Final answer:
For a double exchange to be studied, three gene pairs associated with two alleles must be investigated in the field of genetics, particularly in studying gene linkage and recombination.
Step-by-step explanation:
For a double exchange to be studied, three gene pairs coding for two alleles must be investigated. This refers to the study of gene linkage and genetic recombination through which the arrangement and interaction of alleles for different genes on the same chromosome can be explored. The concept was extensively studied by Thomas Hunt Morgan and is a foundational aspect of genetics.
When examining the inheritance pattern of linked and unlinked genes through a test cross, one would observe different phenotypic ratios based on whether the genes assort independently or are linked (and potentially undergo double exchange or crossing over). An example of this is Mendel's dihybrid cross, which produces a phenotypic ratio that typically totals 16, indicative of a two-gene interaction. However, with gene linkage, the expected phenotypic ratios would deviate from Mendel's 9:3:3:1 due to the genes being physically close on the same chromosome and therefore inheriting together unless a double exchange (or crossover) occurred.