1. Beyond Mendelian Genetics: Incomplete Dominance
Mendel was lucky!
Traits he chose in the
pea plant showed up
very clearly…
One allele was dominant over another, so phenotypes were easy to recognize.
But sometimes phenotypes are not very obvious…

2. Incomplete Dominance Snapdragon flowers come in many colors.
If you cross a red snapdragon (RR) with a white snapdragon (rr)
You get PINK flowers (Rr)!
R R
r r 
Genes show incomplete dominance when the heterozygous phenotype is intermediate (a blend of the two
alleles).
R r

3. Incomplete dominance R r R r
When F1 generation (all pink flowers) is self
pollinated, the F2 generation is 1:2:1
red, pink, white
Incomplete Dominance
R r
R R
R r
r r R r

4. Incomplete dominance What happens if you cross a pink with a white? 
A pink with a red? 

5. copyright cmassengale
Mendelian Genetics
2/23/2019
Codominance
Two alleles are expressed (multiple alleles) in heterozygous individuals.
Example: blood type
Phenotye Genotype(s)
1. type A = IAIA or IAi
2. type B = IBIB or IBi
3. type AB = IAIB
4. type O = ii
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6. copyright cmassengale
Mendelian Genetics
2/23/2019
Codominance Problem
Example: homozygous male Type B (IBIB)
x heterozygous female Type A (IAi) IB IA i
IAIB
IBi
1/2 = IAIB
1/2 = IBi
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7. Another Codominance Problem
Mendelian Genetics
2/23/2019
Another Codominance Problem
Example: male Type O (ii) x female type AB (IAIB) i IA IB
IAi
IBi
1/2 = IAi
1/2 = IBi
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8. copyright cmassengale
Mendelian Genetics
2/23/2019
Codominance
Question: If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents?
boy - type O (ii)
girl - type AB (IAIB)
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9. copyright cmassengale
Mendelian Genetics
2/23/2019
Codominance
Answer: IB IA i
IAIB ii
IAi
Parents:
genotypes = IAi and IBi
phenotypes = A and B
IAi
copyright cmassengale