27. The ratio of observed number of double crossover to expected number of cross over is termed as
(1) Coefficient of coincidence
(2) Coefficient of interference
(3) Hitch hiking
(4) Linkage suppression

Introduction:
In genetics, studying how often crossovers occur between genes is crucial for mapping and understanding inheritance. One key concept is the ratio of the observed number of double crossovers to the expected number of double crossovers, which helps measure crossover interference. This ratio is known as the coefficient of coincidence. This article explains the meaning of this ratio and clarifies related terms to help students and educators grasp these fundamental genetics concepts.

Explanation of the Options:

1. Coefficient of coincidence:
   The coefficient of coincidence (COC) is defined as the ratio of the observed number of double crossovers to the expected number of double crossovers. It quantifies the actual occurrence of double recombinants relative to the theoretically expected occurrence if crossovers were independent events. The formula is:

COC=Observed double crossoversExpected double crossoversCOC=Expected double crossoversObserved double crossovers

If the COC is less than 1, it indicates crossover interference, meaning a crossover in one region reduces the chance of another nearby. This measure is fundamental in genetic linkage analysis.

2. Coefficient of interference:
   The coefficient of interference (I) measures the extent to which one crossover event inhibits another nearby crossover. It is calculated as:

$$I=1-\text{COC}$$

A positive interference (I between 0 and 1) means fewer double crossovers occur than expected, while a negative interference (I < 0) means more double crossovers than expected.

3. Hitchhiking:
   Hitchhiking in genetics refers to the phenomenon where an allele increases in frequency because it is linked to another gene under positive selection. It does not relate to crossover ratios or double crossover observations.

4. Linkage suppression:
   Linkage suppression refers to the reduction or absence of recombination between linked genes, but it is a broader term and does not specifically describe the ratio of observed to expected double crossovers.

Correct Answer:
The ratio of the observed number of double crossovers to the expected number of double crossovers is termed the coefficient of coincidence. Therefore, the correct answer is option (1) Coefficient of coincidence.​

This understanding is essential for studying recombination and genetic interference during meiosis, which aids in constructing genetic maps and interpreting gene linkage data.