Beyond Mendel:
Exceptions/Additions to Mendel’s
Laws
AP Biology
Exceptions to Mendelian Genetics
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Mendel chose traits in peas that showed 2
distinct forms.
Not all genes exhibit such simple inheritance.
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Alleles interact
Genes interact
Segregation of genes on same chromosome
Mitochondrial DNA
Multiple Alleles
A population can have more than 2
alleles for a gene.
 Ex. Labrador Retriever coat color
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Determined by 1 gene with 4 alleles.
Even if more than 2 alleles exist in a
population, any given individual can only
have 2 of them
 (1 from mother, 1 from father)
Multiple Alleles
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Black is dominant to chocolate
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B: black
b: chocolate
Yellow is recessive epistatic (when present,
it blocks the expression of the black and
chocolate alleles)
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Yellow: E or e
* must be ee to produce a yellow lab
BBEE
BbEE
BBEe
BbEe
bbEE
bbEe
BBee
Bbee
bbee
Possible
Genotypes
Problem #1
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How do the multiple alleles act together to
determine the coat color of a mouse?
There are 3 varieties of coat color: black,
brown, and white
They are controlled by 4 different alleles
(similar to the Labradors)
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BbCc x BbCc
B=black, bb = brown
cc is epistatic (white)
Problem #2
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Determine the number of chocolate labs
produced from a black female and a yellow
male
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BbEe x bbee
Codominance
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Both alleles can be
expressed
For example, red
cows crossed with
white will generate
roan cows.
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Roan cows have red
coats with white
blotches.
Incomplete Dominance
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In some cases, an intermediate phenotype is
shown.
Neither allele is dominant.
Ex. Snapdragons – flower color
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3 phenotypes: red, white, pink
Heterozygous condition results in pink flowers
(the intermediate trait).
Incomplete Dominance - Snapdragons
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A white (rr) snapdragon crossed with a red
(RR) snapdragon produces all pink (Rr)
offspring.
Two pinks crossed together (Rr x Rr)
produce 1/4 white, 2/4 pink, 1/4 red
Sex-Linked Genes
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Genes that are located on the X
chromosome.
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Females receive 2 alleles; males receive one.
Ex. Color blindness, hemophilia
Women can be carriers when they carry one
gene for the disorder and one normal gene.
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Carriers can have sons with the disorder.
Normal Male and Female Carrier
Problem #3

Determine the probability of a woman with
hemophilia having children with hemophilia
assuming she marries a normal man.
Pleiotropy
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Some single alleles have more than one
distinguishable phenotypic effect.
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This is called pleiotropy.
Ex. Coloration pattern and crossed eyes of
Siamese cats
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Both caused by the same allele.
Unrelated characters caused by the same protein
produced by the same allele.
Siamese Cat

Siamese cats have a gene
that codes for darker
pigments - this gene is
more active at low
temperatures.
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Parts of the body that are
colder will develop the
darker pigmentation - ears,
feet tail of the siamese cats
Pleiotropy
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Another example
is the gene that
causes pigment
color in rats.
White rats also
have very
sensitive eyes and
often become
blind.
Pleiotropy
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Another example is
Marfan Syndrome.
Marfan Syndrome is a
disease of the
connective tissue.
Symptoms: tall & thin,
long extremities,
deficiencies in eyes and
skeletal system,
enlarged heart
Polygenic Traits
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Individual heritable characters are often
controlled by groups of several genes.
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These genes are called polygenes.
Each allele intensifies or diminishes the
phenotype.
Variation is continuous or quantitative
(adding up)
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Also called quantitative inheritance
Polygenic Traits
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Examples:
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Human hair, eye, and
skin color
Height
Weight
Intelligence
Lethal Genes
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Some genes are lethal when both alleles are
present.
Lethality (death) can occur before or after
birth.
Lethal Genes
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Ex. The “creeper” allele in chickens, which
causes the legs to be short and stunted.
Creeper is a dominant gene.
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Heterozygous chickens display the creeper
phenotype.
If 2 creeper chickens are crossed, one would
expect to have ¾ creeper and ¼ normal
Instead, the ratio is 2/3 creeper and 1/3 normal.
Lethal Genes – Creeper Chickens
Lethal Genes
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Mexican hairless dogs result from a
mutation in a gene that shows lethality.
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hh
Hh
HH
hairy
hairless
lethal
normal trait
one mutation present
two mutations = lethal
Manx Cats
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Cats possess a gene for producing a
tail.
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The tailless Manx phenotype in cats is
produced by an allele that is lethal in its
homozygous state.
The allele interferes with normal spinal
development, in heterozygous cats this
results in lack of a tail.
Blood Types – Multiple Alleles and
Codominance
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4 blood types (humans)
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Blood type is controlled by 3 alleles
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A, B, AB, and O
A, B, O
O is recessive (must have two O alleles to have
blood type O)
A and B are codominant (if inherit an A and B, blood
type is AB)
Crosses involving blood type often use an I to
represent the alleles
Blood Types
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The blood type determines what antibodies are
located within the blood.
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Type A blood has type B antibodies
If Type B blood is put into their body, their immune
system reacts and antibodies clump the blood – can
cause death
Type AB blood has no antibodies, any blood can be
donated to them; they are “universal acceptors”
Type O blood has no antigens, antibodies in the
blood do not react to type O blood, they are
“universal donors”
Blood Type Cross