In a species of pea plants, the allele for green pods is dominant to that for yellow pods. The diagram provided shows two parent pea plants being crossed. What’s the probability, in percent, that the offspring will have a green pod?
The diagram provided shows the genotypes and phenotypes of a heterozygous green pea plant and a homozygous recessive yellow pea plant. An easy way to approach these questions is by using a Punnett square to predict the genotypes and phenotypes of offspring from the parents’ genotypes and phenotypes. Let’s start by looking at the key information in the question and define some key terms to better understand how to answer it.
An allele is an alternative version of a gene. In this case, an allele could be for green pod color, as you can see in the diagram here, or a different allele for yellow pod color, as you can see in this other diagram. The allele for the green pod is dominant and represented by an uppercase G.
“Dominant” is another key term in our question, so let’s define it here. A dominant allele is always expressed in the phenotype if it’s present in the genotype. The fact that the green allele is dominant means that even if the offspring only inherit one allele, they will express the green pod trait, as its dominant allele is always expressed. The allele for the yellow pod is recessive and represented by a lowercase g. A recessive allele is only going to be expressed in the phenotype if two copies are present, or if a dominant allele is absent.
The question has told us that one of the parent pea plants’ genotype is heterozygous for the green traits. This means that they have one dominant allele, which is represented in the genotype, written as an uppercase G, and one recessive allele, which is written as a lowercase g. The phenotype, or observable trait of this pea, is therefore green. In total, there are three different genotypes possible for the pod color characteristic.
Here’s the one we’ve just discussed. The pea could be homozygous for the dominant green pod trait. Homozygous means they have two alleles that are the same. In this case, the genotype would be represented by two uppercase G’s. The phenotype of this pea would also be green. Alternatively, a pea could be homozygous for the recessive yellow pod trait. In this case, the genotype would be represented by two lowercase g’s. The phenotype of this pea would be yellow.
Now, we’re ready to solve this question. And here’s the Punnett square we’ll use to do that. First, let’s find out which alleles the gametes of each parent will contain.
Gametes contain half the genetic material of a normal body cell. So, if the genotype is heterozygous like our green parent pea, the gametes will either contain the dominant allele, an uppercase G, or the recessive allele, a lowercase g. The gametes of parent one will be written on the top row of the Punnett square. The genotype of our homozygous recessive parent contains two lowercase g’s. So, the gametes can only contain a recessive allele, a lowercase g or another lowercase g. Parent two’s gametes are written in the left-hand column of the Punnett square.
Next, we fill in the squares with all the possible genotypes of the offspring produced when a gamete from one parent fertilizes either gamete of the other. The offspring can only inherit one allele from each parent. For example, if the gamete with the dominant allele fertilizes the gamete with the recessive allele, they reproduce heterozygous offspring. The offspring would inherit a genotype with one uppercase G, the dominant allele from one parent, and one lowercase g, the recessive allele from the other.
Let’s fill in the offspring’s genotypes in the rest of our Punnett square. The question asks about the probability of the offspring having a green pod. And we know that only these ticked genotypes with a dominant allele will produce the green pod phenotype. Using this information, we can see that half or two out of four of the offspring have a genotype which would produce a green pod. We multiply this value by 100 to convert our final answer into a percentage.
Therefore, in a cross between one parent who is heterozygous for the dominant green trait and one who is homozygous for the recessive yellow trait, the probability that the offspring will have a green pod is 50 percent.