Which one of the following are the possible blood groups of the offspring of the parents with blood group 0 and AB?

1. Basics of Mendelian Genetics (basic)

To understand how we inherit traits, we must look back at the work of Gregor Mendel, often called the 'Father of Genetics.' In the mid-19th century, Mendel blended his knowledge of science and mathematics to study the inheritance of traits in garden peas. He specifically chose plants with clear, contrasting visible characters — such as tall versus short plants or round versus wrinkled seeds Science, Class X (NCERT 2025 ed.), Heredity, p.130. His breakthrough was realizing that traits are not simply 'mixed' like paint; instead, they are determined by discrete 'factors' (which we now call genes) that remain distinct even when passed through generations.

The foundational principle of Mendelian genetics is that each individual possesses two sets of genes for every trait — one inherited from the mother and one from the father Science, Class X (NCERT 2025 ed.), Heredity, p.131. These different versions of the same gene are called alleles. When an organism has two different alleles for a trait, one often masks the expression of the other. The allele that expresses itself is called dominant, while the one that remains hidden (unless paired with another like itself) is recessive.

How do these genes move from parent to child? This happens through germ cells (sperm and egg). While normal body cells have two copies of every gene, germ cells undergo a special process to ensure they carry only one gene set. This is crucial: if a child inherited two whole sets from each parent, the number of genes would double every generation! Instead, when a sperm and egg fuse during fertilization, they restore the normal double set of genes in the offspring, creating a unique combination of the parents' traits Science, Class X (NCERT 2025 ed.), Heredity, p.131.

Term	Definition
Genotype	The genetic makeup or the specific combination of alleles (e.g., Tt).
Phenotype	The observable physical characteristic (e.g., Tallness).
Homozygous	Having two identical alleles for a trait (e.g., TT or tt).
Heterozygous	Having two different alleles for a trait (e.g., Tt).

Sources: Science, Class X (NCERT 2025 ed.), Heredity, p.130; Science, Class X (NCERT 2025 ed.), Heredity, p.131

2. Genotype vs. Phenotype (basic)

In genetics, understanding the difference between the Genotype and the Phenotype is like distinguishing between a secret recipe and the finished cake. To master this, we must look at how instructions are written in our DNA versus how they are actually expressed in our bodies.

The Genotype is the complete set of genetic instructions an organism carries. It refers to the specific combination of alleles (different versions of a gene) inherited from parents. For instance, in Gregor Mendel's famous experiments, a pea plant might inherit a 'T' allele for tallness from one parent and a 't' allele for shortness from the other. Its genotype would be Tt. While we cannot see the genotype with our naked eyes, it remains the permanent "blueprint" inside every cell Science, Class X (NCERT 2025 ed.), Heredity, p.130.

On the other hand, the Phenotype is the observable physical characteristic or trait—what you actually see. This includes height, eye color, or blood type. Interestingly, the phenotype is the result of the genotype interacting with the environment. Because some traits are dominant, different genotypes can sometimes produce the exact same phenotype. For example, both 'TT' (pure tall) and 'Tt' (hybrid tall) genotypes result in a plant that looks tall. Only when the genotype is 'tt' does the short phenotype appear Science, Class X (NCERT 2025 ed.), Heredity, p.130.

Feature	Genotype	Phenotype
Definition	The genetic makeup or "code" (e.g., Iᴬi, TT, tt).	The observable physical trait (e.g., Blood Type A, Tall, Short).
Visibility	Internal; determined by DNA sequencing.	External; determined by observation.
Inheritance	Passed directly from parents to offspring.	Not inherited directly; it is developed based on the genotype.

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

3. Composition of Human Blood (basic)

To understand how genetics influences our traits, we must first understand the medium that carries many of those traits: human blood. Blood is much more than just a red liquid; it is a complex fluid connective tissue that acts as the body's primary delivery and waste-removal system Science, Class X, Life Processes, p.91. It consists of a straw-colored liquid medium called plasma, in which various specialized cells are suspended. While the heart acts as the pump, blood serves as the vehicle reaching every corner of the body to sustain life.

The composition of blood can be broadly divided into two parts: the fluid plasma and the cellular components (corpuscles). Plasma makes up the bulk of blood volume and is responsible for transporting food (nutrients), salts, carbon dioxide, and nitrogenous wastes in dissolved form. Suspended within this fluid are the Red Blood Corpuscles (RBCs), which contain haemoglobin. This iron-rich protein is vital because it binds with oxygen to carry it to tissues Science, Class X, Life Processes, p.91. Interestingly, haemoglobin levels are not uniform; they vary between children and adults, and typically show differences between men and women due to physiological factors Science, Class X, Life Processes, p.91.

