It is said, that the harmful alleles get eliminated from population over a period of time, yet sickle cell ana

NCERT Exemplar · Q48

Q. It is said, that the harmful alleles get eliminated from population over a period of time, yet sickle cell anaemia is persisting in human population. Why?

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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.

Note

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 …

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Sickle cell anaemia persists because the harmful allele offers a survival advantage against malaria in heterozygous carriers, preventing its elimination by natural selection.

To understand why sickle cell anaemia remains common in certain human populations, we must first revisit the basics of Mendelian genetics and how natural selection works. In a typical scenario, a harmful allele — one that causes a severe, often fatal disease — would be expected to disappear over generations. Individuals carrying two copies of such an allele (homozygous) would have reduced survival or reproductive success, so the allele’s frequency should drop. Yet sickle cell anaemia defies this expectation. Why?

The answer lies in the peculiar nature of the sickle cell allele and its interaction with an environmental factor: malaria. Sickle cell anaemia is caused by a mutation in the haemoglobin gene, leading to abnormal haemoglobin (HbS). Individuals who are homozygous for the sickle cell allele (HbS HbS) suffer from severe anaemia, pain crises, and often die young. That much fits the “harmful allele” story. But the twist is in the heterozygotes — those with one normal allele (HbA) and one sickle cell allele (HbS). These individuals have sickle cell trait, not the full disease. They are largely healthy, and crucially, they are resistant to malaria.

Malaria, caused by the Plasmodium parasite, has been a major cause of death in many tropical and subtropical regions, including parts of Africa, India, and the Mediterranean. The parasite spends part of its life cycle inside red blood cells. In individuals with sickle cell trait, the red blood cells are less hospitable to the parasite — they tend to sickle slightly under low oxygen conditions, which kills the parasite or prevents its multiplication. This gives heterozygotes a strong survival advantage in malaria-endemic areas.

Note

The protective effect is not absolute — heterozygotes can still get malaria, but they are far less likely to die from severe forms of the disease.

Now, here is the key point from the NCERT textbook: natural selection does not act on the allele alone; it acts on the phenotype — the physical expression of the genotype. In a malaria-prone region, the heterozygous genotype (HbA HbS) has a higher fitness than either homozygote. The normal homozygote (HbA HbA) is susceptible to malaria and may die. The sickle cell homozygote (HbS HbS) dies from anaemia. But the heterozygote survives and reproduces. So the sickle cell allele is maintained in the population because it is beneficial in the heterozygous state, even though it is harmful when homozygous. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 42 on this concept.

AP EAPCET 2026Set ap-2026-05-19-FN1 markMCQ
$F_2$
›Reveal solutionSolution
Snapdragon flower colour shows incomplete dominance, giving an F2 genotypic (and here, phenotypic) ratio of 1 red : 2 pink : 1 white, so pink = 50%.

Concept and Intuition

In cases of incomplete dominance, the heterozygote's phenotype is intermediate between the two homozygous parental phenotypes because neither allele is fully dominant — there is a partial/blended expression (e.g., due to partial enzyme/pigment production). In the snapdragon (Antirrhinum) monohybrid cross for flower colour, red (RR) crossed with white (rr) gives an F1 that is entirely pink (Rr), not red, showing the alleles are not fully dominant/recessive. Selfing the pink F1 (Rr × Rr) reproduces the classic Mendelian 1:2:1 genotypic ratio, and because genotype and phenotype track together here, the phenotypic ratio is also 1 red : 2 pink : 1 white.

Step-by-Step Solution

Set up the cross: RR (red) × rr (white) → F1 all Rr (pink), confirming incomplete dominance. …
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AP EAPCET 2026Set ap-2026-05-19-FN1 markMCQ
$I^AI^A$
›Reveal solutionSolution
Testing each option against the observed 25% A : 25% AB : 50% B ratio, only

father $I^AI^B$ × mother $I^BI^O$ reproduces it exactly. Answer: (D).

Concept and Intuition

ABO blood grouping is governed by multiple alleles ( $I^A$ , $I^B$ , $I^O$ ) at a

single locus, where $I^A$ and $I^B$ are co-dominant to each other and both

dominant to $I^O$ . To find parental genotypes from an observed progeny ratio,

we can test each candidate cross by Punnett-square logic and check whether it

reproduces the given proportions.

Step-by-Step Solution

Mother's phenotype is B, so her genotype must be $I^BI^B$ or $I^BI^O$ — this alone doesn't decide between the options, so test the crosses.

Try option (D): father $I^AI^B$ (gametes $I^A$ , $I^B$ , each 1/2), mother $I^BI^O$ (gametes $I^B$ , $I^O$ , each 1/2).

