Question

In a genetic cross between two flowering plants that are heterozygous for the color trait, red flowers (R) are dominant to white flowers (r). What fraction of the offspring should have red flowers?

Answer 1

I'll work you through it. It tells you that the two parents are heterozygous. Start by interpreting that in terms of their genotypes. What do you think their genotypes are?

Answer 2

Heterozygous = two different alleles. R is dominant; therefore it will show the trait of red than white when paired Rr. Recessive, which is r, means that only rr will make white. So, in this case, you are trying to figure out if Rr and Rr have offspring, what fraction would have red flowers.
Use the drawing! This is called a punnet square. You place one pair on top of the square and the other pair to the left of the square. You align the letters and you have a possibility of offspring. According to the chart, it looks that the answer is 3/4 because out of four of the possibilities, 3 of the offspring are to have red due to the alleles. I hope I helped. :)