Law of Dominance: Codominance
In hybridization techniques, two alleles are considered one as dominant character and other as recessive character when these alleles are let to undergo fertilization by crossing techniques the expression of dominant allele will be high compared to that of recessive allele phenotypically.
Hence the dominant character will be expressed Hence it is known as law of dominance. Law of dominance is also said to be Mendel’s first law of inheritance.
Mechanism of Dominance
On undergoing various experiments Mendel evidenced himself that there will be a difference in their genetic characters, even though the phenotypic character resembles as such as their parents.
Because the character does not remain the same as it is being for their parents.
Therefore, there will be something which controls all these characters which are later found as genes which are the units of DNA (Deoxyribo nucleic acid) or RNA (Ribo nucleic acids) accordingly which are present in the chromosomes of an individual and are passed on to the next generation through parents via gametes of male and female.
Hence the individual has 23 pair of chromosomes each pair from one of the parents, each chromosome consists of genes as their functional unit which consists of contrasting characters made up of alleles.
If there are two or more contrasting pair of alleles then it is said to be as allelomorph.
These alleles are produced as the effect of mutation in a wild gene. For example, let us consider a pea plant, where homozygous tall plant has two alleles such as TT on their gene loci in their homologous chromosomes and homologous dwarf plants is represented by the allele tt.
During the process of gametogenesis, the two homologous alleles TT and tt are separated and each chromosome contains a single allele as T and t and it is passed via gametes.
These alleles which are passed through gametes of both the parents (father and mother) combine together during fertilisation.
Thus, the new individual in the F1 generation has two different alleles and it is referred to as heterozygous in condition.
The dominant character is being expressed and the recessive character of the individual gets suppressed this the mechanism why only dominant characters are expressed though the individual has both the alleles.
Variation in Dominance
Mendel studied the dominant and recessive characters in pea plants which helped him to identify seven pairs of genes showing different phenotypes in homozygous and heterozygous condition with lot of variation.
Variation in dominant character is further classified into two types as Incomplete and complete dominance.
In some cases, both the alleles in a heterozygote lacks the character of being both dominant and recessive.
Which means that each trait is capable of obtaining some degree of phenotypic expression from their parents, hence it can also be considered that there will no dominance between two alleles, and they be in equilibrium condition to express their traits.
1. Example of Codominance
Here the coat colour of cattle breeds is taken into account, Where the coat colour of the cattle is chosen as Black and white.
The cattle with Black colour coat have its allele as BB and the white cattle’s coat is denoted as WW.
When these two alleles are crossed the resultant allele obtained is considered as BW, where the coat colour obtained is Spotted (mixture of both).
Where the white colour hair is spread throughout the coat and black patches is scattered on the coat.
In the filial 2 generation the coat colour of the cattle appears to be spotted and also the parental characters of black and white also appears.
2. Example of Codominance
The best example of co-dominance in humans is ABO blood group, it was first discovered by Landsteiner and Levine.
The alleles here are represented as A and B accordingly.
Here three groups are possible which are denoted as A, B and AB which have their alleles as AA, BB and AB accordingly.
The genotype and its characteristic antigen and antibody are listed below.
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- Codominance of visual pigments in hybrid fishes. Science . 1965 Nov 19;150(3699):1055-7.
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