Combine gametes:

$I^A \times I^B \to I^AI^B$ (AB) — 1/4

$I^A \times I^O \to I^AI^O$ (A) — 1/4

$I^B \times I^B \to I^BI^B$ (B) — 1/4

$I^B \times I^O \to I^BI^O$ (B) — 1/4 …
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AP EAPCET 2026Set ap-2026-05-19-AN1 markMCQ
$Q. Mendelian genetic disorder controlled by a single gene on chromosome 11 of each parent is (A) Phenylketonuria (B) Cystic fibrosis (C) Sickle-cell anaemia (D) Cooley's Anaemia$
›Reveal solutionSolution
Cooley's anaemia (beta-thalassemia) is controlled by the single HBB gene on chromosome 11 of each parent — option (D).

Concept and Intuition

NCERT distinguishes the thalassemias: alpha-thalassemia is controlled by two closely linked genes (HBA1, HBA2) on chromosome 16 of each parent, while beta-thalassemia — Cooley's anaemia — is controlled by a single gene, HBB, on chromosome 11 of each parent. The wording of the stem matches this exact statement.

Step-by-Step Solution

Phenylketonuria: PAH gene on chromosome 12 — not chromosome 11.

Cystic fibrosis: CFTR gene on chromosome 7 — not chromosome 11.

Sickle-cell anaemia also involves HBB on chromosome 11, but NCERT does not use the 'single gene ... chromosome 11 of each parent' descriptor for it. …
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AP EAPCET 2026Set ap-2026-05-20-FN1 markMCQ
$Q. Assertion (A): Though the parents contain two alleles during gamete formation, the alleles of a pair segregate from each other. Reason (R): Segregation is a universal phenomenon in all organisms showing sexual method of reproduction. (A) Both (A) and (R) are correct and (R) is the correct explanation to (A) (B) Both (A) and (R) are correct but (R) is not correct explanation for (A) (C) (A) is correct (R) is wrong (D) (A) is wrong (R) is correct$
›Reveal solutionSolution
Mendel's Law of Segregation: alleles separate during gamete formation, and this is universally true across sexually reproducing organisms because it is a direct consequence of meiosis. Both statements true, R explains A.

Concept and Intuition

A diploid organism carries two alleles for every gene (one from each parent). During meiosis, homologous chromosomes — and with them, the two alleles of each gene — separate into different gametes, so any single gamete carries only one allele per gene. This isn't a quirk of Mendel's pea plants; it is a mechanical outcome of the meiotic process itself, which every sexually reproducing organism undergoes to produce haploid gametes.

Step-by-Step Solution

Assertion: alleles of a pair segregate during gamete formation — this is exactly Mendel's Law of Segregation, verified true by countless organisms since. …
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AP EAPCET 2026Set ap-2026-05-20-FN1 markMCQ
$Q. Expression of more than one phenotypic trait by a single gene is known as (A) Pleiotropy (B) Polygenetic inheritance (C) Multiple allelism (D) Lyonisation$
›Reveal solutionSolution
A single gene producing effects on more than one phenotypic trait is the definition of pleiotropy.

Concept and Intuition

Most genes are studied for one visible trait, but many gene products (often enzymes early in a biochemical pathway) influence several downstream processes at once. When a single gene's mutation therefore shows up as changes in multiple, often unrelated, characteristics simultaneously, geneticists call this pleiotropy. This is distinct from polygenic inheritance (many genes controlling one trait), multiple allelism (many alternate forms of one gene, e.g., ABO blood groups), and lyonisation (X-chromosome inactivation in females).

Step-by-Step Solution

Read the definition carefully: 'expression of more than one phenotypic trait by a single gene.'

Match this to the standard genetics term: pleiotropy is defined exactly this way. …
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AP EAPCET 2026Set ap-2026-05-20-FN1 markMCQ
$Q. If blood group of father is A (homozygous) and that of mother is O, these blood groups are not expected in their children. (A) B, AB, A (B) B, AB, O (C) A, O, AB (D) A, B, O$
›Reveal solutionSolution
With father $I^AI^A$ and mother $ii$ , every child is genotype $I^Ai$ (blood group A) — so B, AB and O are all impossible.

Concept and Intuition

ABO blood group is controlled by multiple alleles $I^A$ , $I^B$ , $i$ , with $I^A$ and $I^B$ co-dominant and both dominant over $i$ . A homozygous $I^AI^A$ father can only pass on the $I^A$ allele (he has no other allele to give), and an $ii$ mother can only pass on $i$ . Every offspring therefore receives exactly one $I^A$ and one $i$ , giving genotype $I^Ai$ , phenotype blood group A, with no variation possible.

Step-by-Step Solution

Father's genotype: $I^AI^A$ (homozygous A) — gametes are all $I^A$ .

Mother's genotype: $ii$ (O) — gametes are all $i$ .

