Problem 4 - Campbell Biology

Question

DRAW IT A pea plant heterozygous for inflated pods (Ii) is crossed with a plant homozygous for constricted pods (ii). Draw a Punnett square for this cross to predict genotypic and phenotypic ratios. Assume that pollen comes from the ii plant.

Step-by-Step Solution

Answer

Genotypic ratio: 1 Ii : 1 ii. Phenotypic ratio: 1 inflated pod : 1 constricted pod.

Step 1 - Determine Parental Genotypes

Identify the genotypes of the parent plants. One plant is heterozygous for inflated pods (Ii), and the other plant is homozygous for constricted pods (ii).

Step 2 - Set Up the Punnett Square

Draw a Punnett square, a two-by-two grid, to determine the possible genotypes of the offspring. Label the columns with the alleles from the pollen (ii) and the rows with the alleles from the heterozygous plant (Ii).

Step 3 - Fill in the Punnett Square

Fill in each cell of the Punnett square by combining the alleles from the rows and columns. This will give you the possible genotypes of the offspring.

Step 4 - Determine the Genotypic Ratio

Count the frequency of each genotype in the Punnett square. The possible genotypes are Ii and ii.

Step 5 - Determine the Phenotypic Ratio

Identify the phenotypes associated with each genotype. Ii results in inflated pods, and ii results in constricted pods. Count how many of each phenotype appear in the Punnett square.

Step 6 - Summarize the Results

Summarize the genotypic and phenotypic ratios determined from the Punnett square.

Key Concepts

Genotypic Ratio

Understanding the genotypic ratio is key when working with Punnett squares. It represents the frequency of different genotypes in the offspring of a genetic cross. In our example, we crossed a pea plant heterozygous for inflated pods (Ii) with a plant homozygous for constricted pods (ii).

When we set up the Punnett square and fill in the possible offspring genotypes, we observe these combinations:

• Ii
• Ii
• ii
• ii

This means we have two genotypes: Ii and ii. The genotypic ratio from our Punnett square is 2 Ii : 2 ii. Simplified, this is a 1:1 ratio, indicating an equal likelihood of the offspring having either genotype.

Phenotypic Ratio

Phenotypic ratio shows us the observable traits (phenotypes) that appear in the offspring. The phenotypic ratio is derived from the genotypic ratio by identifying which genotypes correspond to which phenotypes.

For our pea plant cross:

• Genotype Ii results in inflated pods
• Genotype ii results in constricted pods

Based on the filled Punnett square, we have:

• Two plants with genotype Ii (inflated pods)
• Two plants with genotype ii (constricted pods)

This gives us a phenotypic ratio of 2 inflated pods : 2 constricted pods. Simplified, this is a 1:1 ratio, showing that the offspring are equally likely to have either phenotype.

Inheritance Patterns

Inheritance patterns help us predict how traits are passed from parents to offspring. In this example, we focus on simple Mendelian inheritance, where traits are determined by single genes with two alleles. One allele is dominant (I for inflated pods), and the other is recessive (i for constricted pods).

Punnett squares are instrumental in visualizing these inheritance patterns. Each parent's alleles segregate independently, creating a clear layout of possible offspring combinations. By filling in the Punnett square and analyzing the genotypic and phenotypic ratios, we better understand how traits are likely to be inherited.

In summary:

• Genotypic ratio: 1 Ii : 1 ii
• Phenotypic ratio: 1 inflated pods : 1 constricted pods

These ratios reveal the inheritance pattern in this cross, illustrating how the dominant and recessive alleles interact to produce the different traits observed in the offspring.