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 |
2 Incomplete dominance 3 Co-dominance 4 Notes |
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.
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!
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.Co-dominance