For natural selection to cause evolution, it must select for or against one or more of the genotypes for a trait. In the simple case of a trait that is determined by a single gene with two alleles, there are five combinations of genotypes that nature can select:

1. either homozygote (AA or aa but not both)

2. both homozygotes (AA and aa)

3. either homozygote and the heterozygote (AA and Aa or aa and Aa)

4. the heterozygote (Aa)

5. all alleles (AA, Aa, and aa).

The hershey and chase experiments involved the preparation of two different types of radioactively labeled phage. which of the following best explains why two preparations were required? a. the bacteriophage used in the experiments was a t2 phage. b. it was necessary that each of the two phage components, dna and protein, be identifiable upon recovery at the end of the experiment. c. each scientist had his own method for labeling phage, so each conducted the same experiment using a different isotope. d. establishing the identity of the genetic material required observation of two phage generations.