WFB 224 Conservation Biology
Assignment 1
1. (2.5 pts) Under what conditions would you NOT get a 9:3:3:1 ratio of offspring phenotypes in
a dihybrid cross? List three, with the expected ratio.
2. (2.5 pts) A certain mammal has a gene for tail length that has incomplete dominance – TT
individuals have a long tail, Tt have a medium length tail, and tt are tail-less. Another gene for
hair color is dominant-recessive – BB and Bb are black, and bb are white. The genes have no
interaction. What are the genotypic and phenotypic ratios of offspring from a dihybrid cross?
Show your work with a Punnett square.
3. (2.5 pts) A fish breeder is trying to determine whether a particular gene for color carries a
lethal recessive allele. In a hybrid cross, he gets only two phenotypes (black and white), which
might occur due to one of two circumstances: (1) the alleles are dominant:recessive, or (2) the
alleles are co-dominant, but one of the homozygotes is lethal (i.e., one of three possible
phenotypes dies as an embryo). Determine the ratio of the two phenotypes under either of these
possibilities (keep in mind that under the lethal recessive model, all the recessive homozygotes
would be dead before they hatch).
The way to approach this problem is to consider the two outcomes as hypotheses: what would
the offspring ratio look like if the hypothesis of dominance:recessive is true; what would the
offspring ratio be if the hypothesis of a lethal homozygote is true? Are the ratios different?
4. (2.5 pts) A gene is a certain mammal species produces extra toes on one foot; which foot has
extra toes is random. Breeders want to know what proportion of males in a population will have
extra toes on the right side of its body (i.e., right front foot or right hind foot)? Show your work
to demonstrate which probability rules you used.