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A number of deviations were identified in the post-Mendelian era. The normal monohybrid ratio of 3:1 will occur only when the allele have a relationship of dominance and recesiveness between each other. In the absence of such a relationship, the 3:1 becomes modified variously.
Let us understand this with the following examples:
1. Incomplete or Partial Dominance:
When the dominant allele does not mask the phenotypic expression of the recessive allele in a heterozygote, then blending of both the dominant and recessive trait takes place in the F1 and F2 heterozygote. This phenomenon is known as incomplete or partial dominance. However, the alleles maintain their individual identities and segregate from each other during gametogenesis. The phenotypic and genotypic ratio obtained is 1:2:1.
The snap dragon (Antirrhinum majus) and four ‘o’ clock plant (Mirabilis jalapa) are good examples of incomplete dominance. When homozygous red flowered plant is crossed with homozygous white flowered plant, they produce pink flowered F1 progeny. The F1 heterozygote produces the F2 progeny with a phenotypic ratio of 1:2:1 as illustrated in Fig. 6.
In pea, starch synthesis is controlled by one gene, which has two alleles B and b. Homozygous BB produces starch effectively i.e. large starch grains are produced. Homozygous bb produces smaller starch grains. The seeds of BB are round while bb are wrinkled.
In the heterozygous condition, Bb, round seeds are produced indicating that B is dominant. But the starch grains formed are of intermediate size. Therefore, if starch grain is considered B is incompletely dominant. But if seed shape is considered, B is completely dominant over ‘b’. Thus there are genes that controls more than one phenotype.
2. Co-Dominance:
In the phenomenon of co-dominance, both dominant and recessive alleles have the capacity to express themselves phenotypically in a heterozygous condition. A phenotypic and genotypic ratio of 1:2:1 is produced in F2 as in incomplete dominance. Blood group in man exhibits the phenomenon of co-dominance and multiple allelism.
The blood group system in Man is known as the ABO blood group system. The gene that controls blood groups is the ‘I’ gene, which has three alleles, IA, IB and i. The IA and IB allele produces different forms of sugar which is found on the plasma membrane of the red blood cells. The gene ‘i’ does not produce any sugar.
Each individual possesses any two of the three ‘I’ alleles. Table 3 gives the possible combinations of the alleles and the blood group formed by the combination.
Where gene IA and IB are found together, the blood group formed is AB. This is known as co-dominance. The existence of more than two alleles is known as multiple allelism and the alleles are known as multiple alleles.
Co-dominance and multiple alleles can be represented as follows:
(IA = IB) > i
A cross between a man with heterozygous blood group A and a woman with blood group B is represented in Fig. 7.
3. Non-Allelic Interactions:
The interaction between the genes located on the same or different chromosomes for the expression of specific phenotypic traits- of an organism is known as non-allelic interactions or inter-genic interactions. The gene which suppresses or masks the effect or action of a gene at another locus is known as the epistatic gene and the gene which is suppressed is known as the hypostatic gene. However, it is now understood that both the loci and genes, i.e. the epistatic and hypostatic genes could be epistatic to each other. Presently the term epistasis is used for any type of inter-genic genetic interaction.
