Question

A current UC Santa Cruz graduate student who earned their biology degree at SFSU is doing research to replicate classic experiments that investigated the inheritance patterns of traits in the sweet pea plant. In one experiment, they mated a plant with wrinkled seeds (plant A) to a plant with smooth seeds (plant B). Of the resulting 100 offspring plants, they noted that 48 had wrinkled seeds and 52 had smooth seeds. Answer the questions below and make sure you fully explain your logic.

PART 1 (3 points):

Based on the results of the experiment above, can you conclude that the wrinkled phenotype is recessive to the smooth phenotype? Why or why not?

PART 2 (3 points):

In a subsequent experiment, they crossed two of the smooth pea plants that resulted from the original cross. Of the 100 offspring, 24 plants had wrinkled seeds and 76 had smooth seeds. Can you conclude from this which of the two phenotypes is dominant? Explain your reasoning.

PART 3 (3 points):

Based on the results of both crosses (in PART1 and PART2), what were the genotypes of the original two plants (plant A and plant B)? How did you figure this out?

Answer 1

Part 1:

Cross between plant A (wrinkled seed) & plant B (smooth seed) produces wrinkled & smooth seeds in almost 1:1 ratio (As numbers of progeny are 48 & 52 for wrinkled & smooth seed respectively). Now, dominant phenotype must be heterozygous (assuming simple dominant-recessive relationship) to produce 1:1 ratio in the progeny (If dominant phenotype is homozygous, then we will observe all dominant phenotype). Dominant phenotype can be either smooth or wrinkled. Because in either case we will get 1:1 ratio in the progeny. This is shown in below Punnett square.

If smooth is dominant phenotype:

Let, S = allele for smooth phenotype, s = allele for wrinkled phenotype

Gametes	s	s
S	Ss (Smooth)	Ss (Smooth)
s	ss (Wrinkled)	ss (Wrinkled)

Similarly, we will get 1:1 ratio if assume wrinkled phenotype is dominant.

So, it is not possible to determine which phenotype is dominant & which one is recessive.

Part 2:

As cross between two smooth plants produces both smooth & wrinkled seeds, smooth phenotype is dominant because wrinkle allele is hidden in the heterozygous smooth seeds. Also, both smooth plants must be heterozygous in order to produce almost 3:1 ratio for smooth & wrinkled seeds in the progeny (As they produce 76 smooth & 24 wrinkled seeds, that is close to 3:1 ratio). This is shown in below Punnett square.

Let, S = allele for smooth phenotype, s = allele for wrinkled phenotype

Gametes	S	s
S	SS (Smooth)	Ss (Smooth)
s	Ss (Smooth)	ss (Wrinkled)

Part 3:

From the part 2, we determined that both smooth plants are heterozygous. Also, original plant A (Wrinkled seeds) is homozygous recessive (ss genotype; as wrinkled phenotype is recessive that is determined in part 2). Now, plant B (smooth seed) must be heterozygous (Ss genotype) as cross between plant A (wrinkled seed) & plant B (smooth seed) produces wrinkled & smooth seeds in almost 1:1 ratio.