12. Wheat plants can have kernels of white colour or in shades of red i.e. light red, red, dark red and very dark red (purple).
    A researcher made the following cross:

P     Wheat plants              Wheat plants with with white          X      purple (very dark red) kernals                      kernals
F1	All plants with red kernals
F2	1/16	Plants with purple kernals
	4/16	Plants with dark red kernals
	6/16	Plants with red kernals
	4/16	Plants with light red kernals
	1/16	Plants with white kernals

The following conclusions are made from the results obtained:
A. It is a dihybrid cross where white colour is coded by gene A and the purple colour is coded by gene B
B. Two genes, both cod’ng for the colour of kernel and each gene having two alleles, one that produced red pigment and the other that produced no pigment.
C. Four genes, one coding for no pigment. which is epistatic over the other genes. The remaining three genes have 2 alleles each, one that produced red pigment and the other that produced no pigment.
D. The trait is influenced by the environment leading to the observed variation in kernel colour.
Which of the above conclusion(s) is/are correct?
(1) A only      (2) B only
(3) C only      (4) C and D only

Introduction
Wheat kernel colour is a classic example of polygenic (quantitative) inheritance, where multiple genes with additive effect produce a continuous spectrum of phenotypes from white to purple (very dark red). In CSIR NET and similar exams, this concept is frequently tested using crosses that yield a 1:4:6:4:1 F2 phenotypic ratio for different shades of red and white.

Problem restatement and given data

• Parental cross (P): wheat plants with white kernels × wheat plants with purple (very dark red) kernels.

• F1: all plants with red kernels (intermediate phenotype).

• F2 phenotypic ratio:

  • 1/16 plants with purple kernels

  • 4/16 plants with dark red kernels

  • 6/16 plants with red kernels

  • 4/16 plants with light red kernels

  • 1/16 plants with white kernels.

These five phenotypic classes in 1:4:6:4:1 ratio are typical of a quantitative trait controlled by two additive genes showing independent assortment.

Step 1: Determine number and type of genes
For a polygenic trait controlled by n pairs of additive genes, the number of F2 phenotypic classes is $2n+1$.

• Here, number of phenotypic classes = 5, so $2n+1=5$, giving $n=2$.
  Thus, wheat kernel colour in this question is controlled by two genes, each with two alleles, showing additive (cumulative) effect.

Let the two loci be A/a and B/b, where each dominant allele (A or B) contributes one “dose” of red pigment and each recessive allele (a or b) contributes no pigment.

• White parent: aabb (no red alleles).

• Purple (very dark red) parent: AABB (four red alleles, maximum pigment).

• F1: AaBb with two red alleles, giving intermediate red colour.

In F2, the different combinations of A and B produce 0, 1, 2, 3, or 4 dominant alleles, mapping neatly to white, light red, red, dark red, and purple, respectively, in 1:4:6:4:1 ratio.

Now evaluate each conclusion:

Option A
Statement: “It is a dihybrid cross where white colour is coded by gene A and the purple colour is coded by gene B.”

• In a simple Mendelian dihybrid cross where one gene controls white and another controls purple, the expected F2 ratio would be 9:3:3:1 or other standard epistatic ratios, not 1:4:6:4:1.

• Here, both genes contribute to various shades of the same trait (red pigment intensity), not separate discrete colours like “white by A” and “purple by B”.

• Therefore, Option A misinterprets the genetic basis and is incorrect.

Option B
Statement: “Two genes, both coding for the colour of kernel and each gene having two alleles, one that produced red pigment and the other that produced no pigment.”

• This matches the classical description of polygenic inheritance for wheat kernel colour: two loci with alleles that either add red pigment or contribute none.

• Each dominant allele (A or B) increases redness, and each recessive allele (a or b) is non-contributory, giving additive effect and the 1:4:6:4:1 F2 distribution.

• Thus, Option B correctly explains the system and is a valid conclusion.

Option C
Statement: “Four genes, one coding for no pigment, which is epistatic over the other genes. The remaining three genes have 2 alleles each, one that produced red pigment and the other that produced no pigment.”

• If four genes were involved with one epistatic gene masking pigment production, the segregation pattern would resemble epistatic Mendelian ratios (like 9:3:4, 12:3:1) or more complex multi-locus patterns, not a simple 1:4:6:4:1.

• The question explicitly uses the formula 1/4n1/4n with F2 smallest class 1/16, demonstrated in standard solutions to infer only two genes (n=2), not four.

• Hence, Option C conflicts with the observed ratio and is incorrect.

Option D
Statement: “The trait is influenced by the environment leading to the observed variation in kernel colour.”

• Polygenic traits are indeed often influenced by environment, but in this classical genetic experiment, the 1:4:6:4:1 ratio arises from segregation of additive genes under controlled conditions, not primarily from environmental variation.

• The question’s ratio and analysis are entirely explainable by gene dosage without invoking environmental effects, so attributing the variation mainly to environment is not a correct conclusion here.

• Therefore, Option D is not supported by the given data.

Final answer
Only conclusion B is correct because wheat kernel colour in this cross is controlled by two additive genes, each with a pigment (red) and a non-pigment allele, producing five phenotypic classes in a 1:4:6:4:1 F2 ratio.