Problem 12 - Campbell Biology

Question

In maize (corn) plants, a dominant allele \(I\) inhibits kernel color, while the recessive allele \(i\) permits color when homozygous. At a different locus, the dominant allele \(P\) causes purple kernel color, while the homozygous recessive genotype \(p p\) causes red kernels. If plants heterozygous at both loci are crossed, what will be the phenotypic ratio of the offspring?

Step-by-Step Solution

Answer

12 colorless: 3 purple: 1 red

Step 1: Identify Parental Genotypes

Determine the genotype of the parents. Both parents are heterozygous at both loci: \[ I i \underline{\phantom{xxx}} P p \].

Step 2: Set Up the Punnett Square for Each Locus

Create a 4x4 Punnett Square to examine all possible combinations of alleles from the two loci. List all combinations for both loci: \[ I P, I p, i P, i p \]. Pair these for both parents.

Step 3: Determine Genotypes of Offspring

Fill out the Punnett square with the combinations of parental alleles. Each cell will represent a genotype of the offspring.

Step 4: Identify Phenotypic Outcomes

Recall that the presence of the dominant allele \( I \) inhibits color, and only genotypes with \( i i \) permit color. Identify how many of each genotype are colored and separate those that are purple \( P \) and red \( pp \).

Step 5: Calculate Phenotypic Ratios

Count the phenotypes:- Colorless (kernel color inhibited due to \( I \)): 9 (\( I P \)), 3 (\( I Pp \)), 3 (\( I pp \)).- Colored: 3 (Purple: \( i i P \)), 1 (Red: \( i i pp \)). This results in a phenotypic ratio.

Key Concepts

Punnett Square

One of the main tools used in genetics problem-solving is the Punnett Square. This is a diagram that helps predict the genetic variations that will result from a cross or mating. It's especially useful when dealing with problems involving multiple genes. To set up a Punnett Square:
- List the possible alleles from one parent on the top.
- List the possible alleles from the other parent on the side.
- Combine the alleles for each cell in the grid to see all potential genotypes of the offspring.

For example, when crossing maize plants that are heterozygous at both loci (i.e., genotypes are \( I i \) and \( P p \)), you create a 4x4 Punnett Square to account for all possible combinations of these alleles. Ensure to fill out each cell by combining the alleles from the top and the side parent's contributions.

Dominant and Recessive Alleles

Alleles are different versions of a gene. They can be dominant or recessive. Dominant alleles are represented with a capital letter, and they mask the effects of recessive alleles, which are represented by a lowercase letter.

• A dominant allele will show its trait even if there is just one copy (i.e., Aa or AA).
• A recessive allele will only show its trait if there are two copies (i.e., aa).

In the case of maize plants:

• The allele \( I \) is dominant and inhibits kernel color.
• The allele \( i \) is recessive and permits color if homozygous (i.e., \( ii \)).
• The allele \( P \) is dominant and causes purple color.
• The allele \( p \) is recessive and causes red kernels when homozygous (i.e., \( pp \)).

Understanding the dominance and recessiveness of alleles helps us predict which traits will appear in the offspring based on their genotypes.

Phenotypic Ratio

The phenotypic ratio is the relative number of offspring manifesting a particular trait or combination of traits. This is a result of the different genotypic combinations that can occur from a Punnett Square.

For example, in the cross of heterozygous maize plants (genotype \( I i P p \)), if we map all the possible combinations using the Punnett Square, some patterns emerge:

• Kernel color is inhibited if at least one \( I \) is present, regardless of the \( P \) or \( p \) alleles leading to colorless kernels.
• If the genotype is \( ii \), color is permitted, and the kernel color will depend on the \( P \) or \( pp \) alleles.

With this, the phenotypic ratio can be calculated:

• Colorless kernels (inhibited by \( I \)): 12 (dominant \( I \) present in combinations.
• Purple kernels (\