chromosomal Abnormalities
• Chromosomal abnormalities include:
• nondisjunctions (failure of chromatids
to separate during meiosis)
• deletions of parts of chromosomes
• Most chromosomal abnormalities cause
cell death, but a few are survivable.
Cri-du-chat syndrome is caused
by a deletion of a large segment
of chromosome 5.
Cri-du-chat children often have
small head circumference and
are severely cognitively
challenged. Some may have
heart defects, muscular or
skeletal problems, or vision
problems.
Trisomy 21 results in Down
Syndrome.
Using what you know about
meiosis, explain how a
fertilized human egg cell can
end up with three copies of
chromosome 21.
If a person with Down
Syndrome planned to have a
child, could the child inherit
Down Syndrome?
Nondisjunction of the sex chromosomes is more
often survivable than nondisjunctions of somatic
chromosomes. As long as the fetus has at least one
X chromosome, it can survive.
Klinefelter syndrome
produces an XXY male.
At puberty, Klinefelter
males fail to fully develop
secondary sex
characteristics. Men with
this syndrome may or may
not be sterile. They have a
tendency to gain weight
easily and their muscle
mass is underdeveloped,
but mental function is
usually normal.
Turner syndrome occurs when a
girl inherits only one X
chromosome.
Turner children are often short,
and may show swelling in the
hands and feet. Some have
heart defects, but most are
cognitively normal. Hormone
therapy at puberty can help
Turner girls grow to normal
height and develop secondary
sex characteristics.
Recap
• Traits inherited by classic Mendelian
genetics are by far in the minority.
• Incomplete dominance, sex linkage, and
multiple alleles involve single-gene traits
that show unique patterns of inheritance.
• Most traits involve multiple genes and
gene-environment interactions.