Mendelian Exceptions

1.31.13

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 ->