Dominance relationship
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- For other non-genetic uses of the term "dominance", see Dominance.
In genetics, dominance relationships control whether an offspring will inherit a characteristic from the father, the mother, or some blend of both. More technically, they control the ways genes interact to express themselves as phenotypes in a diploid or polyploid individual.
There are three kinds of dominance relationships:
- Simple dominance
- Incomplete dominance
- Co-dominance
Traits inherited in a dominant-recessive pattern are often said to "follow Mendelian inheritance".
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Chromosome redundancy
The dominant/recessive relationship is made possible by the fact that most higher organisms are diploid: that is, most of their cells have two copies of each chromosome -- one copy from each parent. Polyploid organisms have more than two copies of each chromosome, and follow similar rules of dominance, but for simplicity will not be discussed here.
Humans, a diploid species, typically have 23 pairs of chromosomes, for a total of 46. In regular reproduction, half come from the mother, and half come from the father (see meiosis for further discussion of how this happens, and chromosome for less usual possibilities in humans).
Relationship to other genetics concepts
Although humans have only 46 chromosomes, it is estimated that those 46 contain 20,000-25,000 genes, each of which is related to some biological trait of the organism. Many genes are strung together in a single chromosome. The other chromosome of the pair will have genes for the same functions -- for example, to control height, eye colour, and hair colour.
However, since one chromosome came from each parent, it is quite unlikely that the genes will be identical. The specific variations possible for a single gene are called alleles: for a single eye-colour gene, there may be a blue eye allele, a brown eye allele, a green eye allele, etc. Consequently, a child may inherit a blue eye allele from their mother and a brown eye allele from their father. The dominance relationships between the alleles control which traits are and are not expressed.
Simple dominance
Consider the simple example in peas of flower color, first studied by Gregor Mendel. The dominant allele is purple and the recessive allele is white. In a given individual, the two corresponding alleles of the chromosome pair fall into one of three patterns:
- both alleles purple
- both alleles white
- one allele purple and one allele white
If the two alleles are the same (homozygous), the trait they represent will be expressed. But if the individual carries one of each allele (heterozygous), only the dominant one will be expressed. The recessive allele will simply be suppressed.
Latent recessive traits appearing in later generations
It is important to note that an individual showing the dominant trait may have children who display the recessive trait. If a purple-colored parent is homozygous, they will always pass on the dominant allele, and therefore their offspring will always be purple colored, regardless of the contribution of the other parent. However, if the purple-colored parent is heterozygous, they will have a 50/50 chance of passing on the recessive white-colored allele to their offspring.
It is therefore quite possible for two parents with purple flowers to have a white flowers among their progeny. In that situation, we can conclude that both parents were heterozygous (carrying the recessive allele).
However, unless there is a spontaneous genetic mutation, it is not possible for two parents with white flowers to have a purple-colored offspring. Since white flowers are recessive, both parents must have only white-colored alleles to pass on.
Punnett square
Main article: Punnett square
The genetic combinations possible with simple dominance can be expressed by a diagram called a Punnett square. One parent's alleles are listed across the top and the other parent's alleles are listed down the left side. The interior squares represent possible offspring, in the ratio of their statistical probability. In this example, P represents the dominant purple-colored allele and p the recessive white-colored allele. If both parents are purple-colored and heterozygous, it would look like this:
| P | p | |
| P | P P | P p |
| p | p P | p p |
In the PP and Pp cases, the offspring is purple colored due to the dominant P. Only in the pp case is there expression of the recessive white-colored phenotype.
Traits governed by simple dominance
(not an exhaustive list)
| Dominant | Recessive |
| 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 |
| Facial Dimples | No Facial Dimples |
| Able to taste PTC | Unable to taste PTC |
| Earlobe hangs | Earlobe attaches at base |
| Middigital hair (fingers) | No middigital hair |
| No hitchhiker's thumb | Hitchhiker's thumb |
| Tip of pinkie bends in | Pinkie straight |
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
In incomplete dominance (sometimes called partial dominance), a heterozygous genotype creates an intermediate phenotype. In this case, both the dominant and recessive alleles are expressed, creating a blended or combined phenotype. A cross of two intermediate phenotypes can result in the reappearance of either the parent phenotypes or the blended phenotypes.
The classic example of this is the colours of carnations.
| R | R' |
|---|---|
| R RR | RR' |
| R' RR' | R'R' |
R is the allele for red pigment. R' is the allele for no pigment.
Thus, RR offspring make a lot of red pigment and appear red. R'R' offspring make no red pigment and appear white. RR' and R'R offspring make a little bit of red pigment and therefore appear pink.
An example of incomplete dominance in humans is mordan, a trait that is exhibited when eye color alleles from the maternal and paternal chromosomes are blended. This usually occurs when one parent has green eyes and the other parent has brown eyes–the child will have dark blue eyes.
Co-dominance
In co-dominance, neither phenotype is dominant. Instead, the individual expresses BOTH phenotypes. The most important example is in Landsteiner blood types. 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.
Another example involves cattle. If a homozygous bull and homozygous cow mate (one being red and the other white), then the calves produced will be roan-colored, with a mix of red and white hairs.
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 |
Amongst the very few co-dominant genetic diseases in humans, one relatively common one is A1AD, in which the genotypes Pi00, PiZ0, PiZZ, and PiSZ all have their more-or-less characteristic clinical representations.
Most molecular markers are considered to be co-dominant.
Other factors
It is important to note that most genetic traits are not simply controlled by a single set of alleles. Often many alleles, each with their own dominance relationships, contribute in varying ways to complex traits.