Question

Onion bulb shows white coloration instead of red-yellow pigmentation when an allele of another gene ‘I’ is present. It is an example of
(a) Recessive epistasis
(b) Dominant epistasis
(c) Dominant-recessive epistasis
(d) Collaborative-supplementary gene

Answer 1

The allele of one gene masks the effect of other genes is called epistasis and in onion the allele that masks the other gene is recessive. It is the process of genetics in which the presence or absence of one gene affects the gene mutation. The term Epistasis is known to be adapted from the Greek meaning “standing upon.”

Complete answer:
Once the associate degree gene at one locus will mask the expression of each allele at another locus. In alternative words, the expression of 1 gene is disguised by another cistron. This can be conjointly noted as a straightforward epistasis process. Epistasis is mainly of two types: dominant epistasis and recessive epistasis. Recessive epistasis, the Recessive allele of one gene masks the other allele or second gene's effect. In onion, the recessive gene responsible for white color masks the gene responsible for red-yellow color. So, it appears white in color.

Additional information:
- Types of epistasis: Dominant epistasis, recessive epistasis, Duplicate recessive genes, Duplicate Dominant genes, Duplicate genes with cumulative effect, and Dominant Recessive interaction
- Locus of the recessive allele(aa) of one gene hides the effect of the locus of another gene (BB, Bb, or bb) and itself express phenotypically
- when the epistatic locus has Dominant alleles( AA or Aa) then alleles of the B locus express themselves.
- 9:3:3:1 ratio is modified to a 9:3:4 ratio. For example, in an onion bulb white color (B) is Dominant over red-yellow color (b). However, that gene expressed in the phenotype is dependent on the gene that controls the deposition of bulb color in onion.

So, the correct answer is option (a) 'recessive epistasis'.

Note:
If the effect of the Dominant allele of one gene is hidden by another gene and expresses itself phenotypically it is called dominant epistasis. The hypostatic allele is expressed only when the genes of the locus contain two recessive alleles. In dominant epistasis the 9:3:3:1 ratio is modified to 12:3:1.