In a laboratory population of diploid, sexually reproducing organisms a certain trait is determined by a single autosomal gene and is expressed as two phenotypes. A new population was created by crossing 51 pure-breeding (homozygous) dominant individuals with 49 pure breeding (homozygous) recessive individuals. After four generations, the following results were obtained.

1 51 49 100

2 280 0 280

3 240 80 320

4 300 100 400

5 360 120 480

a) Identify an organism that might have been used to perform this experiment, and explain why this organism is a good choice for conducting this experiment.

b) On the basis of the data, propose a hypothesis that explains the change in the phenotype frequency between generation 1 and generation 3.

c) Is there evidence indicating whether or not this population is in Hardy-Weinberg equilibrium? Explain.

Part A (maximum 4 points) Choice of organism

1 pt name of organism that could be used to produce the kind of data shown:

Drosophila melanogaster, or fruit fly, housefly, mouse, dog, cat, rabbit, slug, named diploid plant e.g. maize, pea, or Brassica; any other organism which reproduces sexually, is diploid, reproduces often, and has a reasonably short life cycle. (Peas accepted only because they may have been crossbred by the experimenter.)(not: long-lived, prokaryotic, fungal(except diploid yeast), polyploid, protistan, or human organisms)

1-3 pts Reasons for choice:

large number of offspring/generation

reasonably short life cycle/generation

easily maintained organisms -or- easily controlled conditions

clear, easily identified phenotypic traits/clear gender dimorphism

interbreed freely (without inbreeding)

Part B (maximum 4 points) on the basis of data, hypothesis to explain change from generation 1 - 3; Mendelian genetics

1 pt correct formulation of a hypothesis; (if...then) logical statement

1 pt explanation of genotypic change from generation 1 to 2 (AA x aa --> Aa)

1 pt explanation of genotypic change from generation 2 to 3

(Aa x Aa-->1/4 AA, 1/2 Aa, 1/4 aa) or Punnett square

or 1 pt for only description of phenotypic change if neither of the above two pts are given

1 pt explanation of dominance (not just use of the word)/ explanation of heterozygosity

1 pt explanation of Mendel's law of segregation

Part C (maximum 4 points) evidence for Hardy-Weinberg equilibrium

1 pt yes, with some correct explanation

1 pt recognition that, at equilibrium, allele and genotype frequencies do not change

1 pt describes Hardy-Weinberg equilibrium (p2 + 2pq +q2 = 1 after 1 generation)

1 pt calculation of p and/or q (q2 = 0.25; q = 0.5)

1 pt elaboration: H-W only maintained if population is large, randomly mating, has no(net) mutation, no migration, or no selection for alleles in question (min. of 3 stated)