Question

In tomato plants, the allele for purple stems is dominant to allele for green stems and the allele for red fruit is dominant to the allele for yellow fruit. Create a Punnett Square and state the genotypic and phenotypic ratios for the offspring if two tomato plants heterozygous for both traits are crossed.

Answer 1

So let's call the allele that is dominant for purple stems is P, and the recessive allele yielding green stems is p. And let's say that the allele that is dominant for red fruit is R and the recessive allele for yellow fruit is r.

The two plants are each heterozygour for both traits, meaning that the cross is going to be:

PpRr x PpRr

So let's setup a Punnet Square:

$\left[\begin{array}{ccccc}&PR&Pr&pR&pr&PR&PPRR&PPRr&PpRR\4&PpRr&Pr&PPRr&PPrr&PpRr&Pprr\\pR&PpRR&PpRr&ppRR&ppRr&pr&PpRr&Pprr&ppRr&pprr\end{array}\right]$

The genotypic ratios (alleles) and phenotypic ratios are as follows:

PPRR (Purple stem, Red fruit): $\frac{1}{16}$

PpRR (Purple stem, Red fruit): $\frac{2}{16}$ or $\frac{1}{8}$

ppRR (Green Stem, Red fruit): $\frac{1}{16}$

PPRr (Purple stem, Red fruit): $\frac{2}{16}$ or $\frac{1}{8}$

PPrr (Purple stem, Yellow fruit): $\frac{1}{16}$

PpRr (Purple stem, Red fruit): $\frac{4}{16}$ or $\frac{1}{4}$

pprr (Green stem, Yellow fruit): $\frac{1}{16}$

ppRr (Green stem, Red fruit): $\frac{2}{16}$ or $\frac{1}{8}$

Pprr (Purple stem, Yellow fruit): $\frac{2}{16}$ or $\frac{1}{8}$

So then we take the like phenotypic ratios and add them together:

Purple stem, Red fruit: $\frac{1}{16} +\frac{2}{16} +\frac{2}{16} +\frac{4}{16}=\frac{9}{16}$

Green stem, Red fruit: $\frac{1}{16}+ \frac{2}{16} =\frac{3}{16}$

Purple stem, Yellow fruit: $\frac{1}{16}+ \frac{2}{16} =\frac{3}{16}$

Green stem, Yellow fruit: $\frac{1}{16}$