Question

The trait of medium-sized leaves in iris is determined by the genetic condition $P{P}^{\prime }$. Plants with large leaves are $PP$, whereas plants with small leaves are $P^{\prime }{P}^{\prime }.$ A cross is made between two plants each with medium-sized leaves. If they produce 80 seedlings, what would be the expected phenotypes, and in what numbers would they be expected? What is the term for this allelic relationship?

Short answer

Answer: The expected phenotypes are 20 large leaves, 40 medium-sized leaves, and 20 small leaves. This allelic relationship is known as "incomplete dominance" or "partial dominance."

Step-by-step solution

Set up the Punnett square

First, we need to set up the Punnett square for both parent plants with medium-sized leaves. Their genetic makeup would be $P{P}^{\prime }$.

Fill in the Punnett square

Each cell of the Punnett square should be filled with the results of combining the letters from each parent. Write the dominant allele "P" first in each cell, followed by the recessive allele "P'". $$\begin{array}{cccc}& & \mathbf{\text{Parent 1}}& \\ & & P& P^{\prime }\\ \text{cline}2-4\mathbf{\text{Parent 2}}& P& PP& P{P}^{\prime }\\ & P^{\prime }& P{P}^{\prime }& P^{\prime }{P}^{\prime }\end{array}$$

Calculate expected phenotypic ratios

Now we need to analyze the results of the Punnett square to find the phenotypic ratios. Use the given genotype information to determine the phenotype of each genotype combination. 1. $PP$: Large leaves 2. $P{P}^{\prime }$: Medium-sized leaves 3. $P^{\prime }{P}^{\prime }$: Small leaves We can see in the Punnett square that there is 1 $PP$, 2 $P{P}^{\prime }$, and 1 $P^{\prime }{P}^{\prime }$. The ratio is 1:2:1 for large : medium : small leaves.

Determine the expected numbers of each phenotype

There are 80 seedlings produced, so apply the phenotypic ratio to the total of 80 seedlings: 1. Large leaves: $80\times \frac{1}{4}=20$ 2. Medium-sized leaves: $80\times \frac{2}{4}=40$ 3. Small leaves: $80\times \frac{1}{4}=20$ The expected numbers of each phenotype are 20 large leaves, 40 medium-sized leaves, and 20 small leaves.

Identify the allelic relationship term

The term for this allelic relationship is "incomplete dominance" or "partial dominance," as neither allele is completely dominant over the other, resulting in an intermediate phenotype (medium-sized leaves) when both alleles are present in the offspring.

Key concepts

Punnett Square

A Punnett Square is a simple yet powerful tool used in genetics to predict the genotypic and phenotypic outcome of a cross between two organisms. It is essentially a grid that allows us to visualize the combinations of alleles from the parent organisms and how they can combine in their offspring.
To construct a Punnett Square, start by writing the alleles from one parent across the top and the alleles from the other parent along the side. Each cell within the grid represents a possible genotype for the offspring, determined by combining the parent's alleles.
For example, when crossing two iris plants with the genotype $P{P}^{\prime }$, each parent can pass down either $P$ or $P^{\prime }$. By filling in the squares, you can derive the potential genotypes: $PP$, $P{P}^{\prime }$, and $P^{\prime }{P}^{\prime }$.
This visualization helps us understand the probabilities of different trait expressions in the offspring and is especially useful when dealing with more complex genetic conditions.

Incomplete Dominance

Incomplete dominance is a fascinating genetic scenario where neither allele in a genotype completely masks the effect of the other. Instead, the heterozygous genotype produces a third, distinct phenotype that is often a blend or intermediate of the two homozygous phenotypes.
In the case of the iris plant example, the genotypes $PP$, $P{P}^{\prime }$, and $P^{\prime }{P}^{\prime }$ result in distinct leaf sizes. Large leaves ($PP$), small leaves ($P^{\prime }{P}^{\prime }$), and most interestingly, medium-sized leaves ($P{P}^{\prime }$) represent the middle ground.
This mixed characteristic shows how alleles interact in an incomplete dominance pattern, resulting in diversity beyond the classic dominant-recessive traits. It's a clear example of nature's complexity and how genetic variations contribute to the richness of plant traits.

Phenotypic Ratios

Phenotypic ratios are used to express the frequency of different phenotypes appearing in the offspring from a genetic cross. These ratios give insight into the likelihood of inheriting particular traits based on the genetic makeup of the parents.
In the provided iris plant example, the phenotypic ratio derived from the Punnett Square is 1:2:1. This means for every four offspring, one will have large leaves, two will have medium leaves, and one will have small leaves.
To translate this ratio into expected numbers, multiply each part of the ratio by the total number of offspring. Here, with 80 seedlings, that results in 20 seedlings with large leaves, 40 with medium leaves, and 20 with small leaves.
Understanding phenotypic ratios is crucial in predicting how traits are passed on in a given population, helping experts and breeders to make informed decisions.