Home > Dominance relationship

In genetics, dominance relationships control whether a child 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

1 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).

2 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.

3 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.

3.1 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 heterzygous (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.

3.2 Punnett square

Main article: Punnett squareA Punnett square is a tool in genetics developed by British geneticist Reginald Punnett, and which biologists use to this day to predict the probability of possbile genotypes of offspring. The above example Punnett square represents the possible genotypes

The genetic combinations possible with simple dominance can be expressed by a diagram called a Punnett squareA Punnett square is a tool in genetics developed by British geneticist Reginald Punnett, and which biologists use to this day to predict the probability of possbile genotypes of offspring. The above example Punnett square represents the possible genotypes. 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:

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.