3. Extensions to Mendelian Inheritance

You have four guinea pigs for a genetic study. One male and one female are from a strain that is pure-breeding for short brown fur. A second male and female are from a strain that is pure-breeding for long white fur. You are asked to perform two different experiments to test the proposal that short fur is dominant to long fur and that brown is dominant to white. You may use any of the four original pure-breeding guinea pigs or any of their offspring in experimental matings. Design two different experiments (crossing different animals and using different combinations of phenotypes) to test the dominance relationships of alleles for fur length and color, and make predictions for each cross based on the proposed relationships. Anticipate that the litter size will be 12 for each mating and that female guinea pigs can produce three litters in their lifetime.

In shorthorn cattle, coat color may be red, white, or roan. Roan is an intermediate phenotype expressed as a mixture of red and white hairs. The following data were obtained from various crosses:

Does the roan phenotype illustrate a case of incomplete dominance or a case of codominance? Explain.

In four o’clock plants, many flower colors are observed. In a cross involving two true-breeding strains, one crimson and the other white, all of the generation were rose color. In the , four new phenotypes appeared along with the and parental colors. The following ratio was obtained:

Propose an explanation for the inheritance of these flower colors.