Question

In pea plants, purple flowers \((P)\) are dominant to white \((p),\) and yellow peas ( \(Y\) ) are dominant to green \((y) .\) What are the possible genotypes and phenotypes for a cross between PpYY and ppYy pea plants? How many squares would you need to complete a Punnett square analysis of this cross?

Short answer

The possible genotypes from the cross are PpYY and PpYy. The corresponding phenotypes are Purple flower and Yellow peas. The Punnett square analysis would require a 2x2 square, so 4 squares are needed.

Step-by-step solution

Understanding the Genotypes

First, let’s understand what each letter in a genotype represents. 'P' denotes purple flowers and 'p' white, with 'P' being the dominant trait. Hence, a pea plant with a 'P' will have purple flowers regardless of the other allele. 'Y' signifies yellow peas and 'y' green, 'Y' being the dominant trait. Therefore, a pea plant with a 'Y' will have yellow peas, irrespective of the other allele.

Setting up the Punnett square

To set up a Punnett square, you first need to determine all the possible gamete combinations that could occur. A gamete will carry one allele from each pair. For the PpYY genotype, there can be two gamete types: PY and PY (since both alleles for pea color are same it does not make a difference). For the ppYy genotype, there can also be two gamete types: pY and py.

Filling in the Punnett square

Now that the gametes are known, the Punnett square can be populated. It will be a 2x2 square resulting in four possible combinations i.e., PpYY, PpYy, PpYY and PpYy.

Determining the genotypes and phenotypes

The genotypes from the cross as per Punnett square are PpYY and PpYy. If looked closely, the phenotypes resulting from these genotypes are purple flower with yellow peas, because 'P' and 'Y' are dominant traits.

Key concepts

Genotype

Genotype refers to the genetic makeup of an organism, specifically the combination of alleles inherited from the parents. Alleles are different versions of a gene, and each organism gets one allele from each parent for every gene. In our exercise with pea plants, for example, the genotype PpYY consists of two different genes. The first gene has alleles 'P' for purple flowers and 'p' for white flowers, while the second gene has alleles 'Y' for yellow peas.
A genotype is not always as straightforward to understand as it may seem. The presence of dominant and recessive alleles can influence which traits are expressed in the phenotype. The genotype PpYY, for instance, would actually result in the phenotype of a pea plant with purple flowers and yellow peas because the 'P' and 'Y' alleles are dominant. A Punnett square can be a valuable tool in predicting the genotype of offspring, as it lays out all the possible combinations of alleles that can occur from the parental gametes.

Phenotype

Phenotype is the term used to describe the observable characteristics or traits of an organism, which are the result of the expression of the genotype in a given environment. In other words, while genotype is like the blueprint, phenotype is the actual physical structure that's been built.
In our pea plant example, the dominant alleles 'P' and 'Y' mask the presence of the recessive 'p' and 'y' alleles, resulting in purple flowers and yellow peas, respectively. This is the phenotype. Even though the genotype may contain alleles for white flowers (p) and green peas (y), they won't be a part of the phenotype if the dominant alleles 'P' and 'Y' are present. It is crucial to distinguish between genotype and phenotype when analyzing inheritance, because two organisms may have the same phenotype but different genotypes.

Dominant and Recessive Alleles

In genetics, some traits are controlled by alleles that are considered either dominant or recessive. A dominant allele is one that will express its trait in the presence of another, different allele. This means that only one dominant allele is necessary for the trait to be observable in the phenotype. For example, the allele for purple flowers ('P') in pea plants is dominant over the allele for white flowers ('p').
On the other hand, a recessive allele will only express its trait when it is paired with another recessive allele. If an organism has one dominant and one recessive allele for a trait, it will exhibit the trait of the dominant allele. It is only when there are two recessive alleles (for example, 'pp') that the recessive trait (white flowers) will appear in the phenotype. Understanding this concept is vital for interpreting the results of a Punnett square and predicting the probability of an offspring's traits.

Gamete Combinations

Each parent contributes gametes, or sex cells, that carry one allele for each gene. Gamete combinations are all the different possible gametes that can be formed from the parent's genotype. In our textbook example, the pea plant with the genotype PpYY can only create gametes with the combinations PY, since 'Y' alleles are identical. However, the ppYy pea plant can form gametes with two different combinations: pY and py.
When performing a Punnett square analysis, it is crucial to list all possible gamete combinations for each parent to understand how the alleles can combine when fertilization occurs. A Punnett square shows all potential combinations of maternal and paternal gametes and, therefore, helps in predicting the genotypic outcome of a cross. In the case of the PpYY and ppYy pea plants, a 2x2 Punnett square is used since each parent can form two types of gametes. This determines the probability of the offspring inheriting certain combinations of alleles from their parents.