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Genetics:
Complex Inheritance,
Sex Linkage,
X-Inactivation
AP Biology
Unit 3
Incomplete Dominance
• Heterozygous phenotype
is a blend of the 2
homozygous phenotypes
• Ex. Red flower crossed
with white flower 
heterozygous flower is
pink
Epistasis
• When one gene product
affects the expression
of another gene.
• B and b are fur color
alleles
• bb = brown fur
• Fur color will only be
expressed if the C gene
is also present
Polygenic Inheritance
• Two or more genes work
together to create a
single phenotype
• Eye color, skin color are
good examples
• Opposite is pleiotropy
(where one gene affects
several different
phenotypes) ex. PKU
• Nature vs. Nurture
• Virtually all human diseases have some
genetic component
Pedigrees
• Used to trace the
genotypes for a
particular trait in a
family
• Can help determine
the probability that
future offspring will
have a trait.
Sample Pedigrees
Widow’s Peak =
Dominant Trait
W = widow’s peak
• What is the
grandfather’s
genotype?
• Ww
Sample Pedigrees
• Attached earlobes =
Recessive Trait
F = unattached
f = attached
• What is the genotype of
these grandparents?
• Both Ff
Sex Linkage
• When a trait is
carried on the X or
Y chromosomes, it
is called a sexlinked trait
• Don’t confuse this
with linked genes =
when 2 genes are on
the same
chromosome
XY
X
XX
Y
XX
X
XY
Sex-linked genes and Punnett Squares
• You have to include the
X and Y chromosomes
in the Punnett Square
• Superscripts on the X
and Y denote which
allele is present
– XA, Xa
Red-Green Colorblindness
• Gene that controls this (opsin gene) is on
the X chromosome
• Colorblindness is caused by a recessive
allele (mutation in the opsin gene)
• Who is more likely to be color blind– men
or women?
– Men: only 1 X chromosome – if they have
the recessive allele they don’t have another
X to make up for it.
Sex-limited Trait
• Trait whose expression depends on the sex
of the individual
• Not found on the X or Y chromosomeNOT the same thing as sex-linked trait.
• Ex. Milk production in females, pattern
baldness in males (triggered by hormones)
X-inactivation in female mammals
• In females, one of the two X chromosomes
in each cell becomes inactive during
embryonic development
• Why would one X chromosome inactivate
itself in females?
– Cells of females and males would have same
effective dose of genes on the X chromosome
• Inactive X chromosomes are called Barr
bodies
Example: Calico (Tortoiseshell) cats
• animation
Example in humans
• Anhidrotic dysplasia
• X linked mutation prevents the development
of sweat glands
• A woman who is heterozygous will have
patches of normal skin and patches of skin
without sweat glands
• Difficulty controlling body temperature
Practice Problem #5
• A normal (not colorblind male) marries a
woman who is a carrier for the
colorblindness allele
• What are the chances their son will be color
blind?
• What are the chances their daughter will be
colorblind?
Answer #5
• The man is XBY, the woman is XBXb.
• 50% chance that son will be colorblind
• 0% chance that daughter will be colorblind,
but she could be a carrier.
Practice Problem #6
• Is this trait on the X or Y
chromosome?
• Is it dominant or recessive?
• If individual A marries an
unaffected male, what are
the % chances her children
will have the trait?
• If individual B marries an
unaffected woman, what are
the chances their children
will have the trait?
A
B
Answer #6
• Trait must be on the X chromosome (X*).
only men would have it if on Y
chromosome.
• Dominant trait– determine the genotypes of
males  look at their mother’s to help
determine recessive or dominant.
• Individual A must be X*X (since her
mother was unaffected). 50% chance
children will have trait (girls or boys).
• Individual B must be X*Y. All daughters
will have trait, none of sons will.