Beyond transport, blood has a sophisticated defense and repair mechanism. White Blood Cells (WBCs) function as the body's internal army, fighting off infections. Meanwhile, platelets act as a specialized repair crew. If a blood vessel is injured, platelets circulate to the site and plug the leak by helping the blood to clot, preventing excessive blood loss and maintaining the integrity of the circulatory system Science, Class X, Life Processes, p.94.

Component	Primary Function
Plasma	Transports nutrients, CO₂, and nitrogenous wastes in dissolved form.
RBCs	Carries oxygen (via haemoglobin) to all body cells.
Platelets	Clots blood at the site of injury to prevent leakage.

Sources: Science, class X (NCERT 2025 ed.), Life Processes, p.91; Science, class X (NCERT 2025 ed.), Life Processes, p.94

4. Immune System: Antigens and Antibodies (intermediate)

To understand genetics and evolution, we must first understand how our body identifies 'self' from 'non-self' at a molecular level. This is the domain of the immune system. An Antigen is essentially a molecular 'ID card' or marker, usually a protein or polysaccharide, found on the surface of cells, viruses, or bacteria. When your body detects a foreign antigen, it triggers an immune response. This is why when the body encounters a pathogen for the first time, the response is initially low, but the system 'remembers' it. Upon a second exposure, the response is far more rapid and powerful (Science, Class VIII NCERT, Health: The Ultimate Treasure, p.45). In response to these foreign antigens, specialized white blood cells produce Antibodies (also known as immunoglobulins). These are Y-shaped proteins that circulate in the blood plasma (Science, Class X NCERT, Life Processes, p.91). Each antibody is highly specific to a particular antigen—a concept often called the 'lock-and-key' mechanism. The antibody binds to the antigen, effectively 'tagging' the invader for destruction by other immune cells or neutralizing it directly. From a genetic perspective, these markers are inherited. A classic example is the ABO Blood Group system. Here, antigens are located on the surface of Red Blood Corpuscles (RBCs), while the corresponding antibodies are found in the plasma (Science, Class X NCERT, Life Processes, p.91).

Feature	Antigen	Antibody
Nature	Usually proteins/sugars on cell surfaces.	Proteins (Immunoglobulins) in plasma.
Function	Acts as a label or 'trigger' for the immune system.	Identifies and neutralizes foreign objects.
Location	Found on pathogens or Red Blood Cells.	Produced by B-cells; circulate in blood/fluids.

It is also important to note that environmental factors can influence this delicate system. For instance, excessive UV-B radiation has been shown to decrease the immune response to certain antigens, potentially increasing susceptibility to infectious diseases and skin cancers (Environment, Shankar IAS Academy, Ozone Depletion, p.271).

Sources: Science, Class VIII NCERT (Revised ed 2025), Health: The Ultimate Treasure, p.45; Science, Class X NCERT (2025 ed.), Life Processes, p.91; Environment, Shankar IAS Academy (10th ed.), Ozone Depletion, p.271

5. The Rh Factor and Blood Compatibility (intermediate)

While the ABO system identifies the sugars on your red blood cells, the Rh Factor (Rhesus factor) identifies a specific protein, known as the D antigen. If this protein is present on the surface of your red blood cells, you are Rh-positive (Rh+); if it is absent, you are Rh-negative (Rh-). This distinction is vital because blood is a fluid connective tissue that facilitates the transport of oxygen and nutrients throughout the body, and the immune system must recognize these cells as 'self' rather than 'foreign' Science, class X (NCERT 2025 ed.), Life Processes, p.91.

From a genetic perspective, the Rh factor follows a simple Mendelian inheritance pattern where the Rh-positive trait is dominant and the Rh-negative trait is recessive. A person only needs one 'Rh-positive' allele to express the protein. This aligns with Mendel's experiments showing that certain traits can mask others in the phenotype Science, class X (NCERT 2025 ed.), Heredity, p.133. Therefore, a couple who are both Rh-positive can still have an Rh-negative child if both parents carry the hidden recessive allele (genotype Rr).

