Question

1. smooth pod shape is dominant over constricted in pea plants. fill in the punnett square and determine the expected genotypic and phenotypic ratios from crossing homozygous recessive and homozygous dominant parents.

genotypes: ________________________ genotypic ratio: __________

Explanation:

Step 1: Determine Parent Genotypes and Gametes

The homozygous dominant parent (smooth) has genotype \( SS \), so gametes are all \( S \). The homozygous recessive parent (constricted) has genotype \( ss \), so gametes are all \( s \).

Step 2: Fill Punnett Square

Each cell in the Punnett square is the combination of a gamete from the top (parent 1) and a gamete from the side (parent 2). So all cells are \( Ss \).

Step 3: Determine Genotypes of Offspring

From the Punnett square, all offspring have genotype \( Ss \).

Step 4: Determine Genotypic Ratio

Count the number of each genotype. \( SS = 0 \), \( Ss = 4 \), \( ss = 0 \), so ratio is \( 0:4:0 \) (or \( Ss \) only, 4 copies).

Step 5: Determine Phenotypic Ratio

Since \( S \) is dominant, \( Ss \) shows the smooth phenotype. No offspring have the constricted (\( ss \)) phenotype. So smooth:constricted = \( 4:0 \).

Answer:

Punnett Square Filling:

Let the dominant allele for smooth pod shape be \( S \) and the recessive allele for constricted pod shape be \( s \). The homozygous dominant parent has genotype \( SS \) (so it can only produce \( S \) gametes), and the homozygous recessive parent has genotype \( ss \) (so it can only produce \( s \) gametes).

	\( S \)	\( S \)
\( s \)	\( Ss \)	\( Ss \)

Genotypes:

All offspring will have the genotype \( Ss \).

Genotypic Ratio:

Since all four offspring in the Punnett square have the genotype \( Ss \), the genotypic ratio is \( 0:4:0 \) (for \( SS:Ss:ss \)) or more simply, all \( Ss \), so we can also express it as \( Ss:Ss:Ss:Ss \) which simplifies to a ratio of \( 4Ss \) (or in the standard \( SS:Ss:ss \) format, \( 0:4:0 \)).

Phenotypic Ratio:

Since \( S \) is dominant over \( s \), all individuals with genotype \( Ss \) will have the smooth pod shape phenotype. The homozygous recessive (\( ss \)) phenotype (constricted) is not present, and the homozygous dominant (\( SS \)) phenotype (smooth) is also not present in the offspring (offspring are all \( Ss \)). So the phenotypic ratio of smooth (dominant) to constricted (recessive) is \( 4:0 \) (or \( 1:0 \) when simplified, but since we have 4 offspring in the Punnett square, \( 4 \) smooth to \( 0 \) constricted).