Beyond Mendel: Exceptions/Additions to Mendel’s

Laws

AP Biology

Exceptions to Mendelian Genetics

Mendel chose traits in peas that showed 2 distinct forms.

Not all genes exhibit such simple inheritance.– 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– 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

Black is dominant to chocolate – B: black– b: chocolate

Yellow is recessive epistatic (when present, it blocks the expression of the black and chocolate alleles) – Yellow: E or e– * must be ee to produce a yellow lab

BBEE BbEE BBEe BbEe

bbEEbbEe

BBeeBbeebbee

Possible Genotypes

Problem #1

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)– BbCc x BbCc – B=black, bb = brown– cc is epistatic (white)

Problem #2

Determine the number of chocolate labs produced from a black female and a yellow male – BbEe x bbee

Codominance

Both alleles can be expressed

For example, red cows crossed with white will generate roan cows.

– Roan cows have red coats with white blotches.

Incomplete Dominance

In some cases, an intermediate phenotype is shown.

Neither allele is dominant. Ex. Snapdragons – flower color

– 3 phenotypes: red, white, pink– Heterozygous condition results in pink flowers

(the intermediate trait).

Incomplete Dominance - Snapdragons

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

Genes that are located on the X chromosome.– 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. – 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

Some single alleles have more than one distinguishable phenotypic effect.– This is called pleiotropy.

Ex. Coloration pattern and crossed eyes of Siamese cats – 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. – Parts of the body that are

colder will develop the darker pigmentation - ears, feet tail of the siamese cats

Pleiotropy

Another example is the gene that causes pigment color in rats.

White rats also have very sensitive eyes and often become blind.

Pleiotropy

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

Individual heritable characters are often controlled by groups of several genes.– These genes are called polygenes.

Each allele intensifies or diminishes the phenotype.

Variation is continuous or quantitative (adding up) – Also called quantitative inheritance

Polygenic Traits

Examples:– Human hair, eye, and

skin color– Height – Weight– Intelligence

Lethal Genes

Some genes are lethal when both alleles are present.

Lethality (death) can occur before or after birth.

Lethal Genes

Ex. The “creeper” allele in chickens, which causes the legs to be short and stunted.

Creeper is a dominant gene.– 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

Mexican hairless dogs result from a mutation in a gene that shows lethality.

– hh hairy normal trait– Hh hairless one mutation present– HH lethal two mutations = lethal

Manx Cats

Cats possess a gene for producing a tail.– 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

4 blood types (humans) – A, B, AB, and O

Blood type is controlled by 3 alleles– 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

The blood type determines what antibodies are located within the blood.– 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