Patterns of Intermediate Inheritance Exceptions to Mendel’s Principles
Beyond Mendel: Exceptions/Additions to Mendel’s Laws
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Transcript of Beyond Mendel: Exceptions/Additions to Mendel’s Laws
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