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Incomplete Dominance and Co-

DominanceTaking it a step beyond Mendel

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Complete DominanceBefore Mendel, the prevailing theory of inheritance was

blending inheritance – that children appear as a combination of their parents

We learned that Mendel disproved this theory. The cross of purple and white only yielded purple or white flowers, no other colours.

Complete dominance occurs when only one allele is expressed, regardless of the presence of another

Is this always the case in nature?

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Blending is Common in Nature!

There are exceptions to the dominance ruleIn snapdragons, a homozygous red flower and a

homozygous white flower will produce pink flowers!P Generation

F1 Generation

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The genes of snapdragonsThe gene that codes for flower colour is called CIt has two possible alleles, Red – CR , and White

– CW

Genotype PhenotypeCRCR RedCRCW PinkCWCW White

Note the style of writing genes! The C represents the Gene, the superscript represents the allele

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The ratio stays the same

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Incomplete DominanceIn the case of snapdragons, neither of the alleles

is dominant. Both red and white pigments are expressed and neither masks each other.

Heterozygote looks like a blend of the homozygotes

Incomplete dominance is a situation where neither allele dominates the other and both exercise an influence on phenotype, resulting in partial expression of BOTH traits.

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CodominanceSometimes two alleles will both be equally

dominant in the offspringCodominance is the situation where both

alleles are expressed fully to produce offspring with a third genotype

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Codominance

Note how we also use superscripts here to show that both genes are equally

dominant!

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Comparing the twoIncomplete Dominance

Resulting offspring looks like a HOMOGENEOUS BLEND of both parents

Neither allele is dominant, both have some influence on the phenotype

Complete Dominance Resulting offspring looks

like a MECHANICAL MIXTURE of both parents

Both alleles are dominant, both are completely expressed, but on different parts of the organism

x x

Incomplete dominance: Co-dominance:

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Blood TypesHuman blood is both a dominant and codominant

traitPossible Blood Types (Phenotypes):

A, B, AB, OPossible Alleles:

IA , IB , iEveryone still only inherits two alleles (one from

each parent)

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Human Blood TypesGenotype Phenotype

(Blood Type)

IA IA A

IA i A

IB IB B

IB i B

IA IB AB

ii OCodominance

Dominance

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Human Blood Types

Antigen – a molecule recognized by the immune system

Antibody – fight any foreign material in the body; they link the foreign substances together to create a clump of cells

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Blood DonationWhen you receive a blood

transfusion the concern is that your body’s antigens will attack the foreign red blood cells leading to an immune response

The appropriate donor blood cells will not have a an antigen that can be recognized by an antibody in the recipients blood

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Type O Blood – Universal DonorType AB Blood – Universal RecipientDONATE BLOOD!!!

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You think you got it? Let’s see…

Q: If a woman has blood type AB and a man has blood type A, what possible blood types will their children have?

Hint: There is only one possible genotype for mom but two for dad…

Solution:Mom’s genotype: IAIB Dad’s genotype: IAi or IAIA

Consider the following crosses: IAIB x IAi and IAIB x IAIA

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Solutions Continued Cross #1 (homozygous

dad): IAIB x IAIACross #2 (heterozygous

dad): IAIB x IAi

IA

IB

IA i

IA IA

IA IB

IA i

IB i

All children from these two parents must be type A, type B or type AB

IA

IB

IA IA

IA IA

IA IA

IA IA

IB IB

All children from these two parents must be type A or type AB