Codominance PDF

# Codominance In codominance, both alleles of a character are equally dominant and both of them express their character in the F₁ generation. None is masked. Here characters of the parents are mixed together in the offspring. Codominance is an allelic interaction. In heterozygotes, both alleles e...

# Codominance In codominance, both alleles of a character are equally dominant and both of them express their character in the F₁ generation. None is masked. Here characters of the parents are mixed together in the offspring. Codominance is an allelic interaction. In heterozygotes, both alleles express their characters simultaneously. These genes are called codominant genes. In codominant inheritance, the monohybrid ratio is 1:2:1. The codominant genes are represented by superscripts. Eg. L^, L^, etc. **Examples of Codominance** 1. Coat colour in short horn cattle 2. ABO blood group 3. MN blood group 4. Sickle cell anaemia ## Inheritance of Coat Colour in Short Horn Cattle * **Parents** * Red (RR) x White (rr) * **F₁ individual**: Rr (Roan) * **F₁ crossed**: Rr x Rr * **F₂**: * RR (Red) * Rr (Roan) * rr (White) ## ABO Blood Group ABO blood group in man is a codominant character. Human beings are classified into 4 groups in the ABO system: A, B, AB, and O. - The homozygous A group has the genotype L^L^. The gene L^ produces antigen A in A group persons. - A Group: **L^L^** - The homozygous B group has the genotype L^L^. The gene L^ produces antigen B in B group persons. - B Group: **L^L^** Similarly the homozygous B group has the genotype L^L^. The gene L^ produces antigen B in B group persons. When an A group person marries a B group person, the F₁ children will have the genotype L^L^. They are AB group. They contain both antigen A and B. The gene L^ produces antigen A and the gene L^ produces antigen B simultaneously. When two AB group individuals marry, the F₂ individuals are produced in the ratio of 1A: 2AB: 1B instead of the usual Mendelian ratio of 3: 1. This is due to the codominance of the genes L^ and L^. ## MN Blood Group Human beings are classified into three types based on MN blood group. They are M, N, and MN groups. * **M Group** * genotype: L^L^ * The gene L^ produces antigen M on the surface of RBC. * **N Group** * genotype: L^L^ * The gene L^ produces antigen N on the surface of RBC. * **Parents**: * M Group (L^L^) x N Group (L^L^) * **F₁**: L^L^ (MN group) * **F₂**: * L^L^ (M group) * L^L^ (MN group) * L^L^ (MN group) * L^L^ (N group) When M and N group persons marry, their children will have the genotype L^L^. The L^gene produces M antigen and L^ gene produces Nantigen simultaneously and so they belong to MNblood group. When two MN individuals marry, the F₂ individuals are produced in the ratio of 1M group: 2MN group: 1N group instead of the usual Mendelian ratio of 3: 1. This is due to the codominance of the genes L^ and L^. ## Sickle Cell Anaemia Sickle cell anaemia is a hereditary blood disease characterised by the presence of sickle shaped RBC under low oxygen pressure. The sickle cell anaemic persons have the genotype Hb^Hb^. The gene Hb^ produces defective haemoglobin (Hb^) causing sickling of RBC (S = Sickle). The normal persons have the genotype Hb^Hb^. The gene Hb^ produces adult haemoglobin, the normal haemoglobin.(A = Adult) The heterozygous individuals (Hb^Hb^) produce both normal haemoglobin and defective haemoglobin. So the genes Hb^ and Hb^ are codominant. ## Pleiotropism The production of many characters by a single pair of genes is called pleiotropism. It is the multiple effect of a pair of genes. It is the antithesis of Mendel who said that a single character is controlled by a pair of genes. In pleiotropism, a single pair of genes controls many characters. The genes pp for phenylketonuria produce the accumulation of phenylalanine in the blood. In addition, it produces many other characters such as mental retardation, widely spaced incisors, pigmented patches on skin, excessive sweating, non-pigmented hairs and eyes, etc. Similarly, the potato mutant gene suppresses the growth of meristematic tissue, axillary shoot and petals. It produces apocarpous pistil and dialatory anthers.