Q.(a) "The influence of both the alleles in a heterozygous state is clearly expressed in codominance." Explain with the help of inheritance of ABO blood group in humans.
🔒You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.
🔒 Start your 14-day free trial to unlock the full solution →Part (a)Concept understanding — Mendelian Genetics Basics
Mendelian Genetics Basics
Imagine you have a box of coloured beads — red and white. If you pick one bead from the box, you get either red or white. Now imagine that the colour of your eyes, or the shape of your earlobe, is decided by something like that: a tiny "packet" inside your cells that comes in two versions, and you inherit one from each parent. That is the core idea of Mendelian genetics.
The everyday intuition
You have probably noticed that children often look like their parents — same hair colour, same dimples, same height. But they are never exact copies. Why? Because each parent contributes half of the "instructions" for building a child. Those instructions come in pairs, one from mother and one from father. Sometimes one instruction overrides the other; sometimes they blend. Gregor Mendel, a 19th-century monk, figured out the rules by watching pea plants — tall vs short, yellow vs green seeds — and counting what appeared in the next generation.
The precise meaning
Mendelian genetics is the study of how traits are passed from parents to offspring through genes. A gene is a unit of heredity — a stretch of DNA that codes for a specific characteristic, like flower colour. Each gene comes in different versions called alleles. For every gene, you inherit two alleles: one from your mother, one from your father.
If the two alleles are identical, you are homozygous for that trait. If they are different, you are heterozygous. In a heterozygous pair, one allele may be dominant — it shows up in the appearance — and the other recessive — it stays hidden unless both alleles are recessive.
Mendel's key insight was that traits are not blended like paint. Instead, alleles remain separate and are passed on intact. A recessive allele can skip a generation and reappear later, unchanged.
Why it matters
Mendelian genetics is the foundation of modern biology. It explains:
- Why some diseases run in families (like cystic fibrosis or sickle-cell anaemia)
- How plant and animal breeders create new varieties
- Why you might have your grandmother's eyes but not your mother's
The NCERT textbook states that Mendel's work established the laws of inheritance — the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment. These laws describe how alleles separate during the formation of eggs and sperm, and how different genes are inherited independently of one another.
Key terms at a glance
- Gene: a unit of heredity on a chromosome
- Allele: a variant form of a gene
- Dominant: the allele that expresses itself even when paired with a different allele
- Recessive: the allele that expresses itself only when paired with an identical recessive allele
- Homozygous: having two identical alleles for a gene
- Heterozygous: having two different alleles for a gene …
Part (b)Concept understanding — Lac Operon Catabolite Repression
Imagine you are a factory manager. You have two raw materials: a high-grade fuel that your machines run on perfectly, and a low-grade backup fuel that works but is harder to use. As long as the good fuel is available, you would never waste time and energy switching to the backup. But if the good fuel runs out, you immediately switch to the backup to keep production going.
That is exactly what catabolite repression does inside a bacterium like E. coli. It is the cell's way of saying: "Use the best fuel first; don't bother with the second-best until you absolutely have to."
The Two Fuels: Glucose and Lactose
E. coli bacteria love glucose. It is their favourite energy source — easy to break down, gives quick energy. Lactose (milk sugar) is harder to digest; the cell needs to build special enzymes (like β-galactosidase) to break it down. These enzymes are coded by the lac operon.
The cell has a simple rule: If glucose is present, do not waste energy making lactose-digesting enzymes. That is catabolite repression. It is a global regulatory mechanism that ensures glucose is used first, even when lactose is also available.
Catabolite repression is sometimes called the glucose effect. It is not unique to the lac operon — it affects many operons that break down alternative sugars. But the lac operon is the classic textbook example.
How It Works: The Molecular Switch
The key player is a molecule called cAMP (cyclic AMP). Its level inside the cell is inversely related to glucose concentration:
- When glucose is high: cAMP levels are low.
