A married couple adopted a male child. A few years later, twin boys were born to them. The blood group of the couple is AB positive and O negative. The blood group of the three sons is A positive, B positive, and O positive. The blood group of the adopted son is

1. Foundations of Mendelian Genetics (basic)

Welcome to your first step in understanding the blueprint of life! To understand how we inherit traits from our parents, we must look back at the work of Gregor Mendel. Often called the 'Father of Genetics,' Mendel was a monk who combined his love for science and mathematics to solve a mystery that had baffled humanity for centuries: how do characteristics pass from one generation to the next? While others had studied inheritance before, Mendel was the first to use statistical counting to track traits over multiple generations Science, class X (NCERT 2025 ed.), Heredity, p.130.

Mendel chose the garden pea plant (Pisum sativum) for his experiments because it had clear, contrasting characters. For instance, a pea plant was either tall or short; its seeds were either round or wrinkled; and its flowers were either violet or white Science, class X (NCERT 2025 ed.), Heredity, p.130. He discovered that these traits don't just 'blend' together like paint. Instead, they are determined by discrete 'factors' (which we now call genes). Every individual carries two copies of these factors for each trait—one inherited from each parent.

One of his most vital findings was the concept of Dominance. Mendel observed that in a pair of contrasting traits, one often masks the other. For example, if a plant has one gene for 'Tall' (T) and one for 'Short' (t), the plant will physically appear tall. In this case, 'Tall' is the dominant trait, and 'Short' is the recessive trait Science, class X (NCERT 2025 ed.), Heredity, p.130. For a recessive trait to actually show up in the physical appearance (the phenotype), the individual must possess two copies of that recessive gene (tt).

Term	Description	Example (Pea Plant)
Dominant Trait	A trait that is expressed even if only one copy of the gene is present.	Tall (T)
Recessive Trait	A trait that is expressed only when two copies of the gene are present.	Short (t)
Genotype	The internal genetic makeup (the letters).	TT, Tt, or tt
Phenotype	The external observable appearance.	Tall or Short

Sources: Science, class X (NCERT 2025 ed.), Heredity, p.130

2. Chromosomes, Genes, and Alleles (basic)

To understand how traits are passed down from parents to children, we must first look at the physical structures inside our cells. Imagine your genetic information as a massive biological library. In this library, Chromosomes are the 'books.' Instead of a single, tangled thread of DNA, our genetic material is organized into separate, independent pieces Science, class X (NCERT 2025 ed.), Heredity, p.132. Most human cells contain 23 pairs of these chromosomes—one set inherited from the mother and one from the father. This pairing is crucial because it ensures that every cell has two copies of every instruction, providing stability to the species' DNA.

A Gene is a specific section or 'instruction' within that chromosomal book. It is the fundamental unit of heredity that codes for a specific trait, such as your hair texture or your blood group. However, genes aren't exactly the same in everyone. This is where Alleles come in. An allele is simply a version or variation of a gene. For example, while the "blood type gene" exists in all humans, one person might carry an allele for 'Type A' while another carries an allele for 'Type B'.

Because we inherit one chromosome from each parent, we always possess two alleles for every gene (one maternal and one paternal). These two alleles work together to determine the final trait we see. If the two alleles are the same, the trait is straightforward; if they are different, the interaction between them determines which trait is expressed Science, class X (NCERT 2025 ed.), Heredity, p.133.

Term	Level of Detail	What it is
Chromosome	Macro	The large structure made of DNA that carries many genes.
Gene	Functional	A specific DNA sequence that controls a trait.
Allele	Variant	A specific form or "flavor" of a gene.

Sources: Science, class X (NCERT 2025 ed.), Heredity, p.132; Science, class X (NCERT 2025 ed.), Heredity, p.133

3. The Human Circulatory System & Blood Components (basic)

Hello! It is wonderful to have you here as we dive into one of the most vital systems in the human body. Imagine our body as a vast, bustling city; the circulatory system is the intricate network of highways and delivery routes that keeps everything running. It is essentially a transport system consisting of a powerful pump (the heart), a vast network of tubes (blood vessels), and a fluid medium called blood that carries life-sustaining supplies to every single cell Science-Class VII, Life Processes in Animals, p.133.

