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Dominance relationship

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.

Table of contents
1 Simple dominance
2 Incomplete dominance
3 Co-dominance
4 Notes

Simple dominance

Simple dominance is where one gene's allele is completely over another, the recessive one. The individual is thus a carrier of the recessive gene but the recessive gene is not expressed at all. In a recessive trait, the trait can skip generations. 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
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)

DominantRecessive
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:

DiseaseGene is...
Polydactylismdominant
Marfan syndromedominant
Some types of Dwarfismrecessive
Tay-Sachs diseaserecessive

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:

NameGene is incompletely
Brachydactylydominant2
Sickle cell anemiarecessive3

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.

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

Notes

  1. It is possible for a spontaneous mutation to cause brown eyes in blue-eyed parents' child. In general, however, this is true.
  2. One copy of the gene causes stubby or webbed fingers. Two copies results in almost certain death.
  3. 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.