Discussion
If red and white seed colour shows a ratio of 1 : 4 : 6 : 4 : 1 in F2 generation, it is due to :
Answer: D
The ratio shows a polygenic trait controlled by two genes.
The ratio sums to 16, the number of F2 combinations from two gene pairs (4 × 4). The five classes, 1 : 4 : 6 : 4 : 1, correspond to plants carrying 4, 3, 2, 1 and 0 'red' alleles, each allele adding equally to colour intensity. This is Nilsson-Ehle's classic result for kernel colour in wheat.
Three genes would give 64 combinations and seven classes (1 : 6 : 15 : 20 : 15 : 6 : 1). Duplicate genes give 15 : 1 and epistatic genes give ratios such as 9 : 3 : 4 or 12 : 3 : 1, not five graded classes.
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