In classical genetics, one map unit (one centiMorgan) between two genes is equivalent to what?

Introduction / Context:
Genetic mapping allows scientists to estimate the relative positions of genes on chromosomes based on how often they are inherited together. Early geneticists used the concept of recombination frequency to build linkage maps. To express distances on these maps, they introduced a unit called the map unit or centiMorgan. This question asks you to recall what one map unit corresponds to in terms of recombination between genes, which is a fundamental idea in classical genetics and linkage analysis.

Given Data / Assumptions:

• The question focuses on the definition of a map unit in genetic mapping.
• The options mention recombination frequency, physical distance, and specific nanometer values.
• We assume classical linkage mapping using crossing over in meiosis.
• We recognise that recombination frequency is measured as a percentage of recombinant offspring.

Concept / Approach:
A map unit, also called a centiMorgan (cM), is a unit of genetic distance. It is defined operationally, not as a fixed physical distance, but in terms of recombination: one map unit corresponds to a 1 percent recombination frequency between two loci. That means that in a large number of offspring, 1 percent show a recombinant combination of traits due to crossing over between those loci. Physical distance in base pairs varies depending on the region of the chromosome and does not have a simple fixed nanometer equivalent for one centiMorgan. Therefore, the correct approach is to choose the option that equates one map unit with a 1 percent recombination frequency.

Step-by-Step Solution:
Step 1: Recall that recombination frequency is calculated as the number of recombinant offspring divided by the total number of offspring, multiplied by 100 to get a percent. Step 2: Remember that in classical mapping, if two genes show a 1 percent recombination frequency, they are said to be one map unit or one centiMorgan apart. Step 3: Recognise that this definition is statistical and based on observed genetic data, not on direct physical measurements like nanometers or base pairs. Step 4: Understand that physical distance corresponding to one centiMorgan can differ substantially between species and between different parts of the same chromosome. Step 5: Evaluate the options and identify the one that directly states the 1 percent recombination frequency relationship, without adding misleading physical distance claims. Step 6: Conclude that one map unit equals a 1 percent recombination frequency between two genes.

Verification / Alternative check:
Genetics textbooks and many exam questions define a centiMorgan explicitly as the distance between two gene loci that results in a 1 percent recombination frequency. They often state that 1 centiMorgan is equivalent to 1 map unit and is associated with a 1 percent chance of crossing over between two loci per meiosis. Discussions of physical distance typically emphasise that there is no fixed conversion, because recombination rates vary along chromosomes. This consistent definition across sources supports choosing the recombination frequency option as the correct answer.

Why Other Options Are Wrong:
The physical distance between a pair of homologous chromosomes is a vague description and does not match the precise definition of a map unit. Homologous chromosomes contain many genes and large regions, far more than one map unit, so this option is incorrect.

Exactly both a 1 percent recombination frequency and a fixed nanometer distance is misleading because while one map unit does equal a 1 percent recombination frequency, it does not correspond to a single universal physical distance in nanometers, which can vary greatly, so this combined statement is not correct.
One nanometer of physical distance between two genes suggests a fixed physical separation for each map unit, which is not true. Genetic maps are based on recombination rates, not on a uniform nanometer scale across the genome, making this option wrong.

Common Pitfalls:
A common mistake is to assume that genetic distance and physical distance must have a simple, fixed numerical relationship, leading to confusion between base pair or nanometer measurements and recombination based units. Another pitfall is to focus on the word distance and forget that in genetics, map distance is defined statistically. To avoid these errors, remember that map units and centiMorgans are defined by how often recombination occurs, and that physical distance can vary even when genetic distance is the same.

Final Answer:
In classical genetics, one map unit (one centiMorgan) between two genes is equivalent to a 1 percent recombination frequency between those genes.