Midterm 1 Answers :University of British Columbia - BIOL 234

1. (10 marks) In corn Zea mays, Les (“lesion”) mutants have patches of dead cells all
over the leaves, and wild-type levels of sugar in the corn kernels. Sug mutants with the
“sugary” phenotype make extra sweet corn kernels, and have wild-type leaves (no
patches of dead cells). A pure-breeding Les strain was mated with a pure-breeding Sug
strain. The F1 corn from this cross all had lesions with normal sweetnes levels in the
kernels. These Les F1 plantss were allowed to self, and 2000 F2 plants were scored,
revealing 4 distinct phenotypic classes: wild-type (no lesions and normal sugar), Sug (no
lesions, with extra sweetness), Les (lesions with normal sugar) and Les Sug (lesions with
extra sweetness).
F2 phenotype Numbers observed
Wild-type 378
Les 1125
Sug 125
Les Sug 372
a. List what you can conclude based on this F1 data alone

b. Provide a complete genetic explanation for the above F2 data. Your explanation
should include the genotypes of the original pure-breeding parents, the F1 plants, and
the F2 offspring listed in the table above. Define all genes and alleles, and allelic
relationships. Show all of your work.

c. Draw the chromosomes of the F1 corn plants at metaphase of meiosis I, just prior
to anaphase

1. A cross was made between two strains of plants that are agriculturally important.
One strain was disease resistant but herbicide sensitive; the other strain was
disease sensitive but herbicide resistant. A plant breeder crossed these purebreeding
plants and allowed the F1 generation to self-fertilize and produce an F2
generation. The following results were obtained:
F1 generation: all offspring are disease sensitive and herbicide resistant
F2 generation: 229 disease sensitive and herbicide resistant
72 disease sensitive and herbicide sensitive
78 disease resistant and herbicide resistant
21 disease resistant and herbicide sensitive
total 400
a. List what you can conclude based on this F1 data alone.