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%
