Question

The second generation of a purebred cross of a h.o.m.o.zygous recessive organism, with a h.o.m.o.zygous dominant organism, contains a ratio of approximately ____ expressed dominant traits to recessive traits.
a. 1:1
b. 1:3
c. 2:1
d. 3:1

Answer 1

Answer:

d. 3:1

Explanation:

A purebred organism is an organism that contains identical alleles for a particular gene e.g rr or RR. According to this question, in a cross between a purebred dominant organism e.g RR and a purebred recessive organism e.g rr, each parent will produce R and r gametes respectively.

Using the gametes in a punnet square (see attached image), a heterozygous organism (Rr) will be produced in the F1 generation. If these heterozygous organism is self-crossed i.e. Rr × Rr, the following F2 offsprings will result;

RR, Rr, Rr, and rr.

RR, Rr and Rr will exhibit dominant traits while rr will exhibit recessive traits. This means that the F2 generation contains a ratio of approximately 3:1 expressed dominant traits to recessive traits.