DOMINANCE RELATIONS.

Dominance.

Dominance is a physiological effect of an allele over its partner allele on the same gene locus.

Types.

There are four types of dominance relations among alleles, each indicating a different style of their functional effect upon each other.

1.Complete dominance

2.Incomplete dominance

3.Cociominance

4. Over dominance.

Complete Dominance.

When one allele (R) is completely dominant over the other (r), presence of the recessive allele is functionally hidden, so the heterozygote (Rr) has the same round phenotype as (RR) homozygote.

Note.

The contrasting pairs of alleles for all the seven characters chosen by Mendel showed complete dominance. After Mendel, Further breeding experiments were carried out on different plants and animals. Many novel phenotypes and phenotypic ratios were observed that could not be explained on the basis of complete dominance. Incomplete Dominance When the phenotype of the heterozygote is intermediate between phenotypes of the two homozygotes, it is called incomplete or partial dominance Discovery. In 1899 Carl Correns was working on a flowering plant named 4 O‘clock.

Experiment. Parents.

He crossed a true breeding Red flowered plant with a true breeding white flowered 4 O‘clock, F1. all the F1 hybrids had pink flowers. This new phenotype had a shade intermediate between those of the parents due to an intermediate amount of pigment in petals.

Correns self-fertilized F1 pink F2 .

It showed all three phenotypes of flowers in the ratio of Punnet Square. (1 red : 2 pink : 1 white.) Punnett square indicates that the phenotypic ratio is the same as the genotypic ratio. There is absolutely no need of a test cross. Result. Red was homozygous for red alleles, and white was homozygous for white alleles. But when allele for red and allele for white were present together in the same plant, neither of them masked the effect of other; rather these alleles showed incomplete dominance in the form of pink colour. Use of letters. As there is no truly dominant allele, the usual capital and small letter distinction for dominant and recessive trait is not necessary. Numbering. Both the alleles are represented by the same letter ‗R‘ but are numbered differently to distinguish white from red. Allele for red is designated as R|, and the allele for white as R2. Do these results make Mendel’s principles invalid? The flower colour does show blending at phenotypic level in F1, which is quite contrary to what Mendel observed. But the re-appearance of red and white flowers in F2 confirms that blending does not occur at genetic level.