Cross: $I^AI^A \times ii \Rightarrow$ all offspring $I^Ai$ — phenotype A, with 100% certainty. …
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AP EAPCET 2026Set ap-2026-05-20-AN1 markMCQ
$Q. If blood group of mother is B (homozygous) and that of father is A (homozygous), these blood groups are absent in their children. (A) A, AB, B (B) A, AB, O (C) B, O, A (D) AB, O, B$
›Reveal solutionSolution
This is a classic ABO blood group cross using co-dominant alleles $I^A$ and $I^B$ .

Concept and Intuition

The ABO blood group system involves three alleles at one locus: $I^A$ and $I^B$ are co-dominant to each other and both dominant over $i$ (the recessive allele for O). A homozygous B individual has genotype $I^B I^B$ and can only contribute the $I^B$ allele to offspring. A homozygous A individual has genotype $I^A I^A$ and can only contribute the $I^A$ allele. Since neither parent carries the recessive $i$ allele, no child can be blood group O; and since every child receives one $I^A$ and one $I^B$ , every child is genotype $I^A I^B$ = blood group AB, so no child can be pure A or pure B either.

Step-by-Step Solution

Mother: $I^B I^B$ (homozygous B) → gametes are all $I^B$ .

Father: $I^A I^A$ (homozygous A) → gametes are all $I^A$ .

Cross: every offspring gets $I^A$ from father and $I^B$ from mother → genotype $I^A I^B$ → phenotype AB, with 100% probability. …
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AP EAPCET 2025Set ap-2025-05-19-AN1 markMCQ
$Q. If two heterozygous tall garden pea plants are crossed, the expected genotypic ratio in their off spring is (A) 3 : 1 (B) 1 : 1 (C) 1 : 2 : 1 (D) 1 : 0$
›Reveal solutionSolution
This tests the classic Mendelian monohybrid cross genotypic ratio. The answer is (C) 1 : 2 : 1.

Concept and Intuition

When two heterozygotes for a single gene are crossed (Tt × Tt), each parent contributes either the dominant (T) or recessive (t) allele with equal probability (1/2 each) to the gametes. Combining gametes via a Punnett square yields four equally likely combinations: TT, Tt, Tt, tt — i.e., genotypes in the ratio 1 TT : 2 Tt : 1 tt. This is distinct from the phenotypic ratio, which collapses TT and Tt into the same "tall" phenotype, giving 3 tall : 1 dwarf (3:1).

Step-by-Step Solution

Set up the cross: Tt (tall, heterozygous) × Tt (tall, heterozygous).

Gametes from each parent: T or t, each with probability 1/2. …
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AP EAPCET 2025Set ap-2025-05-19-FN1 markMCQ
$_2$
›Reveal solutionSolution
Only incomplete dominance (snapdragon flower colour) and codominance (lentil seed-coat pattern) give matching 1:2:1 phenotypic and genotypic ratios at F2 — complete dominance (pea height) gives 3:1 phenotypically despite 1:2:1 genotypically, and a homozygous x heterozygous cross gives 1:1. Answer: (C), I and III.

Concept and Intuition

In a standard monohybrid self-cross (Aa x Aa to F2), the genotypic ratio is always 1 AA : 2 Aa : 1 aa. Whether the phenotypic ratio also comes out 1:2:1 depends entirely on whether the heterozygote (Aa) looks different from both homozygotes:

If dominance is complete (as in pea height, Tall dominant over dwarf), Aa looks identical to AA, so the two dominant genotypic classes merge phenotypically — giving the familiar 3:1 phenotypic ratio, even though the underlying genotypic ratio is still 1:2:1. Phenotypic and genotypic ratio do not match here.

If dominance is incomplete (as in snapdragon flower colour: red x white gives pink F1), the heterozygote has its own distinct, intermediate phenotype — so phenotypic classes align exactly with genotypic classes, giving 1 red : 2 pink : 1 white, matching the 1:2:1 genotypic ratio.

If the trait is codominant (as in lentil seed-coat pattern: dotted x spotted gives a distinct dotted-and-spotted heterozygote pattern in which both parental patterns are simultaneously visible), the same logic applies — the heterozygote's phenotype is unique, so phenotypic and genotypic ratios both come out 1:2:1.

A cross between a homozygous and a heterozygous individual (e.g., AA x Aa, or Aa x aa) is a test/back-cross type, not a self-cross producing an "F2" generation in the usual sense, and produces a 1:1 ratio of genotypes/phenotypes (not 1:2:1).

