Crossover Events And Recombinant Chromatids

Genes A and B are located on the same chromosome arm in the following order: centromere, gene A, gene B, telomere. Which of the following recombinant chromatids could be produced by a crossover event between genes A and B in a heterozygote in which the dominant alleles are linked on one homologous chromosome and the recessive alleles are linked on the other?

I. A B

II. a B

III. A b

IV. a b