Biology · Molecular Basis of Inheritance · NEET
DNA polymorphism is a change (mutation) in a DNA sequence that is passed on to the next generation and is present in a population at a frequency greater than 0.01. In simple words, it means the same spot (locus) in the DNA has more than one version (allele) among people. NCERT calls a variation a DNA polymorphism only if more than one variant occurs at a locus with frequency greater than 0.01. Most of this variation lies in the non-coding, repetitive parts of DNA, so it usually does not change the phenotype but is very useful to identify a person.
VNTR stands for Variable Number of Tandem Repeats. It is a short DNA sequence that is repeated one after another (tandemly) many times. The word 'variable number' is the key: the copy number of the repeat is different from chromosome to chromosome and from person to person. This makes VNTR show a very high degree of polymorphism, which is why it is used in DNA fingerprinting.
Both are correct, but be precise. NCERT says the VNTR belongs to a class of satellite DNA that is called mini-satellite. So satellite DNA is the big family, and mini-satellite is the specific type to which VNTR belongs. In NEET, if the question asks the exact class, the answer is mini-satellite.
About 99.9 per cent of the base sequence is the same in all humans, so normal genes are almost identical and cannot tell two people apart. VNTR repeat numbers, however, vary a lot between people (the size of a VNTR ranges from 0.1 to 20 kb). This high variation gives each person a nearly unique pattern of band sizes after hybridisation with a VNTR probe, so VNTR is ideal for identifying individuals.
Every polymorphism starts as a mutation, but not every mutation is a polymorphism. A mutation is any change in DNA. It becomes a DNA polymorphism only when it is inheritable (passed through germ cells) AND is present in the population at a frequency greater than 0.01. So polymorphism is a common, heritable, population-level variation, while a mutation can be rare or even present in a single individual.
DNA polymorphism in repetitive DNA (specifically VNTR) forms the basis of DNA fingerprinting. NEET 2022 asked exactly this: 'DNA polymorphism forms the basis of:' and the official answer is DNA fingerprinting. NCERT also states that polymorphism is the basis of genetic mapping of the human genome as well as of DNA fingerprinting, so read the options carefully.
DNA polymorphism forms the basis of:
DNA fingerprinting involves identifying differences in some specific regions in DNA sequence, called as:
Which of the following is not required for any of the techniques of DNA fingerprinting available at present?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
VNTR stands for Variable Number of Tandem Repeats. It is a short DNA sequence repeated one after another (tandemly), and the number of repeats varies between individuals.
According to NCERT, the size of a VNTR varies from 0.1 kb to 20 kb. This large variation in size is what produces the different band patterns seen in DNA fingerprinting.
VNTR belongs to a class of satellite DNA called mini-satellite. A small DNA sequence is arranged tandemly in many copy numbers, and this copy number varies from chromosome to chromosome.
A variation is called a DNA polymorphism when more than one variant (allele) at a locus occurs in the population with a frequency greater than 0.01, and the variation is inheritable.
The identification uses the 0.1 percent that differs. This difference is greatest in repetitive VNTR regions, where the number of repeats is nearly unique in each person, giving each individual a distinct pattern.
