Incomplete Dominance
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In some cases, an intermediate phenotype is shown (meaning three phenotypes)
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.
Sickle cell disease is incompletely dominant in humans. AA x aa = Aa (sickle cell trait), where
some blood cells will have abnormal shapes
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.
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This mode of inheritance is called incomplete dominance.
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.
Mendel's laws are not compromised here, he just happened to find in peas examples of
complete dominance only.
Polygenic Traits
Traits controlled by many genes, resulting phenotypes are in a range with a
central average
Each allele intensifies or diminishes the phenotype
Examples: height, skin color, seed color in wheat
(P) AABBCC x aabbcc
(F1) AaBbCc x AaBbCc
(F2) Seven possible phenotypes
A bell curve indicates polygenic inheritance, with a center average and a small
number of extremes
Epistatic Alleles
Indicate the genotypes and phenotypes found in labrador retrievers
Labrador Retriever Genetics
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
Show the crosses
1. A black lab (BBEe) x yellow lab (bbee)
2. A chocolate lab (bbEe) x black lab (BbEe)
3. Two black labs (BBEE x BbEe)
Sex-Linked Genes
Genes located on the X chromosomes (some cause diseases)
The Y is much smaller, contains few genes
Sex Linked Disorders
Color Blindness (red-green)
Hemophilia ("bleeders disease)
Duchenne Muscular Dystrophy (weakening/loss of skeletal muscles)
Crosses Involving Sex - Linked Genes
**remember the Y is wimpy, no genes go there **
Consider the Hemophilia - this disease is caused by an allele on the X
chromosome
Muliple Allele Traits
Traits that are controlled by more than two alleles. Blood type in humans is
controlled by three alleles: A, B, and O
Phenotype
Genotype
A
AA or AO
B
BB or BO
AB
AB only
O
OO only
Examples of Blood type crosses
Blood Transfusions
Blood can only be transferred to a body of a person who's immune system will
"recognize" the blood. A and B are antigens on the blood that will be
recognized. If the antigen is unfamiliar to the body, your body will attack and
destroy the transfused blood as if it were a hostile invader (which can cause
death).
O is like a blank, it has no antigens. O is called the universal donor because a
person can receive a transfusion from O blood without having an immune
response
AB is the universal acceptor, because a person with AB blood has both the A
and B antigens already in the body, A and B blood can be transfused to the
person (as well as O) and the body will recognize it and not attack.
Polygenic Traits
Traits controlled by many genes: hair color, height weight, intelligence
Sex Influenced Traits
Traits are influenced by the environment. Pattern baldness affects men
because testosterone activates the genes.
Environmentally Influenced Traits
Siamese cats have dark ears and feet due to the temperature. Height in
humans is influenced by the environment (diet)