The 3:1 ratio obtained in the figure 2-19, differ from the ratio 3:1 obtained by Mendel.
The difference in the figure is the inheritance pattern, which shows a clear difference between the sexes. This is due to the sex linked gene, for the eye color. In Mendel’s experiments it was autosomal genes.
In F2 generation, all the females have red eyes (w+/-) and half the males have red (w+/Y) and half males are white (w/Y). Hence, the result of ratio 3:1 marks.