A hybrid allotetraploid species (2n1 + 2n2 = 60) was backcrossed to one of the suspected parents (2n1 = 30). What is the ploidy of the the F1?

15 chromosomes (n1)

30 chromosomes (2n1)

45 chromosomes (2n1 + n2)

60 chromosomes (2n)

90 chromosomes (2n1 + 2n2)

45 chromosomes (2n1 + n2)

Explanation:

The allotetraploid 2n1+2n2=60 has 60 chromosomes total, two sets of them of type 1 and two sets of chromosomes from the other type (2).

If the suspected type 1 parent is diploid with 30 chromosomes, then the allotetraploid has 30 type 1 chromosomes and 30 type 2 chromosomes.

During meiosis, the germ cells reduce the total chromosome number by half.

The gametes produced by the allotetraploid will have 15 type 1 chromosomes and 15 type 2 chromosomes. The gametes produced by the suspected parent will have 15 type 1 chromosomes.

When these gametes combine during the backcross, the F1 zygote will have a diploid number of 30 type 1 chromosomes (15 coming from the allotetraploid and 15 coming from the suspected parent) and 15 type 2 chromosomes (which will be haploid because they all came from the allotetraploid).