Intro to Molecular, Cellular and Developmental Biology: Spring 2010

A woman with detached earlobes (dominant trait) and a man with attached earlobes have a son with detached earlobes. Their son has a child with a woman with detached earlobes who has a mother with attached earlobes. What is the probability that they will have a son with attached earlobes?

In snapdragons, red flower color (R) is incompletely dominant over white flower color (R¹); the heterozygotes produce pink flowers. A red snapdragon is crossed with a white snapdragon, and the F₁ are intercrossed to produce the F₂.

1. Give the genotypes and phenotypes of the F₁ and the F₂, along with the expected proportions.
2. If the F₁ are backcrossed to the white parent, what will the genotypes and phenotypes of the offspring be?
3. If the F₁ are backcrossed to the red parent, what will the genotypes and phenotypes of the offspring be?

In watermelons, bitter fruit (B) is dominant over sweet fruit (b), and yellow spots (S) are dominant over no spots (s). The genes for these two traits assort independently. A homozygous plant that has bitter fruit and yellow spots is crossed with a homozygous plant that has sweet fruit and no spots. The F₁ are intercrossed to produce the F₂.

1. What will be the phenotypic ratios in the F₂?
2. If an F₁ plant is backcrossed with the bitter, yellow spotted parent, what phenotypes and proportions are expected in the offspring?
3. If an F₁ plant is backcrossed with the sweet, nonspotted parent, what phenotypes and proportions are expected in the offspring?

In sailfin mollies (fish), gold color is due to an allele (g) that is recessive to the allele for normal color (G). A gold fish is crossed with a normal fish. Among the offspring, 88 are normal and 82 are gold.

1. What are the most likely genotypes of the parents in this cross?
2. Assess the plausibility of your hypothesis by performing a Chi-square test.