Mendel observed traits that: A. inheritedB. are determined by one geneC. have two phenotypes only D. one phenotype dominant, one recessive. Can you think of traits that do NOT behave this way? Explain your choice.

Cases when Mendel’s Law of dominance does not apply:

1) Incomplete DominanceOne allele is partially dominant

over the other.

2) Co-DominanceBoth alleles contribute to the

phenotype.

INCOMPLETE DOMINANCE

Case of the Snapdragons

How to make a pink flower?

R1R1 Generation:

P

F1

Incomplete dominance:

R2R2

R1R2

R1R2 R1R2R1R1 R2R2

?

R1R2 R1R2

Comparing punnet squares:- one of Mendel’s peas

(complete dominance)- one of Snapdragon Flowers

(incomplete dominance) (R1=Red, R2=White, R1R2=pink).

P p

P

p

R1

R1

R2

R2

Pp x Pp R1R2 x R1R2

DominanceIncomplete Dominance≠

Complete dominance:Phenotypic ratio would be 3:1 (Purple:White)

Incomplete dominance: Phenotypic ratio would be 1:2:1 (Red:Pink:White flowers), just like the genotypic ratio.

CO-DOMINANCE

Case of the Roan horsesAnd ABO blood groups

Roan Horses

Roan fur: An example of co-dominance

Both (brown and white) alleles show up

Chromosome 9 3 Alleles:

AntigenA none

AntigenB

IA IB i

Human ABO Blood Groups:- Three possible alleles: iIA IB

IB

i

IA

IBIA

Genotype: Phenotype:

IAii

IB iIA IB

ABOAB

AB

ABO Punnet Square exercise, handout

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  Caucasians

African American

Hispanic Asian

O + 37% 47% 53% 39%O - 8% 4% 4% 1%A + 33% 24% 29% 27%A - 7% 2% 2% 0.5%B + 9% 18% 9% 25%B - 2% 1% 1% 0.4%

AB + 3% 4% 2% 7%

AB - 1% 0.3% 0.2% 0.1%