Dominance relationship
- For other non-genetic uses of the term "dominance", see Dominance.
| Table of contents |
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2 Incomplete dominance 3 Co-dominance 4 Notes |
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 children could 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.
| 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.