In a testcross involving two linked genes, 18 out of 100 offspring are recombinant. What is the genetic distance between the two loci?
A0.18 cM
B1.8 cM
C18 cM
D82 cM
Recombination frequency = (recombinant offspring / total offspring) × 100 = (18/100) × 100 = 18%. By definition, 1 cM corresponds to 1% recombination, so the genetic distance is 18 cM. Options A and B confuse the percentage conversion; option D confuses recombinants with parental-type offspring.
Question 2 True / False
If two genes show a 50% recombination frequency in a testcross, they is expected to be located on different chromosomes.
TTrue
FFalse
Answer: False
50% recombination means the two genes assort independently — but this can happen either because they are on different chromosomes OR because they are very far apart on the same chromosome (with so many crossovers between them that recombinant and parental gametes are equally likely). The 50% value is a ceiling imposed by random crossover placement, not proof of separate chromosomes.
Question 3 Short Answer
Why is the maximum measurable recombination frequency between two loci 50%, regardless of how far apart they actually are on a chromosome?
Think about your answer, then reveal below.
Model answer: When two loci are very far apart, crossovers occur so frequently between them that every gamete has roughly equal chances of being recombinant or parental. Multiple crossovers between the loci cancel each other out statistically, capping the observable recombination frequency at 50% — identical to what you would see for unlinked genes.
This 50% ceiling is why genetic distances estimated from two-point crosses become inaccurate for loci far apart: double (and higher-order) crossovers produce parental-type gametes and are therefore invisible in a simple recombination count. Three-point crosses and mapping functions like Haldane's or Kosambi's correct for this by accounting for the probability of multiple crossovers.