Definition of Dominance
The physiological effect of an allele over its partner allele located on the same gene locus is called Dominance. These relationships between alleles refer to how the alleles for a single locus interact to produce a phenotype.
The dominant and recessive relationships are possible due to the fact that higher organisms are diploid i.e., they have sets of chromosomes. Their cells have two copies of each chromosome, one copy from each parent.
Polyploid organisms contain more than two copies of each chromosome and follow the same rules for dominance.
Let’s take an example:
Each human cell carries 46 chromosomes which means 23 pairs. There is a single-sex chromosome and 22 pairs of autosomes. Each chromosomal pair contains the same genes. The specific variations are possible for a single gene.
A single eye color gene contains 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.
Basically, the dominance relationships between the alleles control which traits are to be expressed and which are not to be expressed.
Take an example of dominance for pea plant flower color:
Let say, flower color in sweet pea is controlled by a single gene with two alleles. The three genotypes for this are PP, Pp, and pp. The flower color purple PP and white pp are not dependent on the dominance relationship.
The heterozygote Pp, however, can have various colors such as purple, white, or pink. The exact color which the flower has reflect the dominant relationships.
Types of Dominance
There are four types of dominance relationships:
- Complete Dominance
- Incomplete Dominance
- Co-dominance
- Over Dominance
1. Complete Dominance
In heterozygous conditions, the relationship between two alleles in which one allele (let say R) is completely dominant over the other (let say r) allele, so, the presence of the recessive allele is fundamentally hidden. This is called complete dominance.
In this case, if the two alleles are the same i.e., homozygous the trait will be expressed. But if the individual carries one of each allele i.e., heterozygous then only the dominant one will be expressed. The recessive allele will be suppressed.
Dominant traits are recognized by the fact that they do not skip the generations like recessive.
Examples
- Tumor Suppressor genes
Examples of dominant genes include the tumor suppressor genes BRCA1 and BRCA2. Mutations in these genes lead to the development of breast cancer in humans. It disables tumor-suppressing functions of the proteins.
- Huntington’s disease
Another example of an autosomal disorder is Huntington’s disease. This is a neurological disorder. The mutant gene results in abnormal protein. This mutant protein contains a large number of glutamine amino acids.
2. Incomplete Dominance
The interaction between two alleles that are expressed more or less equally and the heterozygote is different from their homozygote is called incomplete dominance.
Or
The type of dominance in which heterozygote is intermediate between the phenotypes of the two homozygotes is called incomplete dominance.
Example
- Cattle Coat Color
In cattle, the alleles for red coat color and white coat color interact to produce the intermediate between both of them. This new color is called roan. In this, both red and white coat color alleles are not dominant.
So, genes are represented by capital letters and superscripts. RR symbolized red cattle and white cattle is symbolized by R’R’. the roan cattle color is symbolized by RR’.
3. Co-dominance
The dominance in which both alleles of a gene are expressed in heterozygous conditions is called co-dominance. In this type, neither phenotype is dominant. Instead, the individual expresses both phenotypes.
Example
- Landsteiner blood types
The most important example is Landsteiner’s blood type. There are three alleles for blood type genes. A, B, and i. The i form O type and is recessive to both A and B. When a person has both A and B type, the blood type is AB.
4. Over Dominance
The type in which over dominant heterozygote exceeds in quantity than the phenotypic expression of both the homozygotes is called over dominance.
Example
In Drosophila – the fruit fly, the heterozygote W+/ W- has more quantity of fluorescent pigments in eyes than wild W+/ W- or white eye WW homozygote.