Final answer:
A three-point testcross is used to determine the recombination frequency between three genes by crossing an organism with unknown genotype expressing dominant phenotypes for three traits with a triple homozygous recessive organism. This method helps construct genetic linkage maps.
Therefore, option B is correct.
Step-by-step explanation:
The purpose of a three-point testcross is to determine the recombination frequency between three genes. In this genetic analysis, an organism expressing the dominant phenotypes (but with an unknown genotype) for three different genes is crossed with a triple homozygous recessive organism (known genotype). By analyzing the phenotypes of the offspring, geneticists can infer the positions of genes on a chromosome and the distances between them. This technique is instrumental in constructing genetic linkage maps, which depict the order of genes and the relative distances between them on chromosomes.
A three-point testcross goes beyond the simpler test cross, which typically involves crossing an individual expressing a dominant trait (but of unknown genotype) with a homozygous recessive individual. This helps to determine if the dominant-expressing organism is heterozygous or homozygous for the trait. The test cross is a fundamental principle in Mendelian genetics that validates Gregor Mendel's postulate that pairs of unit factors (genes) segregate equally.