35. A Variable Number of Tandem Repeats (VNTR) locus has 15 different alleles. The number of genotypes possible in a population for this VNTR is ________.
Variable Number of Tandem Repeats (VNTR): Calculating the Total Number of Genotypes for Multiple Alleles
Introduction
Variable Number of Tandem Repeats (VNTRs) are highly polymorphic DNA sequences consisting of short nucleotide motifs repeated consecutively at a specific chromosomal location. Different individuals possess different numbers of repeat units, making VNTR loci extremely useful for DNA fingerprinting, forensic investigations, paternity testing, population genetics, and biodiversity studies. Because numerous alleles may exist at a single VNTR locus, these markers exhibit a very high level of genetic variation.
Unlike simple Mendelian traits involving only two alleles, VNTR loci often possess many alleles within a population. Consequently, the number of possible genotypes increases rapidly as the number of alleles increases.
Correct Answer
Correct Answer: 120
Detailed Explanation
When a gene possesses multiple alleles, the total number of possible genotypes consists of two categories:
- Homozygous genotypes, where both alleles are identical.
- Heterozygous genotypes, where the two alleles are different.
If a locus contains n alleles, the total number of possible genotypes is calculated using the formula:
Total Genotypes = n(n + 1) / 2
This formula includes both homozygous and heterozygous combinations.
Step-by-Step Calculation
Step 1: Identify the Number of Alleles
The VNTR locus contains:
n = 15 alleles
Step 2: Apply the Formula
Total Genotypes = n(n + 1) / 2
= 15 × 16 / 2
= 240 / 2
= 120
Therefore, a VNTR locus with 15 alleles can produce 120 different genotypes.
Alternative Method
The same answer can be obtained by calculating homozygous and heterozygous genotypes separately.
Homozygous Genotypes
Each allele can form one homozygous genotype.
Number of homozygous genotypes = 15
Heterozygous Genotypes
The number of ways to choose two different alleles from fifteen is:
15C2 = (15 × 14) / 2 = 105
Total Genotypes
Total = Homozygous + Heterozygous
= 15 + 105
= 120
Both methods produce the same result.
Calculation Summary
| Parameter | Value |
|---|---|
| Number of Alleles | 15 |
| Homozygous Genotypes | 15 |
| Heterozygous Genotypes | 105 |
| Total Genotypes | 120 |
Formula for Multiple Alleles
| Formula | Purpose |
|---|---|
| n | Number of homozygous genotypes |
| nC2 = n(n−1)/2 | Number of heterozygous genotypes |
| n(n+1)/2 | Total number of genotypes |
Examples of Genotype Calculation
| Number of Alleles (n) | Total Genotypes |
|---|---|
| 2 | 3 |
| 3 | 6 |
| 4 | 10 |
| 10 | 55 |
| 15 | 120 |
What Makes VNTRs Highly Polymorphic?
VNTR loci contain tandemly repeated DNA sequences whose repeat number varies among individuals. Because many repeat-length variants exist in a population, numerous alleles can occur at a single locus. This high allelic diversity greatly increases the number of possible genotypes, making VNTRs excellent molecular markers for distinguishing individuals.
Applications of VNTR Analysis
| Application | Importance |
|---|---|
| DNA Fingerprinting | Individual identification |
| Forensic Science | Crime investigation |
| Paternity Testing | Biological relationship analysis |
| Population Genetics | Study of genetic diversity |
| Conservation Biology | Assessment of genetic variation |
Biological Significance
VNTR markers provide an exceptional level of genetic variation because they possess multiple alleles at a single chromosomal locus. This variability enables researchers to identify individuals with high accuracy, investigate evolutionary relationships, estimate genetic diversity, study inheritance patterns, and construct population genetic databases. VNTRs have become indispensable tools in molecular biology, forensic science, medical genetics, and biodiversity research.
Final Answer
Number of alleles = 15
Total genotypes = n(n + 1)/2
= 15 × 16 / 2
= 120
Correct Answer: 120
