Download AP Exceptions to Mendel

Exceptions to Mendel
Many Genes Have Multiple Alleles
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A population might have more than two alleles for a given gene.
In labrador retriever, coat color is determined by one gene with four different alleles. Five
different colors result from the combinations of these alleles.
Even if more than two alleles exist in a population, any given individual can have no more
than two of them: one from the mother and one from the father.
Genes That Interact
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More than one pair of genes may interact to produce the phenotype.
A. Epistasis: a gene at one locus alters the phenotypic expression of a gene at a
second locus.
B. The homozygous recessive condition masks the effect of a dominant allele at
another locus.
C. Albino animals inherit allelic pair (aa) preventing production of melanin,
expression of eye, hair, color.
Labrador Retriever Genetics
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Black is dominant to chocolate B or b
Yellow is recessive epistatic (when present, it blocks the expression of the black and
chocolate alleles) E or e
Phenotype
Possible
Genotypes
BBEE
BbEE
BBEe
BbEe
bbEE
bbEe
BBee
Bbee
bbee
Task: Determine the number of chocolate labs produced from a black female and
a yellow male (BbEe x bbee)
See if you can figure out
how the multiple alleles
act together to determine
the coat color of a mouse.
There are three varieties
of coat color: black,
brown, and white are
controlled by four
different alleles (similar to
the labradors)
This is an example of
epistasis!
Page 263 in text!
Polygenic Inheritance
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Polygenic inheritance occurs when a trait is controlled by several allelic pairs at different
loci.
Allelic pairs at different loci on a chromosome or on different chromosomes all control
one trait.
Gene alleles can be contributing or non-contributing.
Contributing alleles have an addictive effect, resulting in quantitative variations.
Examples include seed color in wheat and skin color and height in humans.
Polygenic traits are subject to environmental effects that cause intermediate phenotypes;
so they produce continuous variations whose frequency distribution forms a normal (bellshaped) curve. (page 259 in text)
Codominance
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Both alleles can be expressed
For example, red cows crossed with white will generate roan
cows. Roan refers to cows that have red coats with white
blotches.
This phenotype might seem to support the blending theory.
(The blending theory predicts pink F1 progeny.)
The F2 progeny, however, demonstrate Mendelian genetics.
When the F1 roan individuals self-fertilize, the F2 progeny
have a phenotypic ratio of 1 red:2 roan:1 white.
This mode of inheritance is called incomplete dominance.
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The phenotypic outcomes for cow color and incomplete dominance in general can be
explained biochemically.
One allele of the gene codes for an enzyme that functions in the production of the red
color. The other allele codes for the gene to make white color. If both alleles are present,
both are expressed, resulting in a cow that has some red and some white.
MendelÕs laws are not compromised here, he just happened to find in peas examples of
complete dominance only.
Incomplete Dominance
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In some cases, an
intermediate phenotype is
shown
Neither allele is dominant
In snapdragons, flower
color can be red, pink, or
white. The heterozygous
condition results in pink
flowers (or an
intermediate trait)
A white snapdragon
crossed with a red
snapdragon produces all
pink offspring
Two pinks crossed
together produce 1/4
white, 2/4 pink, and 1/4
red
When dealing with
incomplete dominance and codominance it does not matter what letter you use, as long
as the heterozygous condition always denotes the intermediate trait. In the diagram R is
used, but you could also use W or even P. Ww = pink, Pp = pink if these letters are used.
Polygenic Traits
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Polygenes mediate quantitative inheritance
Individual heritable characters are often found to be controlled by groups of several genes,
called polygenes.
Each allele intensifies or diminishes the phenotype.
Variation is continuous or quantitative (Òadding upÓ) - also called quantitative
inheritance
Pleiotropy
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Some single alleles have more than one distinguishable phenotypic effect - This is called
pleiotropy.
An example is the coloration pattern and crossed eyes of Siamese cats, which are both
caused by the same allele. These unrelated characters are caused by the same protein
produced by the same allele.
Another example is the gene that causes pigment color in rats. White rats also have very
sensitive eyes and often become blind.
Sex Linked Genes
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Some genes are located on the X chromosome. Females receive two alleles for these genes,
but males only receive one.
In humans, hemophilia and color blindness are sex-linked traits.
When doing a punnet square, use large X's and Y's to denote male and female, use
superscript letters to designate the alleles.
If the parent is a male, the genotype is automatically known. A colorblind male has to be
b, since he only has one allele and colorblindness is recessive. A normal male must then be
B
Females can be heterozygous for the colorblindness trait - they are called carriers. A female
can be BB - normal, Bb - carrier, or bb - colorblind
The following shows a cross between a normal man and a woman who is a carrier.
Task: Determine the probability of a woman with
hemophilia having children with hemophilia assuming
she marries a normal man.
Lethal Genes
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Some genes are lethal when both alleles are present.
Lethality can occur before or after birth
An example is the "creeper" allele in chickens, which
causes the legs to be short and stunted.
Creeper is a dominant gene, heteroyzous chickens
display the creeper phenotype
If two creeper chickens are crossed, one would expect
to have (from mendelian genetics) 3/4 of the offspring
to be creeper and 1/4 to be normal
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Instead the ratio obtained is 2/3 creeper and 1/3 normal.
This occurs because homozygous creeper chickens die.
Blood Types - Multiple Alleles and Codominance
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In humans, there are four blood types
(phenotypes): A, B, AB, and O
Blood type is controlled by three alleles. A, B,
O
O is recessive, two O alleles must be present
for the person to have type O blood
A and B are codominant. If a person receives
an A allele and a B allele, their blood type is
type AB
Crosses involving blood type often use an I to
denote the alleles - see chart.
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When doing blood type crosses, you will need
to know whether at type A or B person is
heterozygous or homozygous. Type O's are
automatically OO and type AB is automatically
AB. Crosses are performed the same as any
other.
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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 bodies, their immune system reacts
as if it were a foreign invader, the antibodies clump the blood - can cause death.
Type AB blood has no antibodies, any blood can be donated to them - they are called the
"universal acceptors"
Type O blood has no surface markers on it, antibodies in the blood do not react to type
O blood, they are called the "universal donors"
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