When two crossovers occur between two genetic loci, it can give the appearance that none have taken place, resulting in undercalculation of the recombination frequency. this is known as a(n): ?

The process is known as double cross-over. It is the two chromosomal cross-overs, in a region of study, as a result of which t appears that no crossover has taken place, and leads to the miscalculation of a recombination frequency. 

Wikipedia aswell can be used to solve the following questions