Types of dominances
Simple dominance or complete dominance
Consider the simple example of flower color in peas,
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 (
PP)
both alleles white (
pp)
one allele purple and one allele white (
Pp)
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.
Simple dominance in pedigrees Dominant traits are recognizable by the fact that they do not skip generations, as recessive traits do. It is therefore quite possible for two parents with purple flowers to have white flowers among their progeny, but two such white offspring could not have purple offspring (although very rarely, one might be produced by mutation). In this situation, the purple individuals in the first generation must have both been heterozygous (carrying one copy of each allele).
Incomplete dominance
Discovered by Karl Correns, incomplete dominance (sometimes called partial dominance) is a heterozygous genotype that creates an intermediate
phenotype. In this case, only one allele (usually the wild type) at the single locus is expressed, and the expression is doseage dependent. Two copies of the gene produce full expression, while one copy of the gene produces partial expression in an intermediate phenotype. A cross of two intermediate phenotypes (= monohybrid heterozygotes) will result in the reappearance of both parent phenotypes and the intermediate phenotype. There is a 1:2:1 phenotype ratio instead of the 3:1 phenotype ratio found when one allele is dominant and the other is recessive. This lets an organism''s genotype be diagnosed from its phenotype without time-consuming breeding tests.
The classic example of this is the color 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. Both RR'' and R''R offspring make some pigment and therefore appear pink.
A readily visible example of incomplete dominance is the color modifier Merle in dogs.
Codominance In codominance, neither phenotype is completely dominant. Instead, the heterozygous individual expresses
both phenotypes. A common example is the ABO blood group system. The gene for blood types has three alleles: A, B, and i. i causes O type and is recessive to both A and B. The A and B alleles are codominant with each other. When a person has both an A and a B allele, the person has type AB blood.
When two persons with AB blood type have children, the children can be type A, type B, or type AB. There is a 1A:2AB:1B phenotype ratio instead of the 3:1 phenotype ratio found when one allele is dominant and the other is recessive. This is the same phenotype ratio found in matings of two organisms that are heterozygous for incomplete dominant alleles.
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 codominant 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 markersdominant.
A roan horse has codominant follicle genes, expressing individual red and white follicles.