Question

In a typical monohybrid cross, the $F_2$ generation is written as 3 : 1 for phenotype but expressed as 1 : 2 :1 for genotype. Explain with the help of an example.

Answer 1

In a monohybrid cross e.g: when a tall plant with the dominant allele T having genotype TT is crossed with a dwarf plant having recessive allele with genotype tt, then the F1 generation will have all tall plants with genotype Tt and a cross between two F1 generation hybrids will result in one homozygous tall plant (TT), two heterozygous tall plants (Tt) and one dwarf plant (tt). So, the genotypic ratio will be 1:2:1. However, since three of the plants are tall and one dwarf, the phenotypic ratio can be said to be 3:1. This can be shown as in the image.