Monday, September 28, 2009

Dominance, Incomplete Dominance, Co-Dominance, Test Cross and Back Cross


If two different alleles of the same gene (T and t) come together by fusion of gametes, they co exist as such although one of them expresses itself called dominant and the other is unable to express itself and is called recessive. If F1 plants are tall therefore the allele of tallness is dominant and allele of dwarfness is recessive. Therefore Tall character is dominant on recessive.


In case of flower colour Mendel found that red colour of flower was dominant over white colour of flowers. White coloured flowers reappeared in F2 following Mendel’s principle of segregation. Later a deviation to this rule was observed in a plant species commonly called four o clock (Mirabilis Jalapa). The plant produces flowers with red and white colours. When pure breeding red (R) flowered plants were crossed with pure breeding white (r) flowered plants, F1 plants were Pink (Rr) flowered. Appearance of pink flowers, an intermediate shade between red and white provided relief to those who believe in blending inheritance. But when F1 hybrids were crossed (Rr × Rr), F2 generation showed a phenotypic ratio of 1 red: 2 pink: 1 white instead of typical 3: 1 ratio. Although the ratio contradicted Mendel’s principles but it provides strong support to the concept of particular inheritance.

Appearance of red and white factors (genes) in F2 indicates that these have not been altered or blended white present together in the pink flowered individuals. F2 reds and whites are same as the parental red and white. This was declared a case of ‘incomplete dominance’. In this case neither of the genes is dominant to the other. Each expresses itself in the presence of its allele to produce intermediate effect. Another important aspect of incomplete dominance is that phenotypic ratio (1: 2: 1) is the same as genotypic ratio (1 homozygous dominant: 2 heterozygous dominant: 1 homozygous recessive) resulting from cross when two monohybrids are crossed, when dominance is incomplete, a cross of two monohybrids (Rr × Rr) gives a phenotypic ratio of 1: 2: 1 which is identical to genotypic ratio. The heterozygous individual shows incomplete dominance. Where as when the dominance is complete the phenotypic (3: 1) and genotypic ratio are complete.


In case of blood types in human, both genes (A & B) produce an effect in a heterozygous individual. This is called co dominance. The genes which govern A and B blood types are alleles. Each control the formation of different red blood cell protein or antigen. Antigen in case of person having blood group A and antigen b in individuals with blood group B. Neither gene is dominant to the other. Heterozygous individuals with blood group AB contains both antigens a and b. Both proteins are detected in equal amounts in the red cells.


Tall plant produced in F1 generation may either have TT or Tt genotypes. Mendel devised a simple way to distinguish the genotype. It is known as test cross. The organism of dominant phenotype (tall) but unknown genotype is crossed to homozygous recessive individual. For example a tall plant is crossed with a dwarf plant. If the tall plant is homozygous (TT), all F1 plants would be tall. If the tall plant s heterozygous (Tt) one half of F1 plants would be tall (Tt) and one half dwarf (tt), exhibiting ratio of 1: 1. That ratio is called ‘test cross’.


The term test cross is often used interchangeably with back cross. However, two are not necessarily the same. The test cross is a cross of an individual of dominant phenotype to homozygous recessive. On the other hand back cross is mating of an individual of known phenotype to any one of homozygous parents. For example in the above mentioned example the cross between F1 hybrid to homozygous recessive individual is a test cross, but if we cross the F1 hybrid to homozygous tall (TT), it is a back cross. The back cross may have special value in genetic analysis such as in case of incomplete dominance. Because the homozygous recessive individual is one of the parents therefore test cross and back crosses are regarded same.

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