Incomplete dominance.

The genes for red and white colour do not actually mix in the F pink hybrids as both the pure white, black and blue. A cross between a pure splashed white male and a pure black female fowls yields hybrid fowls with blue colour in F generation. The blue appearance of the hybrids is due to very fine alternating white and black stripes on the feathers. The blue hybrids on crossing with each other give the usual Mendelian ratio for monohybrid cross, i.e., their ratio is 1:2:1. This cross again shows that the genes for black and splashed white colour do not mix up in F hybrids, and segregate in F offspring, and there is no specific gene for blue colour of the fowl.

Black colour is produced only when there are two genes which produce the black pigment melanin. A single melanin-producing gene in a heterozygote give blue colour to the fowl.

Practical Utility of Mendelism. The above case illustrates the practical utility of Mendel’s laws of heredity. Of the three varieties of Andalusian fowl, the blues are in demand for food, but the blacks and whites are ‘wasters’, as the are not eaten. In the past, the poultrymen have been crossing the blues, hoping that they would get all blues. They, however, got only 50% blues from such crosses, the other 50% being wasters. We now know that if blacks and whites are crossed, all the off spring are blue. There is, thus, no need of destroying the so-called wasters.

2. Multiple Alleles (Multiple Allelomorphs) and Codominance.

So far, we have considered genes having only two distinct alleles. If mutation occurs at different sites of a gene in different individuals, the population as a whole will have many different alleles of that gene. Each allele may produce a different phenotype, and various combinations of alleles produce several genotypes and phenotypes in the population. Any one individual contains only two of the different alleles of a gene, one on each chromosome of the homologous pair carrying that gene.

Definition. More than two alternative forms (alleles) of a gene in population occupying the same locus on a chromosome are known as multiple alleles.

Example. A well known examples by multiple alleles is the blood group in man. There are 4 blood groups in human population - A, B, AB and O. These letters refer to a glycoprotein substance, called an antigen, present on the of red blood corpuscles. People with blood group A produce A antigen, those with blood group B form B antigen, those with blood group AB produce both antigens, and those with blood group O form antigen.

The four phenotypes A, B, AB and O are produced by three different alleles I, I and i of the immunogen gene. The alleles I and I code for two different enzymes, each of which attaches a different sugar to a protein on the surface of red blood corpuscles, forming a specific glycoprotein. The allele I produced the glycoprotein A, and the allele I forms the glycoprotein B. The allele i does not code for an enzyme, and does not produce any glycoprotein, and is recessive to the other two alleles. The alleles I and I are equally dominant and do not interfere with the expression of each other. Any person carries only two of these three alleles, one from each parent. The good group A is produced by the genotypes I I and I i, the group B by the genotypes I I and I i, the group AB by the genotype ii. Thus, three alleles of a gene produce six genotypes and four phenotypes.

The alleles I and I are said to be codominant because both are expressed in the phenotype AB. Each produced its antigen and neither checks the expression of the other.

There is codominance as well as dominant-recessive inheritance in the case of the genes for the blood groups in human beings.

Since the blood groups are inherited, and do not change in an individuals lifetime, they are useful indicators of parintage. This can help in paternity law suits or in case of suspected mix- ups of babies in an hospital nursery to some extent.

The human blood groups illustrate both multiple allelism and codominance. However, all cases of multiple allelism do not show codominance.

Incomplete Dominance versus Codominance. IN the incomplete dominance, the heterozygote has phenotype that is intermediate between the phenotypes of the homozygotes. In codominance, the position is somewhat different.