Assignment 2

A testcross is a valuable way to use a genetic cross to determine the genotype of an organism that shows a dominant phenotype but unknown genotype. For instance, using Mendel's peas, a pea plant with purple flowers as the dominant phenotype could have either a homozygous or a heterozygous genotype. With a testcross, the organism with an unknown genotype for a dominant phenotype is crossed with an organism that is homozygous recessive for the same trait. In the animal- and plant-breeding industries, testcrosses are one way in which the unknown genotype of an organism with a dominant trait can be determined. Perform the following experiment to help you understand how a testcross can be used to determine the genotype of an organism.


1 .		Design a female fly with brown eye (BW) color (keep all other traits as wild-type), and design a male fly with ebony body color (E; keep all other traits as wild-type). Mate the two flies. Examine the F₁ offspring from this cross and save your data to your lab notes. Add to your data any comments that you would like. To determine the genotype of an F₁ wild-type female fly, design a male fly with brown eye color and ebony body color, then cross this fly with an F₁ wild-type female fly. Examine the results of this cross and save the results to your lab notes. What was the phenotypic ratio for the offspring resulting from this testcross?

2 .		Based on the phenotypic ratio, determine whether the F₁ wild-type female male was double homozygous or double heterozygous for the eye color and body color alleles. Explain your answer. If your answer was double homozygous, describe an expected phenotypic ratio for the offspring produced from a testcross with a double heterozygous fly. If your answer was double heterozygous, describe an expected phenotypic ratio for the offspring produced from a testcross with a homozygous fly.