Final answer:
In a testcross between a plant of genotype ABab and abab, the proportion of progeny that will be ABab can be determined by considering the recombination frequency between the two loci.
Step-by-step explanation:
In a testcross between a plant of genotype ABab and abab, the two loci are 10m.u. apart. To determine the proportion of progeny that will be ABab, we need to consider the recombination frequency between the two loci. A recombination frequency of 10m.u. indicates that, on average, 10% of the progeny will have a recombination event between the two loci. Therefore, the proportion of progeny that will be ABab is 90% since they represent the non-recombinant offspring.