Step-by-Step Solution

(I) Snapdragon red x white: incomplete dominance to F1 pink; F2 = 1 red : 2 pink : 1 white (phenotypic) and 1 RR : 2 Rr : 1 rr (genotypic) — both 1:2:1. QUALIFIES. …
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AP EAPCET 2025Set ap-2025-05-20-AN1 markMCQ
$Q. If father has blood group A (heterozygous) and mother has blood group B (homozygous), these blood groups are not expected in their children (A) AB, O (B) AB, B (C) A, O (D) B, O$
›Reveal solutionSolution
A father who is $I^Ai$ crossed with a mother who is $I^BI^B$ can only produce children

of blood group AB ( $I^AI^B$ ) or B ( $I^Bi$ ) — groups A and O are impossible from this

particular cross. Answer: (C).

Concept and Intuition

Human ABO blood groups are controlled by three alleles at one locus: $I^A$ and $I^B$ are

co-dominant to each other and both dominant over $i$ ; genotype $I^AI^A$ or $I^Ai$ gives

blood group A, $I^BI^B$ or $I^Bi$ gives blood group B, $I^AI^B$ gives blood group AB, and

$ii$ gives blood group O. To determine which blood groups are possible in offspring, we

need the parents' gamete contributions, not just their phenotypes.

Step-by-Step Solution

Father's genotype: heterozygous A = $I^A i$ . His gametes: $I^A$ or $i$ (each with 50% probability).

Mother's genotype: homozygous B = $I^B I^B$ . Her gametes: only $I^B$ (100%).

Combine gametes: $I^A$ (from father) + $I^B$ (from mother) = $I^A I^B$ → blood group AB.

Combine gametes: $i$ (from father) + $I^B$ (from mother) = $I^B i$ → blood group B.

These are the only two possible offspring genotypes/phenotypes: AB and B. Blood …
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AP EAPCET 2025Set ap-2025-05-20-AN1 markMCQ
$\beta$
›Reveal solutionSolution
Cystic fibrosis (autosomal recessive, not dominant) and PKU (autosomal recessive, not

X-linked) are both misdescribed in I and IV, while thalassemia's haemoglobin-synthesis

defect (II) and sickle-cell anaemia's beta-globin point mutation (III) are both

correctly described. Answer: (B).

Concept and Intuition

Human genetic disorders each have a specific, well-defined inheritance pattern and

molecular basis that are frequently tested together:

Cystic fibrosis is caused by mutations in the CFTR gene and is inherited in an autosomal recessive manner — not dominant.

Thalassemia arises from mutations that reduce or abolish the synthesis of one of the globin chains of haemoglobin (alpha- or beta-thalassemia), correctly described in Statement II.

Sickle-cell anaemia results from a single point mutation in the beta-globin gene (a GAG→GTG change causing glutamic acid to be replaced by valine at position 6), correctly described in Statement III.

Phenylketonuria (PKU) is caused by a defective phenylalanine hydroxylase enzyme and is inherited in an autosomal recessive manner — not X-linked.

Step-by-Step Solution

Statement I: cystic fibrosis inheritance is autosomal recessive, not autosomal dominant as claimed — false. …
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AP EAPCET 2025Set ap-2025-05-20-FN1 markMCQ
$Q. If both parents have B blood groups (both are heterozygous), the expected blood groups in their children. (A) B, O (B) A, AB (C) B only (D) B, AB$
›Reveal solutionSolution
Two heterozygous B-blood-group parents ( $I^BI^O \times I^BI^O$ ) can only produce children with blood group B or O — never A or AB, since neither parent carries the $I^A$ allele. Answer: (A).

Concept and Intuition

Human ABO blood groups are controlled by a single gene with three alleles — $I^A$ , $I^B$ , and $I^O$ — where $I^A$ and $I^B$ are co-dominant to each other and both are dominant over the recessive $I^O$ . A person's phenotype (blood group) depends on which two alleles they carry: $I^AI^A$ or $I^AI^O$ → group A; $I^BI^B$ or $I^BI^O$ → group B; $I^AI^B$ → group AB; $I^OI^O$ → group O. Since ABO inheritance follows simple Mendelian segregation, the possible offspring genotypes (and hence phenotypes) from a cross depend only on which alleles the two parents actually carry.

Step-by-Step Solution

Both parents have blood group B and are stated to be heterozygous, so each parent's genotype is $I^BI^O$ .

Set up the cross: $I^BI^O \times I^BI^O$ .

Each parent contributes either $I^B$ or $I^O$ with equal probability, giving offspring genotypes in the ratio $I^BI^B : I^BI^O : I^OI^B : I^OI^O = 1:2:1$ .

Translate genotypes to phenotypes: $I^BI^B$ and $I^BI^O$ both give blood group B (3 out of 4 parts), and $I^OI^O$ gives blood group O (1 out of 4 parts). …
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Q.It is said, that the harmful alleles get eliminated from population over a period of time, yet sickle cell anaemia is persisting in human population. Why?

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