Explain with suitable examples the complementary factors in inheritance

From Mendel’s experiment on heredity, it was concluded that, there is an unit or factor responsible for every character. The factor later described as genes inherit in mendelian fashion and one gene carry a particular character which can exhibit its phenotypic expression of its own. In post Mendelian experiments it was found that a gene or genes have many fold effects i.e. a single gene can exhibit several phenotypic expression or a single character may results from the interaction of several true. Bateson and Punnett (1905-1908) demonstrated that characters or a trait is governed by the interaction of two or more genes or the phenotypic expression of a trait depends on the combined action of many genes. In such cases normal phenotypic ratio 1:3 or 9:3:3:1 can be modified into various proportions. The modifications are called neo-mendelism or gene interactions.

There are several types of interaction factors which results in the modification of normal phenotypic ratio into 9:7 or 9:3:4 or 12:3:1 or 13:3 or 15:1 etc.

Complementary factors (9:7):-

It has been found that certain characters are produces by interaction of two genes inherited from parental generation. These genes are said to be as complementary genes when these are located on separate gene loci, which inter act to produce a particular or phenotypic trait. But neither of the genes have their phenotypic expression of its own. They could not be expressed phenotypically in the absence of the other. In absence of one or both the genes produce recessive or alternative character. Here the genes are called complementary genes.

Example 1: –

W.Bateson and R.C. Punnett (1904) worked on Lathyrus odoratus (Sweet pea) to explain the phenomenon of complementary interaction factors. They selected two true breeding varieties of white flowered sweet pea plants with genotype “CCrr” and “RRcc” respectively. They made artificial crosses between two. In F, generation all red flowered plants were obtained with genotype “CcRr”.

In F₂ the F₁ plants were subjected to be self pollinated. In F₂ it was found that the off springs resulted in the ratio of 9:7. Out of 16, 9 showed phenotypically red flower and 7-appeared as phenotypically white.

F₁ Generation:-

Parental type- White flowered x White flowered

(CCrr) (RRcc)

F₁ generation: – All Red flowered

(CcRr)

F₁– gametes: – CR, Cr, cR, cr

F₂– Generation: (checker board)

Explanation:-

From above experiment it is to be noted that red flowered plants with both complementary “C” and “R” were in 9 off springs. Whereas white flowered plants in which either of the complementary gene “C” or “R” is present are seven in number.

The above checker board shows that red coloration of flower is due to interaction of two complementary factors “C” and “R” The white coloration of flower is due to lacking of either :C” or “R” gene in the genotype of the individual. In F, generation plants with both “C” and’ “R” genes are present together which exhibit the Red Coloration due to the phenotypic expression of the dominant “R” gene along with complementary “C” gene.

Therefore the phenotypic expression is modified from 9:3:3:1 into 9:7.

This surprising result was understood by analyzing the mechanism of color production in flowers Colour in flowers is produced by anthocyanin pigment. Here the gene ‘C’ is responsible for production of enzyme that catalyses the necessary chemicals for production of an intermediate complex in pigment formation. Whereas gene ‘R’ is responsible for production of enzyme which catalyses the intermediate colorless complex into the colour producing pigment anthocyanin. Therefore both ‘C’ gene and ‘R’ gene have their combined effect in production of anthocyanin pigment. But alone ‘C’ gene or ‘R’ gene cannot help in production of anthocyanin pigment.

Example-2:

Emerson and others explained the complementary factor in inheritance of seed colour in maize. In Indian corn varieties some individual variety, there is colourless seed and some bear purple colored seeds due to presence of anthocyanin pigment in aleurone layer and some others have seeds of white color production of anthocyanin pigement is due to interaction of two-non allelic dominant genes. both influencing the colour production. But absence of any one of the dominant gene results in the production of white seeds. Here two genes are said to be complementary to each other. According to Emerson. Production of pigment controlled by a different enzyme influenced by separate dominant gene. It has been observed that absence of any one of the these genes resulted in not production of the enzymes, which causes the inhibition of the step controlled by that particular enzyme and anthocyanin pigment is not produced causing the colourless in seeds.

Suppose genes A and P interact to produce the purple colour By selfing of a purpled coloured variety which is heterozygous for the alleles of the genes A and P with genotype “AaPp” the joint action of these two genes can be explained. This heterozygous hybrid plant can be produced when two white parents. each homozygous dominant for separate genes are mated “AApp” X” aapp” The F phenotypic ratio in this cross comes to purple seeded and whitish seeded plants with ratio of 9:7.