Molecular markers — What best defines a restriction fragment length polymorphism (RFLP) in genomic DNA?

Introduction:
RFLPs were among the first DNA markers used for genetic mapping and linkage analysis. This question asks for the precise definition that distinguishes an RFLP from other molecular biology facts or artifacts.

Given Data / Assumptions:

• Restriction enzymes cut DNA at specific recognition sites.
• Genetic variation can create or abolish these sites.
• Southern blotting historically detected fragment length differences.

Concept / Approach:
An RFLP is a heritable difference in the pattern of restriction fragments caused by sequence polymorphisms that create or destroy restriction sites. The outcome is a change in fragment lengths after digestion with a specific enzyme, observable as altered band sizes on a gel or blot.

Step-by-Step Solution:

Verification / Alternative check:
Classic RFLP mapping correlates Mendelian inheritance of band sizes with traits, confirming the polymorphic restriction site as the source of length differences.

Why Other Options Are Wrong:

• Different enzymes cutting different sequences (EcoRI vs HindIII): that is enzyme specificity, not polymorphism.
• Cystic fibrosis mutation spectrum: a disease example, not a definition of RFLP; some mutations may create/abolish sites, but the statement does not define RFLP.
• All of the above: overinclusive; only one option is the correct definition.
• Band intensity differences from loading: experimental artifact, not genetic polymorphism.

Common Pitfalls:
Confusing general mutation examples or enzyme properties with the specific concept of polymorphic restriction sites causing length variation.

Final Answer:
Different length DNA fragments among individuals due to loss or gain of a restriction site.