Problem 1 - Campbell Biology

Question

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 \(i i\) plant.

Step-by-Step Solution

Answer

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

Step 1: Define the Alleles and Parent Genotypes

Identify the alleles involved in the cross. The allele for inflated pods is represented by 'I' and the allele for constricted pods is represented by 'i'. In this cross, the heterozygous plant has the genotype 'Ii' (inflated pods) and the homozygous plant has the genotype 'ii' (constricted pods).

Step 2: Set Up the Punnett Square

Draw a 2x2 grid. Label the top of the Punnett square with the alleles from the heterozygous 'Ii' plant (I and i). Label the left side of the Punnett square with the alleles from the homozygous 'ii' plant (i and i).

Step 3: Fill in the Punnett Square

Fill in each box of the Punnett square by combining the alleles from the top and the left side. This gives the following combinations: Ii, Ii, ii, ii.

Step 4: Determine Genotypic Ratios

Count the frequency of each genotype: there are 2 'Ii' and 2 'ii'. The genotypic ratio is 1 Ii : 1 ii.

Step 5: Determine Phenotypic Ratios

Since 'I' (inflated pods) is dominant over 'i' (constricted pods), 'Ii' will show inflated pods and 'ii' will show constricted pods. The phenotypic ratio is 1 inflated pods : 1 constricted pods.

Key Concepts

Punnett square

A Punnett square helps predict the genetic outcomes of a cross between two organisms. It’s like a simple table where we combine the alleles from each parent to see all possible offspring.

In this exercise, we have a pea plant heterozygous for inflated pods (Ii) crossed with a plant homozygous for constricted pods (ii). We draw a 2x2 grid:

• Top labels: the alleles from the heterozygous 'Ii' plant (I and i).


• Side labels: the alleles from the homozygous 'ii' plant (i and i).

Filling in the boxes, we see the possible combinations: Ii, Ii, ii, and ii.

Genotypic Ratio

The genotypic ratio shows the relative number of different genotypes in the offspring after a genetic cross.

From our Punnett square, we count the frequencies:

• Two 'Ii' genotypes


• Two 'ii' genotypes

So, the genotypic ratio is 1 Ii : 1 ii. This means there's an equal probability of getting either genotype.

Phenotypic Ratio

The phenotypic ratio tells us about the visible traits of the offspring.

In our Punnett square,

• 'Ii' will have inflated pods, because 'I' (inflated pods) is dominant.


• 'ii' will have constricted pods, as there's no dominant 'I' allele.

Thus, the phenotypic ratio is 1 inflated pods : 1 constricted pods. This means there's an equal chance of either phenotype in the offspring.

Dominant and Recessive Alleles

Alleles come in pairs and can be dominant or recessive. Dominant alleles mask the effect of recessive alleles when paired together.

In our example:

• The allele for inflated pods (I) is dominant.


• The allele for constricted pods (i) is recessive.

That’s why 'Ii' shows inflated pods, even though there's a 'recessive' i allele. Dominant 'I' overshadows the recessive 'i'. Only plants with 'ii' show the recessive trait (constricted pods).