mendelian exceptions
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Mendelian Exceptions1.31.13
Mendelian Exceptions
Mendel got lucky – all 7 traits he studied showed complete (simple) dominance.
Mendelian Exceptions
Mendel got lucky – all 7 traits he studied showed complete (simple) dominance. One allele is completely dominant over the
other allele Homozygous dominant and heterozygous =
same phenotype 1 allele is enough for full expression of
dominant trait
Incomplete Dominance = the norm
Heterozygote = intermediate phenotype
1 allele is not enough for full expression
Incomplete Dominance
Ex: flower color R = red RR x rr r = white
Incomplete Dominance
F2: Rr x Rr
Incomplete Dominance
F2: Rr x Rr
Alleles not blended – still able to separate.
Rr Rr
Rr Rr
1 red2 pink1 white
Codominance – Both alleles are expressed equally
Codominance – Both alleles are expressed equally
Ex: Cows B = black W = white
BB = Black WW = White BW = ???
Codominance – Both alleles are expressed equally
Ex: Cows B = black W = white
BB = Black WW = White BW = Black and white
Other examples: calico cats, zebras, streaked flowers…
Multiple alleles
Allele: Alternate forms of a gene
Only 2 possible alleles in an individual, BUT any # of alleles may be present in a population due to mutations
Blood Types (ABO blood types)
Example of simple dominance, codominance, and multiple alleles
Blood Types (ABO blood types)
Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O
A & B = Dominant O = Recessive
Blood Types (ABO blood types)
Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O
A & B = Dominant O = Recessive
Genotype Phenotype
AA
Blood Types (ABO blood types)
Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O
A & B = Dominant O = Recessive
Genotype Phenotype
AA A
Blood Types (ABO blood types)
Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O
A & B = Dominant O = Recessive
Genotype Phenotype
AA AAO
Blood Types (ABO blood types)
Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O
A & B = Dominant O = Recessive
Genotype Phenotype
AA AAO A
Blood Types (ABO blood types)
Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O
A & B = Dominant O = Recessive
Genotype Phenotype
AA AAO ABB
Blood Types (ABO blood types)
Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O
A & B = Dominant O = Recessive
Genotype Phenotype
AA AAO ABB B
Blood Types (ABO blood types)
Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O
A & B = Dominant O = Recessive
Genotype Phenotype
AA AAO ABB BBO
Blood Types (ABO blood types)
Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O
A & B = Dominant O = Recessive
Genotype Phenotype
AA AAO ABB BBO B
Blood Types (ABO blood types)
Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O
A & B = Dominant O = Recessive
Genotype Phenotype
AA AAO ABB BBO BAB
Blood Types (ABO blood types)
Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O
A & B = Dominant O = Recessive
Genotype Phenotype
AA AAO ABB BBO BAB AB (codom.)
Blood Types (ABO blood types)
Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O
A & B = Dominant O = Recessive
Genotype Phenotype
AA AAO ABB BBO BAB AB (codom.)OO
Blood Types (ABO blood types)
Example of simple dominance, codominance, and multiple alleles Alelles = A, B, O
A & B = Dominant O = Recessive
Genotype Phenotype
AA AAO ABB BBO BAB AB (codom.)OO O
Q: What combination of parents have an equal chance of having any of the 4 blood types?
Q: What combination of parents have an equal chance of having any of the 4 blood types?
Mexican Hairless Dogs
Hairless (H) is completely dominant over hairy (h).
When two hairless are crossed: 2/3 hairless 1/3 hairy Why???
Mexican Hairless Dogs
Hairless (H) is completely dominant over hairy (h).
When two hairless are crossed: 2/3 hairless 1/3 hairy
Lethal Allele
2 copies of a lethal allele = death (usually stillborn)
Hint: if offspring ratio is x/3, think lethal allele (someone is dying)
Lethal Allele
Q: How can we breed hairless dogs without stillborns?
3, 4, 5, etc. trait crosses
If a pea plant that is TTRrPp is crossed with a plant that is TtrrPp, what percent of the offspring will be heterozygous for all three traits?
If a pea plant that is TTRrPp is crossed with a plant that is TtrrPp, what percent of the offspring will be heterozygous for all three traits?
Is there an easier way???
Product Rule Make a simple 4 square cross for each trait, then
multiply the results for each
Epistasis
The process of one gene controlling the expression of another
An epistatic gene can completely mask the effects of another gene 2 genes -> 1 trait
Example: Hair Color
2 genes are responsible for hair color: black-brown gene red-blonde gene
Example: Hair Color
2 genes are responsible for hair color: black-brown gene red-blonde gene
Black-brown is epistatic (dominant) over the red-blonde gene
Two “exceptions” present – incomplete dominance and epistasis
B = black b = brown
R = red r = blonde
-> controls ->
B = black b = brown
BB__ =
R = red r = blonde
-> controls ->
B = black b = brown
BB__ = black
R = red r = blonde
-> controls ->
B = black b = brown
BB__ = black Bbrr =
R = red r = blonde
-> controls ->
B = black b = brown
BB__ = black Bbrr = brown
R = red r = blonde
-> controls ->
B = black b = brown
BB__ = black Bbrr = brown BbR_ =
R = red r = blonde
-> controls ->
B = black b = brown
BB__ = black Bbrr = brown BbR_ = auburn
R = red r = blonde
-> controls ->
B = black b = brown
BB__ = black Bbrr = brown BbR_ = auburn
R = red r = blonde
bbRR =
-> controls ->
B = black b = brown
BB__ = black Bbrr = brown BbR_ = auburn
R = red r = blonde
bbRR = red
-> controls ->
B = black b = brown
BB__ = black Bbrr = brown BbR_ = auburn
R = red r = blonde
bbRR = red bbRr =
-> controls ->
B = black b = brown
BB__ = black Bbrr = brown BbR_ = auburn
R = red r = blonde
bbRR = red bbRr = strawberry blonde
-> controls ->
B = black b = brown
BB__ = black Bbrr = brown BbR_ = auburn
R = red r = blonde
bbRR = red bbRr = strawberry blonde bbrr =
-> controls ->
B = black b = brown
BB__ = black Bbrr = brown BbR_ = auburn
R = red r = blonde
bbRR = red bbRr = strawberry blonde bbrr = blonde
-> controls ->
B = black b = brown
BB__ = black Bbrr = brown BbR_ = auburn
= breeds true
R = red r = blonde
bbRR = red bbRr = strawberry blonde bbrr = blonde
-> controls ->
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