Genetics (Dominance)

Dominance in genetics is a relationship between alleles of one gene, in which one allele is expressed over a second allele at the same locus.

The first allele is dominant and the second allele isrecessive. For genes on an autosome (any chromosome other than a sex chromosome), the alleles and their associated traits are autosomal dominant orautosomal recessive. Dominance is a key concept in Mendelian inheritance and classical genetics. Often the dominant allele codes for a functional protein whereas the recessive allele does not.

Incomplete dominance:
Incomplete dominance (also called partial dominance) occurs when the phenotype of the heterozygous genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes

The law of dominance is not always found to be correct as there are many examples where the complete dominance is absent. In such cases some traits of F1 phenotype is intermediate between those of parental traits.

In incomplete dominance the genes of an allelomorphic pair are not expressed as dominant and recessive but express themselves partially when present together in the hybrid. As a result F1 hybrids show characters intermediate to the effect of two genes of the parents.
A good example of incomplete dominance is seen in Mirabilis jalapa (Four o’clock plant) and Antirrhinum majus. It has also been noticed in Andalusian fowl.
(a) Four o’clock plant:
In Mirabilis jalapa (Four o’clock var. Gulbansi) and Antirrhinum majus (snapdragon or dog flower), there are two types of flower colours i.e., red and white (Fig. 5.16).
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Here, when a cross is made between red flowered (RR) and white flowered (rr) varieties, F1 (Rr) progeny produced is an all pink flowered. When these F1 pink flowers are self pollinated or crossed among themselves to raise F2 generation, they produce red (RR), pink (Rr) and white (rr) flowers giving 1:2:1 ratio. This phenotypic ratio is identical with genotypic ratio because heterozygotes are phenotypically intermediate between two homozygous types.
Table 5.12. F2 genotype and F2 phenotype due to phenomenon of dominance showing relationship among pairs of independent alleles:

Number of heterozygous pairs	Number of types of gametes	Number of F2phenotypes	Number of genotypes
1 2 3 4 10 n	2 4 8 16 1024 2n	2 4 8 16 1024 2n	3 9 27 81 59059 3n

(b) Andalusian fowl:
The Andalusian fowl is found in three colours: Black, white and blue. Pure forms are black (BB) and white (bb). If these two forms are crossed, F1 individuals appear blue (Bb) coloured. The blue hybrids on crossing with each other (Bb x Bb) give rise to one black, two blue and one white in 1:2:1 ratio.
Out of three types blue coloured fowls are favoured as delicacy and are in demand for food, but the blacks and whites are wasters. Previously, poultry men had been destroying blacks and whites.
Crosses were being made only between blues but to get only 50% blues, others 25% blacks and 25% whites (wasters). Now there is no need to destroy blacks and whites. If blacks and whites are crossed all the offsprings are blue (Fig. 5.17).
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However, it should be noted that incomplete dominance does not favour blending theory of inheritance, because 25 per cent each of F2 progeny still exhibit parental red and white flowers. It clearly shows that blending has failed to take place in this case also and alleles of the gene are discrete and or particulate
Gene Interaction:
Gene interaction is the influence of alleles and non-alleles on the normal phenotypic expression of genes.
It is of two types:
1. Intragenic (inter-allelic)
2. Intergenic (non-allelic)
1. Intragenic interaction:
In this case two alleles (located on the same gene locus on two homologous chromosomes) of gene interact in such a fashion to produce phenotypic expression other from normal dominant recessive phenotype e.g., co-dominance and multiple alleles.
2. Intergenic interaction:
Here two or more independent genes present on same or different chromosomes interact to produce a new expression e.g., epistasis, complementary genes, supplementary genes, duplicate genes, inhibitory genes, lethal genes etc.

Co-dominance:

Co-dominance occurs when the contributions of both alleles are visible in the phenotype.

When the dominant character is not able to suppress, even incompletely the recessive character and both the characters appear side by side in F1hybrids, the phenomenon is called co-dominance. In this case the F1generation resembles both parents.
For example, in catties, if a cattle with black coat is crossed to a cattle with white colour, the F1 hybrids possess roan coat. In roan coat, both black and white patches appear separately. So, the alleles which are able to express themselves independently when present together are called co- dominant alleles

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For example, in the ABO blood group system, chemical modifications to a glycoprotein (the H antigen) on the surfaces of blood cells are controlled by three alleles, two of which are co-dominant to each other (I^A, I^B) and dominant over the recessive i at the ABO locus. The I^A and I^Balleles produce different modifications. The enzyme coded for by I^A adds an N-acetylgalactosamine to the membrane-bound H antigen. The I^B enzyme adds a galactose. The iallele produces no modification. Thus I^A and I^B alleles are each dominant to i (I^AI^A and I^Aiindividuals both have type A blood, and I^BI^B and I^Bi individuals both have type B blood. But I^AI^Bindividuals have both modifications on their blood cells and thus have type AB blood, so the I^A andI^B alleles are said to be co-dominant.)

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