Question

In complementary genes, the dihybrid ratio of 9:3:3:1 is modified to –
$
  A.9:7 \\
  B.12:3:1 \\
  C.15:1 \\
  D.13.2 \\
 $

Answer 1

Hint: Complementary genes is a group of genes that complement to or contribute to a single characteristic where both genes can mask the effect of the other.

Complete answer
Complementary genes: Two independent parts of genes, which interact to produce a trait together, but each dominant gene alone does not show its effects, are called complementary genes.
Example: when white flowered sweet peas are crossed they produce plants with purple flower F1 plants on inbreeding or self-fertilization give F2 generation in a ratio of purple flowered to 7 white flowered plants.
Explanation: Two pairs of genes located in different chromosomes regulate the flower colour in their varieties of sweet peas. One dominant gene, say C, controls the synthesis of the raw material for the purple pigment and another dominant gene, say E, controls the formation of an enzyme that converts the raw material into purple pigment.
At least one C gene and one E gene must be present to produce purple flowers; the absence of any one or both results in white flowers.

(i) Here a cross between two white flowered varieties, one is genotypically ccEE and the other CCee. The former lack genes for the formation of the raw material, and the latter lack genes for the formation of enzymes needed to change the raw material into the final pigment. Hence both are white coloured.
(ii)A cross between CCee and ccEE which are purple and genotype is CeEe, as they carry a gene for the raw material as well as a gene for the enzyme required to convert the raw material into the pigment.
(iii)F2 generation shows a ratio of a purple to 7 white coloured flower instead of Mendelian dihybrid ratio of 9:3:3:1.
The correct option is A. 9:7.

Note:Dihybrid cross is a cross between two different lines/genes that differ in two observed traits. It describes a mating experiment between two organisms that are identically hybrid for two traits.