Blood is much more than just a red liquid; it is a specialized fluid connective tissue. It consists of a straw-colored liquid called plasma in which various types of cells are suspended. Each component has a highly specific job to ensure our survival:

• Plasma: This is the fluid part of the blood. It is responsible for transporting food (nutrients), salts, carbon dioxide, and nitrogenous wastes in a dissolved form Science, Class X, Life Processes, p.91.
• Red Blood Corpuscles (RBCs): These are the oxygen-carriers. They contain a pigment called hemoglobin that binds with oxygen and delivers it to tissues.
• White Blood Cells (WBCs): These are the body's primary defense force. They identify and fight off pathogens (harmful microbes) to maintain our health Science, Class VIII, Health: The Ultimate Treasure, p.45.
• Platelets: These are tiny cell fragments that act like a repair crew. If a blood vessel is damaged, platelets circulate and plug the leak by helping the blood to clot at the point of injury Science, Class X, Life Processes, p.94.

From a genetic perspective, which we will explore soon, it is important to note that the surface of our Red Blood Cells contains specific markers called antigens. These markers determine our blood group (such as A, B, AB, or O). While the system for transport is the same for everyone, these tiny biological markers on our RBCs are inherited from our parents, making our blood unique to us. Understanding these components is the first step in unraveling how traits like blood type are passed down through generations.

Component	Primary Function	Physical Form
Plasma	Transport of CO₂, nutrients, and waste	Liquid/Fluid
RBCs	Transport of Oxygen (O₂)	Cellular
Platelets	Blood clotting/Repairing leaks	Cellular fragments

Sources: Science-Class VII, NCERT, Life Processes in Animals, p.133; Science, Class X, NCERT, Life Processes, p.91; Science, Class X, NCERT, Life Processes, p.94; Science, Class VIII, NCERT, Health: The Ultimate Treasure, p.45

4. The Rh Factor and Blood Compatibility (intermediate)

The Rh factor (Rhesus factor) is an inherited protein found on the surface of red blood cells. If your blood has this protein, you are Rh positive (Rh+); if it lacks it, you are Rh negative (Rh-). While the ABO system classifies blood into four main groups, the Rh factor adds a layer of complexity, resulting in the eight common blood types we recognize today (e.g., A+, O-). In genetics, the Rh-positive trait is dominant, while the Rh-negative trait is recessive. This means an Rh-positive individual could have the genotype 'Rh+/Rh+' or 'Rh+/Rh-', whereas an Rh-negative person must be 'Rh-/Rh-'. Understanding Rh compatibility is critical for blood transfusions and pregnancy. If an Rh-negative individual is exposed to Rh-positive blood, their immune system treats the Rh protein as a foreign invader and produces antibodies to attack it. As noted in Science, Class VIII, Health: The Ultimate Treasure, p.45, the immune response is much stronger upon a second exposure to a pathogen or foreign antigen. This principle explains Erythroblastosis Fetalis (Hemolytic disease of the newborn), where an Rh-negative mother's antibodies attack the red blood cells of her Rh-positive fetus during a second or subsequent pregnancy. When we combine Rh factors with ABO inheritance, we use Mendelian principles. For instance, the ABO blood group is determined by three alleles: Iᴬ, Iᴮ, and i. As discussed in Science, Class X, Heredity, p.133, traits like blood groups are inherited through specific alleles from both parents. Because the 'i' allele (Group O) is recessive, an individual with Type AB blood (genotype IᴬIᴮ) possesses no 'i' allele to pass on. Therefore, they cannot biologically father or mother a child with Type O blood (genotype ii), regardless of the other parent's blood type.

Feature	Rh Positive (Rh+)	Rh Negative (Rh-)
Antigen Presence	D-antigen present on RBCs	D-antigen absent
Genotype	Homozygous (DD) or Heterozygous (Dd)	Homozygous recessive (dd)
Transfusion	Can receive from + or -	Can only safely receive from -

Sources: Science, Class X (NCERT 2025 ed.), Heredity, p.133; Science, Class VIII (NCERT Revised ed 2025), Health: The Ultimate Treasure, p.45

5. Multiple Allelism and Co-dominance (intermediate)

In our previous hops, we looked at how one trait typically masks another. However, nature is often more nuanced. While Mendel’s classic experiments suggested that traits are either dominant or recessive Science, Class X (NCERT 2025 ed.), Heredity, p.133, the ABO blood group system in humans introduces us to two fascinating departures: Multiple Allelism and Co-dominance.

Multiple Allelism occurs when a single gene has more than two possible variations (alleles) within a population. While an individual can only carry two alleles (one from each parent), the "menu" of choices in the human population for blood type consists of three: Iᴬ, Iᴮ, and i. This variety allows for the four different blood phenotypes we see in society: A, B, AB, and O. It is a biological reality that as people move and interact, the biological characteristics of populations can shift through the mixing of these genetic markers.

