Final answer:
The recombination frequency is a measure that indicates the likelihood of a crossover event between two genes on the same chromosome during meiosis; it's exemplified by Thomas Morgan's experiments with a 17 percent observed frequency for fruit flies. Option 3.
Step-by-step explanation:
The recombination frequency in genetics is a measure of the likelihood of a crossover event between two alleles that are on the same chromosome during meiosis.
A lower recombination frequency, such as the observed 17 percent example from Thomas Morgan's experiments with fruit flies, indicates that the genes are linked and that crossovers happen occasionally but not every time.
Sturtevant introduced the concept of map units or centimorgans (cM), equating 1 cM to a 1 percent recombination frequency, to predict genetic distances on a chromosome.
If the recombination frequency is less than 50 percent, it demonstrates linkage, which means the genes reside on the same chromosome and crossover sometimes occurs.
But if the recombination frequency is 50 percent or greater, this indicates that the genes are on separate chromosomes or are so far apart on the same chromosome that they assort independently during meiosis.
With the example given, if we calculate the recombination frequency as 50/1000, this would be 0.05, or 5 percent, which is well below the 50 percent threshold for genes to be considered linked.
Recombination frequencies such as 12, 25, 50, and 75 are possible, but when context is given, such as in Thomas Morgan's work, we can determine exact percentages that represent the genetic distance on a chromosome map.
So Option 3.