Final answer:
An allele B1 may hitch-hike to a high relative frequency with advantageous allele A1, when it is in gametic disequilibrium with A1 and if the selection for A1 is strong, overcoming the rate of recombination.
Step-by-step explanation:
We can expect to see an allele, call it B1, at locus B hitch-hiking to a high relative frequency along with the increase in frequency of an advantageous allele, A1, at locus A when B1 is neutral or mildly deleterious, provided it is initially in gametic disequilibrium (i.e., co-inherited) with A1 and selection favoring A1 is much stronger than the rate of recombination between loci A and B. This process is known as genetic hitch-hiking or genetic draft, whereby an allele that is at close genetic proximity to a beneficial allele, and therefore often inherited alongside it, may increase in frequency in the population as a result of the selection for the beneficial allele.
Hitch-hiking can occur when selection on the advantageous A1 allele is so strong that it 'pulls' the nearby B1 allele along with it, even if the B1 allele itself is not advantageous or is slightly deleterious. However, this only works if the two alleles started out linked together in the population and if the rate of recombination, or genetic mixing that happens during sexual reproduction, is not so high as to break apart this association frequently. This is one way in which genetic variation can be maintained in populations, alongside phenomena such as natural selection, mutation, genetic drift, and gene flow.