In a plant, the colour of grains is controlled by 3 genes located on separate chromosomes. Gene 'C' and 'R' independently do not form any colour, but when together, they impart gre

Question

In a plant, the colour of grains is controlled by 3 genes located on separate chromosomes. Gene 'C' and 'R' independently do not form any colour, but when together, they impart greenish-brown colour to the stem, while the grains remain colourless. In presence of an additional allele 'A', the stem as well as grains become violet. In a trihybrid cross, what phenotypic ratio is expected in grains?

TestHub · Accepted Answer

According to the data given, coloured seed can be produced in the plant only when the genotype has at least one dominant C gene and one dominant R gene and one dominant A gene, that is $C_R_A_$

A trihybrid cross is $C c R r A a x C c R r A a$ . The three genes are on separate chromosomes. Thus, they are independently assorted. If we consider the three genes separately: $C c x C c$ will produce 1 CC, 2 Cc and 1 cc. Thus, the proportion of individuals having CC or Cc is $\frac{3}{4}$ .

Similarly, $R r x R r$ will produce 1 RR, 2 Rr and 1 rr. Thus, the proportion of individuals having RR or Rr is $\frac{3}{4}$ .

Similarly, $A a x A a$ will produce 1 AA, 2 Aa and 1 aa. Thus, the proportion of individuals having AA or Aa is $\frac{3}{4}$ .

Thus, the proportion of plants with genotype $C_R_A_$ in trihybrid cross will be $\frac{3}{4} \times \frac{3}{4} \times \frac{3}{4} = \frac{27}{64}$ . Thus, 27 out of 64 progenies will have coloured grains. The remaining 37 out of 64 will have colourless grains.

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