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Dominance relationship

From Academic Kids

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
Contents

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 ~35000 genes. Each gene is related to some biological trait of the organism, and 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, they will not be identical. The specific variations possible for a single gene are called alleles: 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 of the dominant brown eye allele and the recessive blue eye allele. In a given individual, the two corresponding alleles of a chromosome pair must fall into one of three patterns:

  • both blue
  • both brown
  • one brown and one blue

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 brown-eyed parent is homozygous, they will always pass on the dominant trait, and therefore their children will always have brown eyes, regardless of the contribution of the other parent. However, if that brown-eyed parent is heterozygous (and they typically would have no way of knowing), they will have a 50/50 chance of passing on the suppressed blue-eyed trait to their offspring.

It is therefore quite possible for two parents with brown eyes to have a blue-eyed child. 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 blue eyes to have a brown eyed child. Since blue eyes are recessive, both parents must have only blue-eyed 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, B represents the dominant brown-eye gene and b the recessive blue-eye gene. If both parents are brown-eyed and heterozygous, it would look like this:

Bb
BBBbB
bBbbb

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. In this fictional case, the couple's children are three times as likely to have brown eyes as blue.

Traits governed by simple dominance

(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
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

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

Main article: Incomplete dominance

In incomplete dominance, the dominant and recessive traits blend into a middle ground. This is because heterozygous individuals only produce half the amount of the trait.

The classic example of this is the colours of carnations.

R r
R RR rR
r Rr rr

R is the gene for red pigment. r is the gene for no pigment.

Thus, RR offspring make a lot of red pigment and appear red. rr offspring make no red pigment and appear white. Rr and rR offspring make a little bit of red pigment and therefore appear pink.

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, he has type AB blood.

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

Ai
BABB
iAO

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

Mechanisms of dominance

Dominance is caused by the fact that many genes code for proteins, specifically, enzymes. Consider the case where someone is homozygous for a dominant trait. Both alleles code for the same enzyme, which causes a trait. Only a small amount of that enzyme is necessary for a given phenotype. The individual therefore has a surplus of the necessary enzyme. Let's call this case "normal".

Next consider the case where an individual is heterozygous. One allele codes for an "abnormal" (and not necessarily bad) enzyme. The other allele codes for the normal protein. Since only a small amount of the normal enzyme is needed, there is still enough enzyme to show the phenotype. This is why some alleles are dominant over others.

In the case of incomplete dominance, the single dominant allele does not produce enough enzyme, so the "abnormal" enzyme shows through. In codominance, both enzymes are equally effective. Since there are only two alleles, there would be an equal amount of both proteins. Thus, both there would be an equal presence of both phenotypes.

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.

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