Final answer:
An allele (B1) at locus B is expected to increase in frequency with an advantageous allele (A1) at locus A through hitch-hiking when both alleles are closely linked and natural selection favors allele A1, leading to its increased frequency. Low recombination rates and gene flow in populations can contribute to this process.
Step-by-step explanation:
Under certain conditions, the occurrence of an allele (B1) at locus B would be expected to increase in frequency along with the advantageous allele (A1) at locus A due to a process known as hitch-hiking. This can happen when both alleles are located near each other on the same chromosome and natural selection favors allele A1. If allele A1 offers an adaptive advantage, individuals with this allele may have more offspring. As a result, allele B1 may also become more common in the population simply by being linked to allele A1, even if it itself is neutral or slightly deleterious.
Hitch-hiking is more likely to occur when there is a low recombination rate between the advantageous allele and the hitch-hiking allele, allowing them to be inherited together across generations. This phenomenon can be observed in scenarios where one population (Population A) experiences gene flow, leading to the introduction of allele B, and concurrently, allele A is favored by selection within the same population. Conversely, in Population B, where genetic drift results in allele A becoming more prevalent, allele B1 can also increase in frequency if it is closely linked to allele A1.