Feature	Rh-Positive (Rh+)	Rh-Negative (Rh-)
Antigen Present	D Antigen present on RBCs	No D Antigen
Genotype	Dominant (RR or Rr)	Recessive (rr)
Can Receive From	Rh+ and Rh-	Only Rh-

In clinical practice, blood compatibility is strictly monitored because an Rh-negative individual's immune system will produce antibodies against Rh-positive blood, leading to a potentially fatal immune reaction. This is particularly significant during pregnancy. If an Rh-negative mother carries an Rh-positive fetus, her body may develop antibodies during the first birth that could attack the red blood cells of a subsequent Rh-positive fetus—a condition known as Erythroblastosis Fetalis. Understanding these genetic markers is essential for modern maternal healthcare and safe blood transfusions.

Sources: Science, class X (NCERT 2025 ed.), Life Processes, p.91; Science, class X (NCERT 2025 ed.), Heredity, p.133

6. Codominance and Multiple Alleles (intermediate)

In our previous discussions on Mendelian genetics, we often looked at traits governed by two clear-cut options: a dominant version and a recessive version, like the violet and white flowers seen in Mendel's experiments Science, class X (NCERT 2025 ed.), Heredity, p.133. However, nature is often more nuanced. Multiple Allelism occurs when more than two alleles exist for a single gene within a population. While an individual can only carry two alleles (one from each parent), the ABO blood group system in humans is a classic example where three alleles — Iᴬ, Iᴮ, and i — are at play.

This brings us to a fascinating departure from Mendelian dominance called Codominance. In codominance, two different alleles are expressed simultaneously and fully, without one masking the other. In the ABO system, alleles Iᴬ and Iᴮ are both dominant over the recessive allele i. However, when a person inherits both Iᴬ and Iᴮ, they don't produce a 'blend' or show just one; instead, both antigens appear on the surface of their red blood cells, resulting in the AB blood group.

To understand how these interact in a family, consider the following table of genotypes and their resulting phenotypes:

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

This genetic arrangement creates unique inheritance patterns. For instance, if a parent with blood group O (genotype ii) and a parent with blood group AB (genotype IᴬIᴮ) have children, the results are often surprising to students. The O parent can only pass on the i allele, while the AB parent passes on either Iᴬ or Iᴮ. Consequently, the children will have genotypes Iᴬi (Type A) or Iᴮi (Type B). Ironically, in this specific cross, the children cannot have the blood group of either parent!

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

7. Mechanics of ABO Inheritance (exam-level)

In our journey through genetics, we've seen how Mendel’s peas usually followed a 'dominant or recessive' pattern. However, human blood groups offer a more sophisticated look at inheritance. The ABO blood group system is governed by three alleles: Iᴬ, Iᴮ, and i. While each of us only carries two of these (one from each parent), the interaction between these three determines our blood type. This is a classic example of Multiple Allelism and Codominance Science, class X (NCERT 2025 ed.), Heredity, p.131.

The 'i' allele is recessive, while 'Iᴬ' and 'Iᴮ' are dominant over 'i'. This means if you inherit one 'Iᴬ' and one 'i', your blood type is A. However, when 'Iᴬ' and 'Iᴮ' meet, neither can suppress the other. Instead, they are both expressed equally—a phenomenon called Codominance—resulting in the AB blood group. This deviates from the simple dominance Mendel first observed in pea height Science, class X (NCERT 2025 ed.), Heredity, p.133. To have Type O blood, an individual must be homozygous recessive (ii), receiving a recessive allele from both the mother and the father.

Let’s look at how this plays out in a family. Imagine a parent with Type AB (genotype IᴬIᴮ) and a parent with Type O (genotype ii). The Type AB parent can only pass on either an Iᴬ or an Iᴮ allele. The Type O parent can only pass on an 'i' allele. Consequently, the children can only be genotype Iᴬi (Type A) or Iᴮi (Type B). In this specific scenario, it is biologically impossible for them to have a Type O child (because the AB parent has no 'i' to give) or a Type AB child (because the O parent has no 'Iᴬ' or 'Iᴮ' to give).

Genotype	Phenotype (Blood Group)	Nature of Interaction
IᴬIᴬ or Iᴬi	Type A	Iᴬ is dominant over i
IᴮIᴮ or Iᴮi	Type B	Iᴮ is dominant over i
IᴬIᴮ	Type AB	Codominance (Both expressed)
ii	Type O	Recessive (Requires two copies)

Sources: Science, class X (NCERT 2025 ed.), Heredity, p.131; Science, class X (NCERT 2025 ed.), Heredity, p.133

8. Solving the Original PYQ (exam-level)

Review the concepts above and try solving the question.