1. Starting with the pea plants, let's say that the red flower-color allele is dominant and is denoted as R, and that the white flower-color allele is recessive and is denoted as r. You have one parent with genotype RR and the red phenotype. The other parent has genotype Rr, and the red phenotype. Out of 4 offspring, how many will be red?
Well, you know that the getting an R from both parents will give the red flower color. RR can happen in two combinations. So, you have 2(1/2*1/2) = 2*1/4 = 1/2. You also know that the Rr combination gives the red phenotype, and this happens in two combinations, so the math is the same as above. 1/2+1/2 = 1, so that all of the four offspring will have the red phenotype.
2. Now, let's say that you have two parents, each with the Rr genotype. Now, what proportion of offspring will have the red phenotype?
Well, you know that getting an R from both parents gives a red phenotype. This happens in one combination. So you have 1/2*1/2 = 1/4. You also know that the Rr genotype gives red flower-color. This happens in two combinations: 2(1/2*1/2) = 1/2. So, so far, 3/4 of the offspring have red flowers. Then, you know that the rr genotype gives white flowers. This happens in one combination, which means that 1/4 of the offspring have white flowers. Therefore, out of four offspring, three would have red flowers and one would have white.
3. Now let's say that R and r are codominant. The Rr genotype results in a pink flower-color. Out of four offspring, how many will have pink flowers.
Alright, you know that the RR genotype gives red flowers. This happens in one combination, so 1/4 of the offspring will have red flowers. You know that the genotype rr gives white flowers, and this happens in one combination, so 1/4 of the offspring will have white flowers. Lastly, you know that the Rr genotype results in pink flowers. This happens in two combinations, therefore 1/2 of the offspring have pink flowers. This means that among four offspring, on average, 1 has red flowers, two have pink flowers, and one has white flowers.