- When glucose is low: cAMP levels rise.
cAMP binds to a protein called CAP (Catabolite Activator Protein). The cAMP–CAP complex then binds to a specific site near the lac operon's promoter. This binding dramatically increases the rate of transcription — it is like pressing the accelerator pedal.
So here is the logic:
- Glucose present (high): Low cAMP → CAP cannot bind → lac operon is barely transcribed, even if lactose is around. The cell ignores lactose.
- Glucose absent (low): High cAMP → CAP binds → lac operon is fully activated. Now, if lactose is also present, the operon switches on fully and the cell digests lactose.
Catabolite repression is a positive control mechanism. The CAP–cAMP complex activates transcription. This is different from the lac repressor, which blocks transcription when lactose is absent. The lac operon is controlled by two switches: a negative one (repressor) and a positive one (CAP–cAMP). Both must be in the "on" position for maximum expression.
Why It Matters (Exam Perspective)
The NCERT textbook (Class 12 Biology, Chapter 6) presents catabolite repression as a fine-tuning mechanism. It explains that even when the lac repressor is removed (by lactose binding), transcription is still low unless glucose is absent. The CAP–cAMP complex is the "second key" that unlocks full expression.
Key points to remember for exams: …
Part (a)
In codominance, both alleles of a heterozygote are fully and independently expressed — neither is dominant nor blended. The ABO blood group gene has three alleles: I^A, I^B and i. I^A makes the A antigen, I^B makes the B antigen, and i makes no antigen. In the heterozygote I^A I^B, both A and B antigens appear on the red blood cells, giving blood group AB — showing both alleles are expressed together (codominance). (I^A and I^B are each dominant over i; three alleles for one gene also illustrate multiple allelism.) …
Part (a): In codominance both alleles express fully in the heterozygote — genotype I^A I^B makes both A and B antigens (blood group AB).
Part (b): Lactose inactivates the repressor so RNA polymerase can transcribe genes z, y, a (operon ON); the operon is negatively regulated because a repressor normally keeps it off.
Part (a)
Concept-first idea: Codominance is a deviation from simple dominance in which both alleles of a heterozygote are expressed simultaneously and distinctly — there is no masking (unlike dominance) and no blending (unlike incomplete dominance).
ABO blood groups as the example. The gene I has three alleles: I^A, I^B, i.
- I^A → produces A-antigen (a specific sugar) on the RBC surface.
- I^B → produces B-antigen.
- i → produces no antigen.
Dominance relations: both I^A and I^B are dominant over i (so I^A i = group A, I^B i = group B). The key point is the I^A I^B heterozygote: here both antigens A and B are formed on the red cells, giving blood group AB. Neither allele masks the other and there is no intermediate antigen — both are expressed together, the definition of codominance. …
Showing the 12 most recent of 66 on this concept.
- CBSE 2026Set EG1 markQ.Who proposed the chromosomal theory of inheritance?
›Reveal solutionSolution
The chromosomal theory of inheritance was put forward by Sutton and Boveri (1902).
Working independently, Walter Sutton and Theodor Boveri noticed that the behaviour of chromosomes during meiosis exactly parallels the behaviour of Mendel's 'factors' (genes) — they occur in pairs, segregate during gamete formation and assort independently. Around 1902 they proposed the chromosomal theory of inheritance, stating that genes are located on chromosomes, and that the pairing and separation of chromosomes is the physical basis of …
- CBSE 2026Set A1 markMCQQ.Which of the following laws is based on monohybrid cross?(a) Law of dominance(b) Law of segregation(c) Law of independent assortment(d) Both (A) and (B)
›Reveal solutionSolution
A monohybrid cross reveals the Law of Dominance and the Law of Segregation; independent assortment needs a dihybrid cross.
