Mendelian ExceptionsMendelian Exceptions  Mendel got lucky – all 7 traits he studied showed complete (simple) dominance.

Mendelian ExceptionsMendelian 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 DominanceIncomplete Dominance  Ex: flower color  R = red RR x rr  r = white

Incomplete DominanceIncomplete Dominance  F 2 : Rr x Rr

Incomplete DominanceIncomplete Dominance  F 2 : Rr x Rr  Alleles not blended – still able to separate. RR Rr rr 1 red 2 pink 1 white

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, streaked flowers…

Multiple allelesMultiple 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 AAA

Blood Types (ABO blood types)  Example of simple dominance, codominance, and multiple alleles  Alelles = A, B, O  A & B = Dominant  O = Recessive Genotype Phenotype AAA AO

Blood Types (ABO blood types)  Example of simple dominance, codominance, and multiple alleles  Alelles = A, B, O  A & B = Dominant  O = Recessive Genotype Phenotype AAA AOA

Blood Types (ABO blood types)  Example of simple dominance, codominance, and multiple alleles  Alelles = A, B, O  A & B = Dominant  O = Recessive Genotype Phenotype AAA AOA BB

Blood Types (ABO blood types)  Example of simple dominance, codominance, and multiple alleles  Alelles = A, B, O  A & B = Dominant  O = Recessive Genotype Phenotype AAA AOA BBB

Blood Types (ABO blood types)  Example of simple dominance, codominance, and multiple alleles  Alelles = A, B, O  A & B = Dominant  O = Recessive Genotype Phenotype AAA AOA BBB BO

Blood Types (ABO blood types)  Example of simple dominance, codominance, and multiple alleles  Alelles = A, B, O  A & B = Dominant  O = Recessive Genotype Phenotype AAA AOA BBB BOB

Blood Types (ABO blood types)  Example of simple dominance, codominance, and multiple alleles  Alelles = A, B, O  A & B = Dominant  O = Recessive Genotype Phenotype AAA AOA BBB BOB AB

Blood Types (ABO blood types)  Example of simple dominance, codominance, and multiple alleles  Alelles = A, B, O  A & B = Dominant  O = Recessive Genotype Phenotype AAA AOA BBB BOB ABAB (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 AAA AOA BBB BOB ABAB (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 AAA AOA BBB BOB ABAB (codom.) OOO

 Q: What combination of parents have an equal chance of having any of the 4 blood types?

Mexican Hairless DogsMexican Hairless Dogs  Hairless (H) is completely dominant over hairy (h).  When two hairless are crossed:  2/3 hairless  1/3 hairy Why???

Mexican Hairless DogsMexican Hairless Dogs  Hairless (H) is completely dominant over hairy (h).  When two hairless are crossed:  2/3 hairless  1/3 hairy

Mexican Hairless DogsMexican Hairless Dogs  Hairless (H) is completely dominant over hairy (h).  When two hairless are crossed:  2/3 hairless  1/3 hairy HHHh hh

Mexican Hairless DogsMexican Hairless Dogs  Hairless (H) is completely dominant over hairy (h).  When two hairless are crossed:  2/3 hairless  1/3 hairy HHHh hh

Lethal AlleleLethal Allele  2 copies of a lethal allele = death (usually stillborn)  Hint: if offspring ratio is x/3, think lethal allele (someone is dying)

Lethal AlleleLethal Allele  Q: How can we breed hairless dogs without stillborns?

3, 4, 5, etc. trait crosses3, 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?

Is there an easier way???

Product RuleProduct Rule  Make a simple 4 square cross for each trait, then multiply the results for each PPPp pp Rr rr TT Tt

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: Lab Fur ColorExample: Lab Fur Color  2 genes are responsible for fur color:  Black gene  Brown-blonde gene

Example: Hair ColorExample: Hair Color  2 genes are responsible for hair color:  Black gene  Brown-blonde gene  Black is epistatic (dominant) over the red-blonde gene

Polygenic InheritancePolygenic Inheritance  Most traits (especially visible ones) are due to the interaction of multiple proteins, thus multiple genes  As such, most traits do not follow simple Mendelian ratios