Model Answer
0 min readIntroduction
The ABO blood group system, discovered by Karl Landsteiner in 1901, remains a cornerstone of human genetics and has historically played a significant role in forensic science, particularly in disputed paternity cases. This system classifies individuals into four main blood types: A, B, AB, and O, based on the presence or absence of specific antigens on red blood cells. Inheritance follows Mendelian principles, with each child inheriting one allele from each parent. While not definitive proof of paternity, incompatible blood types can definitively *exclude* a man from being the father of a child. This answer will discuss the principles and limitations of using the ABO blood group system in resolving paternity disputes, alongside the increasing prevalence of DNA profiling.
Understanding the ABO Blood Group System and Inheritance
The ABO blood group system is determined by three alleles: *IA*, *IB*, and *i*. *IA* and *IB* are co-dominant, while *i* is recessive. Individuals inherit two alleles, one from each parent. The possible genotypes and phenotypes are:
- *IAIA*: Blood type A
- *IAi*: Blood type A
- *IBIB*: Blood type B
- *IBi*: Blood type B
- *IAIB*: Blood type AB
- *ii*: Blood type O
For example, if a mother has blood type A (*IAi*) and a man claims paternity, the child’s blood type can be determined. If the child has blood type O (*ii*), and the man has blood type A (*IAIA* or *IAi*), the man *cannot* be the father, as he lacks the *i* allele necessary for the child to inherit type O.
Role in Disputed Paternity: Exclusionary Evidence
The ABO blood group system's primary utility in paternity cases lies in its ability to *exclude* a potential father. A man can only be excluded if he lacks a necessary allele for the child’s blood type. Let's illustrate with a table:
| Child's Blood Type | Mother's Possible Genotypes | Father's Genotype Required for Compatibility | Father's Genotype that Excludes Paternity |
|---|---|---|---|
| A | IAIA or IAi | IAIA or IAi | i |
| B | IBIB or IBi | IBIB or IBi | i |
| AB | IAi or IAIB | IBi or IAIB | ii |
| O | IAi or IBi | ii | IA, IB, or IAIB |
Limitations and Error Rates
Despite its usefulness, the ABO blood group system has significant limitations:
- Inability to Prove Paternity: It can only exclude, not confirm. Compatibility does not equal paternity.
- Rare Blood Types: In cases involving rare blood types, the likelihood of exclusion is reduced.
- Technical Errors: Errors in blood typing, though rare, can occur due to improper technique or reagent issues. The accuracy of ABO typing is generally 99.9%, but this can be affected by factors like acquired B phenomenon.
- Mixed Parentage: In cases involving multiple potential fathers, the system provides limited information.
- Chimerism: Extremely rare conditions like chimerism (an individual having two or more genetically distinct cell populations) can complicate interpretations.
The Rise of DNA Profiling
DNA profiling, or deoxyribonucleic acid profiling, has largely superseded ABO blood typing in paternity cases due to its significantly higher accuracy and ability to provide conclusive evidence. DNA profiling analyzes highly variable regions of DNA (Short Tandem Repeats – STRs) to create a unique genetic "fingerprint" for each individual. The probability of paternity using DNA profiling is extremely high, often exceeding 99.999%. India's DNA Technology (Use and Application) Act, 2019 regulates the use of DNA technology, including its application in paternity determination.
Case Study: The Leopold Bloom Case
Case Study: Leopold Bloom Case (Hypothetical)
A woman initiates a paternity suit against a man claiming he is the father of her child. The child has blood type O. The mother has blood type A. The alleged father has blood type AB. Blood typing reveals incompatibility: the father lacks the ‘i’ allele required for the child’s type O blood. This evidence conclusively excludes the man from being the child’s father. However, further investigation using DNA profiling is still recommended to confirm the exclusion.
FAQ
FAQ: What is the difference between blood typing and DNA profiling in paternity testing?
Answer: Blood typing (ABO system) can only exclude a potential father, while DNA profiling can definitively prove or disprove paternity with a very high degree of certainty. DNA profiling analyzes genetic markers that are much more variable than blood type alleles.
Conclusion
The ABO blood group system provided an early and accessible tool for resolving disputed paternity cases, primarily through exclusion. However, its limitations in proving paternity and susceptibility to errors have led to its gradual replacement by more accurate DNA profiling techniques. While still relevant for understanding basic inheritance principles, the role of ABO blood typing in paternity determination is now largely historical, with DNA technology providing definitive and reliable results. The legal and ethical considerations surrounding genetic testing continue to evolve, emphasizing the importance of responsible application of these powerful tools.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.