Mendel's monohybrid cross (a single-character cross) showed that one allele masks the other in the F1 (Law of Dominance) and that the paired factors separate during gamete formation so each gamete carries only one, reappearing in the 3:1 F2 ratio (Law of Segregation). T …
- CBSE 2026Set BOTANY1 markMCQQ.The genotypic ratio of F2 generation of Mendel's monohybrid cross is _____.(a) 3 : 1(b) 1 : 2 : 1(c) 2 : 2(d) 1 : 1
›Reveal solutionSolution
In a monohybrid cross, the F2 phenotypic ratio is 3:1 but the underlying genotypic ratio is 1:2:1.
In Mendel's monohybrid cross (e.g. tall (TT) x dwarf (tt) pea plants), the F1 generation is entirely heterozygous (Tt) and phenotypically tall, since tallness is dominant. When F1 plants are self-pollinated, each parent contributes either a T or a t gamete with equal probability (1/2 each). Using a Punnett square, the F2 offspring genotypes appear in the ratio:
- TT (homozygous dominant) : 1/4
- Tt (heterozygous) : 1/2 (i.e. 2/4, from Tt and tT)
- tt (homozygous recessive) : 1/4 …
- CBSE 2026Set ANNUAL1 markMCQQ.Who rediscovered the results of Mendel's inheritance of characters?(a) Devries, Correns and Tschermak(b) Sutton and Boveri(c) Schleiden, Schwann and Morgan(d) Watson and Crick
›Reveal solutionSolution
Mendel's 1865 findings were ignored for about 35 years until three botanists independently rediscovered the same laws of inheritance in 1900.
Mendel presented his laws of inheritance in 1865, but their significance was not understood at the time and the work was ignored for about 35 years. In 1900, Hugo de Vries, Carl Correns and Erich von Tschermak independently rediscovered Mendel's results while working on different plants, and gave due credit to Mendel's original work. Sutton and Boveri later proposed the chromosomal theory of inherit …
- CBSE 2026Set ANNUAL1 markMCQQ.When the genotype of an organism contains two identical alleles, the organism is considered as(a) Homozygous(b) Heterozygous(c) Hemizygous(d) Pleiotropy
›Reveal solutionSolution
Two identical alleles at a locus (e.g., TT or tt) make the organism homozygous; two different alleles (Tt) make it heterozygous.
In diploid organisms, each gene locus carries two alleles, one inherited from each parent.
- If both alleles are identical (e.g., TT or tt), the genotype is homozygous (true-breeding) for that trait.
- If the two alleles are different (e.g., Tt), the genotype is heterozygous.
- Hemizygous refers to having only a single copy of a gene/allele (as with X-linked genes in a male, who has only one X chromosome). …
- CBSE 2026Set ANNUAL1 markMCQQ.Crosses between F1 offspring and either of their parents are called as(a) Inbreeding(b) Back cross(c) Dihybrid cross(d) Test cross
›Reveal solutionSolution
A back cross is any cross of an F1 hybrid with one of its own parents; a test cross is the specific back cross to the homozygous recessive parent used to determine genotype.
- Back cross: F1 offspring crossed with either parent (dominant homozygous OR recessive homozygous) — used generally in breeding programmes to study inheritance or improve stock.
- Test cross: a specific type of back cross — F1 (of unknown genotype for a trait) crossed specifically with the homozygous recessive parent, used to determine whether the F1 is homozygous or heterozygous for the dominant trait. …
- CBSE 2026Set ANNUAL1 markQ.............. A cross between an organism of an unknown genotype and homozygous recessive organism.
›Reveal solutionSolution
A test cross is a cross between an individual of unknown genotype and a homozygous recessive individual, used to determine whether the unknown individual is homozygous dominant or heterozygous.