Co-dominance is observed when two different dominant alleles are present together and both are fully expressed. In the ABO system, the alleles Iᴬ and Iᴮ are both completely dominant over the recessive allele i. However, when a person inherits Iᴬ from one parent and Iᴮ from the other, neither dominates the other. Instead, they work in tandem to produce the AB blood type. This is distinct from "incomplete dominance" (where traits blend like paint); here, both markers are distinct and present on the surface of the red blood cells.

Genotype	Phenotype (Blood Group)	Genetic Relationship
IᴬIᴬ or Iᴬi	Type A	Iᴬ is dominant over i
IᴮIᴮ or Iᴮi	Type B	Iᴮ is dominant over i
IᴬIᴮ	Type AB	Co-dominance (Both expressed)
ii	Type O	Recessive homozygous

Sources: Science, Class X (NCERT 2025 ed.), Heredity, p.133

6. Inheritance Logic of ABO Blood Groups (exam-level)

To understand the inheritance of blood groups, we must first look at the concept of Multiple Allelism. While Mendel’s classic experiments often dealt with two versions of a trait (like tall or short), the human ABO blood group system is governed by three distinct versions of a gene, known as alleles: Iᴬ, Iᴮ, and i. Because humans are diploid, every individual inherits one allele from each parent, resulting in two alleles that determine their blood type Science, class X (NCERT 2025 ed.), Heredity, p.129.

The interaction between these three alleles follows specific rules of Dominance and Codominance. Alleles Iᴬ and Iᴮ are both dominant over the recessive allele i. However, when Iᴬ and Iᴮ meet, they are codominant—meaning both are expressed equally, resulting in the AB blood group. For a person to have blood group O, they must possess two copies of the recessive allele (genotype ii). This genetic mixing is why children are unique combinations of their parents' traits Science, Class VIII, NCERT (Revised ed 2025), Our Home: Earth, a Unique Life Sustaining Planet, p.222.

In a clinical or forensic context, this logic allows us to determine biological parentage. For instance, consider a parent with Type AB (genotype IᴬIᴮ) and a parent with Type O (genotype ii). The AB parent can only pass on an Iᴬ or an Iᴮ allele, while the O parent can only pass on an i allele. Consequently, their biological offspring can only have the genotypes Iᴬi (Type A) or Iᴮi (Type B). A child with Type O (genotype ii) would require an i allele from both parents—an impossibility if one parent is Type AB.

Parent 1 (AB) Alleles	Parent 2 (O) Allele: i
Iᴬ	Iᴬi (Blood Group A)
Iᴮ	Iᴮi (Blood Group B)

Sources: Science, class X (NCERT 2025 ed.), Heredity, p.129; Science, Class VIII, NCERT (Revised ed 2025), Our Home: Earth, a Unique Life Sustaining Planet, p.222

7. Solving the Original PYQ (exam-level)

This question is a classic application of Mendelian genetics and the multiple alleles system that governs human blood groups. You have just learned that the ABO blood group is determined by three alleles: I^A, I^B, and i. To solve this, you must translate the phenotypes into genotypes. An AB blood group parent has the genotype I^AI^B, meaning they can only pass on either the A or B allele. Conversely, an O blood group parent has the genotype ii and can only contribute the recessive i allele. As detailed in NCERT Class 12 Biology, the cross between I^AI^B and ii results in offspring that are either I^Ai (Blood Group A) or I^Bi (Blood Group B).

As your coach, I want you to focus on the exclusion principle. When we look at the three sons—A positive, B positive, and O positive—we immediately see that the A and B children fit the biological profile of the parents. However, the child with O positive blood group presents a genetic impossibility for this couple. Because the AB parent must contribute either an A or a B allele, they can never produce an O child (who requires two i alleles, one from each parent). Therefore, the O positive son is the adopted child. Don't let the Rh factor (positive/negative) distract you; while an AB+ and O- couple could produce Rh-positive children if the AB+ parent carries the dominant Rh allele, the ABO group itself provides the definitive proof needed here.

UPSC often uses options like "Cannot be determined" to tempt students who feel uncertain about the Rh factor or the Bombay Phenotype exceptions. Option (D) is a trap designed to make you overthink. Options (B) and (C) are incorrect because those blood groups are the expected biological outcomes for this specific parental cross. By identifying that a parent with AB blood can never have a biological child with O blood, you cut through the complexity to find that (A) O positive is the only logical answer.