Wikipedia: Dominance relationship

Dominance relationship
From Wikipedia, the free encyclopedia.

For other non-genetic uses of the term "dominance", see Dominance.

In genetics, dominance relationships are the ways genes interact to express themselves as phenotypes in an individual. There are three kinds of dominance relationships.

Simple dominance

Simple dominance occurs when one gene's allele is completely dominant over another, recessive allele. The individual is thus a carrier of the recessive gene but the recessive gene is not expressed at all; however, a recessive gene may still be passed on through reproduction and have an opportunity to express itself in a later generation. If both parents carry a recessive gene, their parents should inherit the gene. ¹ For dominant traits, people either have it or not. There is no in-between. Also, most genes have more than two alleles, which can have all sorts of dominance relationships.

This can be expressed by a diagram called a Punnett square. It looks like this:

b B

B Bb BB

b bb bB

In this example, B represents the dominant brown-eye gene and b the recessive blue-eye gene. In the BB, Bb and bB cases, the child has brown eyes due to the dominant B. Only in the bb case does the recessive blue-eye trait express itself in the blue-eye phenotype. So in this fictional case, statistically one quarter of this couple's children will have blue eyes and the rest will have brown eyes. Ideally, for this to occur, a large number of people are necessary.

Traits governed by this relationship: (not an exhaustive list)

Dominant Recessive

Brown Eyes Blue Eyes

Curled Up Nose Roman Nose

Clockwise Hair Whorl Counter-clockwise Hair Whorl

Can Roll Tongue Can't Roll Tongue

Widow's Peak No Widow's Peak

Some genetic diseases carried by dominant and recessive alleles:

Disease Gene is...

Polydactylism dominant

Marfan syndrome dominant

Some types of Dwarfism recessive

Tay-Sachs disease recessive

As can be seen from this, dominant alleles are not necessarily more common or more desirable.

Incomplete dominance

This relationship has the same effects as above for homozygous individuals. For heterozygous individuals, the dominant and recessive traits blend into a middle ground.

The classic example of this is the colours of carnation flowers.

     R    w
R    RR   Rw
w    wR   ww

R is red, w is white. Rw is pink, since red is incompletely dominant over white.

Genetic diseases governed by this relationship:

Name Gene is incompletely

Brachydactyly dominant²

Sickle cell anemia recessive³

For these examples, the homozygous traits are more serious than the heterozygous trait. In fact, carriers of SCA are better off!

Co-dominance

In co-dominance, neither phenotype is dominant. Instead, the individual expresses BOTH phenotypes. The most important example is in Landsteiner blood groups.

The gene for blood types has three alleles: A, B, and i. i causes O type and is recessive to both A and B. When a person has both A and B, they have type AB blood.

Example Punnett square for a father with A and i, and a mother with B and i:
A i
B AB B

i A O

There are very few (if any) co-dominant genetic diseases and very few other traits.

Notes

It is possible for a spontaneous mutation to cause brown eyes in blue-eyed parents' child. In general, however, this is true.
One copy of the gene causes stubby or webbed fingers. Two copies results in almost certain death.
Two copies cause the disease. Carriers have an asymptomatic condition called Sickle Cell Trait. Carriers are also strongly resistant to malaria. This provides an interesting example of natural selection.