Pure-breeding sweet peas with purple flowers and round pollen are crossed with pure-breeding sweet peas with red flowers and long pollen. The resulting F1 plants all have purple flowers and long pollen. When one of these plants is test crossed, 20% of the resulting offspring have purple flowers and long pollen. By how many map units are the genes for flower color and pollen shape separated? A. 10 B. 20 C. 40 D. 60 E. None of the above

C. 40

Explanation:

Pure-breeding means that the individuals are homozygous for the genes being analyzed.

From Mendel's Law of Dominance we know that the traits that appear in the F1 are the dominant ones.

I will call:

P_ = purple flowers

pp = red flowers

L_ = long pollen

ll = round pollen

Initial cross:

P Pl/Pl x pL/pL

F1 Pl/pL

Test cross (cross with a homozygous recessive individual):

Pl/pL x pl/pl

Expected progeny:

Pl/pl = Parental (purple flowers, round pollen)

pL/pl = Parental (red flowers, long pollen)

PL/pl = Recombinant (purple flowers, long pollen)

pl/pl = Recombinant (red flowers, round pollen)

20% of the offspring have purple flowers and long pollen (PL/pl).

Every time crossing over happens in the meiosis of the F1 individual, both a PL gamete and a pl gamete form. That means that 20% of the offspring will also be pl/pl, and the total proportion of the offspring that will be recombinants will be 40%.

A distance of 1 map unit corresponds to a recombinant frequency of 1%.

A recombinant frequency of 40% therefore means that 40 map units separate the glower color and pollen shape genes.