Question

Ratio 9:7 is due to
(a) Supplementary genes
(b) Lethal genes
(c) Complementary genes
(d) Epistatic genes

Answer 1

The phenotypic ratio is altered from 9:3:3:1 to any other ratio in cases of linkage or intergenic gene interactions. The ratio 9:7 is seen when two non-allelic genes that show similar effect independently but produce a different trait when seen together.

Complete Step by Step Answer:
Complementary Genes: Complementary genes are non-allelic genes that independently show a similar effect but produce a completely new trait when present together in the dominant form.
The phenotypic ratio in ${F}_{2}$ generation is 9:7.

Additional Information: Intergenic interaction is also called a non- allelic interaction. It occurs between the alleles of different genes that are located on the same or a different chromosome. It includes various types of interactions like Complementary Genes, Supplementary genes, Epistasis, lethal genes, etc.
Supplementary genes: They are a pair of non-allelic genes, one of which produces its effects independently in the dominant state while the second dominant allele alters the expression of the first gene but only in presence of the first dominant allele. The phenotypic ratio in ${F}_{2}$ generation is 9:3:4.
Lethal genes are alleles that cause the death of the organism possessing those genes. It is usually due to a mutation in the genes that are necessary for growth and development. The phenotypic ratio typically is 2:1.
Epistasis: Epistasis is the phenomenon of masking or suppressing the expression of one gene by another non-allelic gene.
It can be of two types: dominant epistasis and recessive epistasis.
- When a dominant allele (gene A) masks the activity of another allelic pair (Bb) it is called dominant epistasis. The phenotypic ratio in ${F}_{2}$ generation is 12:3:1 or 13:3. - When a recessive gene(b) masks the activity of a Dominant allele of an allelic pair (Aa) it is called recessive epistasis. The phenotypic ratio in ${F}_{2}$ generation is 9:3:4.
So, the correct answer is, “Complementary genes”.

Note:
- All these are examples of intergenic gene interaction.
- There is another kind of gene interaction that occurs between two alleles present on the same locus on the two homologous chromosomes. This is called intragenic or interallelic interaction.
- This kind of gene interaction generally does not alter the phenotypic ratios.