Question

The phenotype ratio of a dihybrid cross in which both the genes show incomplete dominance is
(A) 1:2:1
(B) 9:3:3:1
(С) 1:2:1: 2:4:2: 1:2:1
(D) 3:1

Answer 1

Mendel conducted artificial pollination or cross-pollination experiments using several true-breeding pea lines A true breeding line is one that having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations.

Complete answer:
Mendel selected a total of 14 true-breeding pea plant varieties, such as the height of the stem whether tall or dwarf as pairs which were similar except for one character with contrasting traits. It means, Mendel selected 7 characters that are seed shape, seed colour, pod colour in pea plant for carrying out hybridization experiments and to study inheritance
Mendel crossed pea plants say for example stem height and cross tall and dwarf plants and got ration
3:1 but some inheritance differs from this ratio as:
Incomplete dominance:
After the law given by Mendel , a few cases were observed and seen where $\mathop F\nolimits_1$, phenotype is intermediate between dominant and recessive phenotype, it means $\mathop F\nolimits_1$ did not resemble either of the two parents and was in between the two.
A most popular example of incomplete dominance is that of flower colour in Snapdragon (dog flower or Antirrhinum majus). True-breeding red-flowered plants (RR) was crossed with true breeding white flowered plants (rr). $\mathop F\nolimits_1$ offspring (Rr) had pink flowers. Here one allele is incompletely dominant over another so that intermediate phenotype is produced by $\mathop F\nolimits_1$ hybrid with respect to the parents. If the $\mathop F\nolimits_1$is made to undergo self pollination , the plants of $\mathop F\nolimits_2$ generation are of three types red (RR), pink (Rr) and white flowered (rr) in the ratio of 1: 2: 1. In heterozygous condition (Rr), the phenotypic effect of one allele is more pronounced than that of another and then mixing of both colours (red & white) results in the development of pink colour.

Hence, the correct answer is option (C)

Note: Genes usually function or express themselves singly or individually. But many cases are known where two genes of the same allelic pair or genes of two or more different allelic pairs influence one another. This is called gene interaction and exception to mendelian arises due to this