In a test cross, an organism showing a dominant phenotype (whose genotype could be either homozygous dominant, e.g. TT, or heterozygous, e.g. Tt) is crossed with a homozygous recessive organism (tt). If all offspring show the dominant phenotype, the unknown parent was hom …
- CBSE 2026Set ANNUAL1 markMCQQ.A pure tall (TT) plant is crossed with a pure dwarf (tt) plant to produce F1 offspring. The F1 offspring are then self-crossed. What is the ratio of true-breeding tall (TT) to true-breeding dwarf (tt) plants in the F2 generation?(a) 1 : 1(b) 1 : 2 : 1(c) 1 : 3(d) 3 : 1
›Reveal solutionSolution
The F2 of a monohybrid cross gives a 1 TT : 2 Tt : 1 tt genotypic ratio; the two true-breeding types (TT and tt) are therefore in a 1 : 1 ratio.
From the CBSE/NCERT Principles of Inheritance and Variation chapter (Mendel's monohybrid cross):
- Parents: TT (tall) × tt (dwarf).
- F1: all Tt (tall).
- F1 selfed (Tt × Tt) gives the F2 Punnett square:
T t T TT Tt t Tt tt - Genotypic ratio = 1 TT : 2 Tt : 1 tt. …
- CBSE 2026Set ANNUAL1 markMCQQ.In E.coli, the lac operon gets switched on when :(a) lactose is present and it binds to the repressor.(b) repressor binds to operator.(c) RNA polymerase binds to the operator.(d) lactose is absent.
›Reveal solutionSolution
The lac operon is switched ON when lactose (the inducer) binds and inactivates the repressor, so RNA polymerase can transcribe the structural genes; option (a) is correct.
In the CBSE/NCERT Molecular Basis of Inheritance chapter, the lac operon of E. coli is regulated by negative control:
- No lactose: the repressor protein (from the i gene) binds the operator, blocking RNA polymerase — operon OFF. …
- CBSE 2025Set 57/4/11 markMCQQ.In a pea plant (Pisum sativum) inflated pod shape is dominant over constricted pod shape. The expected ratio of phenotypes of the offspring in a cross between both the parents with heterozygous inflated pod shape will be : (A) 1 : 0 (B) 1 : 1 (C) 2 : 1 (D) 3 : 1
›Reveal solutionSolution
A cross between two heterozygotes (Ii × Ii) follows Mendel's monohybrid ratio: three offspring show the dominant phenotype (inflated) for every one showing the recessive (constricted). The answer is 3 : 1.
This is a classic Mendelian monohybrid cross. When both parents are heterozygous for a single trait, each carries one dominant and one recessive allele. The dominant allele masks the recessive in the phenotype, but the recessive can still be passed to offspring. The question tests whether you recognize the signature 3:1 phenotypic ratio that emerges when dominance is complete.
Let's denote the allele for inflated pod shape as I (dominant) and constricted pod shape as i (recessive). Both parents are heterozygous inflated, so their genotype is Ii.
Step-by-step analysis
-
Set up the cross
Parent 1: Ii (heterozygous inflated)
Parent 2: Ii (heterozygous inflated)
-
Determine the gametes
Each heterozygous parent produces two types of gametes in equal proportion: I and i.
-
Construct the Punnett square
Cross the gametes from both parents:
I (from Parent 2) i (from Parent 2) I (from Parent 1) II Ii i (from Parent 1) Ii ii -
Identify the genotypes
The offspring genotypes are:
- II : 1 (homozygous dominant) …
-
- CBSE 2025Set 57/4/11 markMCQQ.Assertion (A) : ABO blood grouping in humans is an example of multiple allelism. Reason (R) : More than two genes in a population govern the same character in ABO blood grouping in humans.
›Reveal solutionSolution
Multiple allelism means more than two alleles of a single gene exist in a population. ABO blood grouping is controlled by three alleles (I^A, I^B, i) of one gene, so Assertion is true. But the Reason says "more than two genes" — that is wrong; it's one gene with multiple alleles. Hence Assertion is true but Reason is false.
The core concept: What multiple allelism really means
When you hear "multiple alleles," the trap is to think it means "many genes." It does not. Multiple allelism refers to a single gene locus that has more than two possible allele variants present in the population. Any one diploid individual still carries only two alleles (one from each parent), but across the population, three or more different versions of that gene exist.
