BICD 100 W’ 04 (L. Smith)
(monkeyflower), plant stems can be either smooth or hairy, and this difference is
determined by a single gene, H (call H the dominant allele and h the recessive allele).
results of the following crosses and below each phenotype given, indicate the genotypes of the
parental and progeny plants in each cross:
(or one of these parents could be Hh)
3/4 hairy, 1/4 smooth
HH & Hh
1/2 hairy, 1/2 smooth
In the pea strains used by Mendel, differences in flower color, pea shape and pea color were
each controlled by a single gene.
Purple flower color was dominant to white flower color, round
peas was dominant to wrinkled peas and yellow peas was dominant to green peas.
a true-breeding strain of peas with white flowers and round, green seeds to another true breeding
strain with purple flowers and wrinkled, yellow seeds.
The resulting trihybrids were then allowed to
self-pollinate to produce an F
In the trihybrids, what color(s) were the flowers, and what were the colors and shapes of the
Only the dominant traits are seen: flowers are purple, peas are yellow and round.
What proportion of the F
progeny had white flowers and round, green seeds?
p(round) = 3/4
p(green) = 1/4
p(white) = 1/4
so p(round, green and white) = 3/4 X 1/4 X 1/4 = 3/64
Of the progeny with these phenotypes (white flowers and round, green seeds), what proportion
will be true breeding?
To be true breeding, an individual must be homozygous for all alleles controlling the traits of
p(WW) = 1/4 p(gg) = 1/4
p(pp) = 1/4
so p(WW and gg and pp) = 1/4 X 1/4 X 1/4 = 1/64
Answer: of the progeny with round, green peas and white flowers, 1/3 will be true breeding.
How would the answers to part a, b, and c be different if the first cross was between a true
breeding strain with white flowers and wrinkled, yellow peas and another true breeding strain with
purple flowers and round, green peas?