28. Ratio of observed double crossover frequency to the expected frequency of double crossover gamete during three point test cross is termed as-
(1) Coefficient of interference
(2) Coefficient of Coincidence
(3) Coefficient of variance
(4) Coefficient of suppression

Introduction:
In genetics, specifically during a three-point test cross, the ratio of the observed double crossover frequency to the expected double crossover frequency is known as the coefficient of coincidence. This concept helps in understanding how crossover events influence each other during meiosis, which is crucial for gene mapping and recombination studies.

Explanation of options:

(1) Coefficient of interference:
This term describes how one crossover event affects the occurrence of another nearby crossover. It is calculated as 1 minus the coefficient of coincidence and reflects the degree to which one crossover suppresses others. Positive interference occurs when a crossover reduces the likelihood of another nearby crossover.

(2) Coefficient of Coincidence:
This is the correct answer for the question. It is defined as the ratio of the observed frequency of double crossovers to the expected frequency of double crossovers in a three-point test cross. It quantifies how actual crossover events match with those predicted by independent assortment.

(3) Coefficient of variance:
This term is unrelated to genetic crossovers. It is a statistical measure that shows the extent of variability relative to the mean of a data set, commonly used in quality control and statistics.

(4) Coefficient of suppression:
This is not a standard term used in genetics related to crossover frequencies or recombination. It does not apply to the context of three-point test crosses or crossover interference.

By calculating the coefficient of coincidence, geneticists can infer how closely linked genes are and how crossover events affect genetic variation. A coefficient less than 1 indicates positive interference, meaning fewer double crossovers than expected, while a value greater than 1 indicates negative interference, where more double crossovers occur than predicted.

This understanding forms the basis for constructing accurate genetic linkage maps and studying genome organization.

References:

• The coefficient of coincidence is the ratio of observed double recombinants to expected double recombinants, used to calculate interference (interference = 1 – coefficient of coincidence).

• Positive interference reduces the probability of nearby crossovers, and the coefficient of coincidence measures this effect.