ABO blood grouping is the classic textbook example. The gene involved is the ABO gene on chromosome 9. It has three common alleles: I^A, I^B, and i. The I^A and I^B alleles are co-dominant (both expressed when together), and both are dominant over i. This gives the six possible genotypes and four blood group phenotypes you know.
Now look at the Reason statement carefully. It says: "More than two genes in a population govern the same character." That is a fundamental error. It is not "more than two genes" — it is more than two alleles of the same gene. The Reason misstates the definition of multiple allelism.
Step-by-step reasoning
-
Check the Assertion (A).
ABO blood grouping in humans is indeed an example of multiple allelism because three alleles (I^A, I^B, i) of a single gene exist in the human population. So Assertion is true.
-
Check the Reason (R). …
-
- CBSE 2025Set 57/5/11 markMCQQ.During a monohybrid cross involving a tall pea plant with a dwarf pea plant, the offspring populations were tall and dwarf in equal ratio. Find out the genotype of parent pea plants. (A) Tt × tt (B) TT × Tt (C) tt × tt (D) Tt × Tt
›Reveal solutionSolution
The parent pea plants must have genotypes Tt (heterozygous tall) and tt (homozygous dwarf) to produce tall and dwarf offspring in an equal 1:1 ratio.
To understand the genotypes of the parent pea plants, we first need to recall the fundamental principles of Mendelian genetics, specifically concerning a monohybrid cross. A monohybrid cross involves studying the inheritance of a single pair of contrasting characters, such as plant height in pea plants. Gregor Mendel's experiments established that certain traits are dominant and others are recessive.
In pea plants, tallness is a dominant trait, while dwarfness is a recessive trait. This means that if an allele for tallness (let's denote it as 'T') is present, the plant will be tall, even if it also carries an allele for dwarfness. The allele for dwarfness (let's denote it as 't') will only express itself if two copies of it are present in the plant's genetic makeup.
NoteAn allele is an alternative form of a gene. For instance, the gene for plant height has two alleles: 'T' for tall and 't' for dwarf.
A plant's genotype refers to its genetic constitution (the combination of alleles it possesses), while its phenotype is the observable characteristic (what it looks like).
For the trait of height in pea plants, the possible genotypes and their corresponding phenotypes are:
- TT: Homozygous dominant, Phenotype: Tall
- Tt: Heterozygous, Phenotype: Tall (because 'T' is dominant over 't')
- tt: Homozygous recessive, Phenotype: Dwarf
We are given a monohybrid cross between a tall pea plant and a dwarf pea plant. The offspring population shows tall and dwarf plants in an equal ratio, meaning 1 tall plant for every 1 dwarf plant, or a 1:1 ratio. Our task is to determine the genotypes of the parent plants that would produce this specific offspring ratio.
Let's consider the dwarf parent first. Since dwarfness is a recessive trait, a pea plant can only be dwarf if its genotype is homozygous recessive, meaning it must have two copies of the dwarf allele.
- Therefore, the genotype of the dwarf parent is tt.
Now, let's consider the tall parent. A tall pea plant can have one of two possible genotypes: TT (homozygous tall) or Tt (heterozygous tall). We need to test which of these, when crossed with a tt dwarf plant, yields a 1:1 ratio of tall to dwarf offspring.
Scenario 1: Tall parent is homozygous dominant (TT)
If the tall parent is TT, the cross would be TT × tt.
- The TT parent produces only 'T' gametes.
- The tt parent produces only 't' gametes.
- When these gametes combine, all offspring will have the genotype Tt.
- Since Tt plants are tall, all offspring would be tall.
- The phenotypic ratio would be 100% Tall, or 1 Tall : 0 Dwarf. …
🎓Unlock everything free for 14 days
- ✓Full step-by-step solutions
- ✓Concept-first explanations
- ✓Methods, shortcuts & mistakes
- ✓PYQ mapping + timed mock tests
Full access for 14 days. No credit card required.