Double crossover among three chromatids
Independent assortment of two
loci (A & B)
on separate chromosome pairs results in equal proportions
of Parental and
Recombinant progeny.
Mendel crossed AABB with aabb parents,
who produce AB and ab gametes,
respectively. The F1 generation are
all AaBb. Independent assortment of the
two loci in gamete formation produces equal frequencies of
AB, Ab, aB, and ab gametes:
the AB and ab types are called parental,
and the Ab and aB are called recombinant.
Crossing the F1 with
an aabb tester strain [bottom left] allows the genotype
of the F2
progeny
to be determined directly from inspection of their phenotype
[bottom right] since it is determined by the contribution
of the F1
parent.
The F2
progeny
are expected to show equal
proportions of the two parental types and of the two
recombinant types,