Alternative Forms of Dominance

Not all organisms follow the simple genetic rules discovered by Mendel.

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Mendel’s Peas exhibited:

Complete Dominance

The dominant allele will TOTALLY hide the recessive allele.

Tt = Tall

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Incomplete Dominance

• Neither allele is dominant.
• Each form is equally dominant.
• Both expressed simultaneously in a blended or intermediate phenotype.

Incomplete Dominance

Given: In Snapdragons R = Red

W = white

Perform an F1 cross:

(Heterozygous X Heterozygous)

RW X RW

R

R

W

W

R

W

W

W

Genotypic ratio:

RR : RW : WW

1 : 2 : 1

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Phenotypic ratio:

Red : Pink : White

1 : 2 : 1

W

R

R

R

Codominance

• Neither allele is dominant.
• No blending occurs.
• Both traits are shown in a speckled phenotype.

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Codominance

Given: Guinea Pigs B = Black

W = White

Perform an F1 cross:

(Homozygous X Homozygous)

BB X WW

W

B

W

B

B

W

B

B

Genotypic ratio:

BW

4

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Phenotypic ratio:

Black and White Spots

4

W

B

W

W

Blood Type

There are 3 alleles for blood type.

I^A I^B i^O

Blood types A and B are co-dominant with each other, but are both completely dominant over the recessive O type blood.

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Blood Type

Blood type A can either be

homozygous, I^AI^A or heterozygous, I^Ai^O .

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Blood type B can either be

homozygous, I^BI^B or heterozygous, I^Bi^O.

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Blood type AB will always be written I^AI^B .

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Blood type O is recessive and will always be written i^Oi^O .

Sex-Linked Traits

• Traits that are carried on the X chromosome and not the Y are considered sex-linked traits. 
• For example, color-blindness, hemophilia, and muscular dystrophy are all traits that are carried on the X chromosome.

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If the father has the affected X chromosome:

If the mother has the affected X chromosome: