Mendelian Disorders Pedigree

8 MCQs9-step worked example
Source: NCERT Genetics and EvolutionPYQ coverage: NEET 2021, 2022, 2025Official key: NTA-verifiedLast updated: 25 Sep 2026

Mendelian Disorders Pedigree, explained for NEET

NEET repeatedly confuses two X-linked recessive disorders that share the same carrier-to-son inheritance logic but not the same numbers: colour blindness and haemophilia. Learn to keep them apart before the pedigree symbols and the four named disorders.

Pedigree analysis (NCERT Class 12 Biology, Chapter 4, page 72) is the tool: since controlled genetic crosses cannot be run on humans, inheritance is traced through a family's history over generations, recorded with standard symbols as a family tree. It reveals whether a trait is dominant or recessive and whether it is autosomal or sex-linked.

Genetic disorders split into two categories (page 73): Mendelian disorders, caused by alteration of a single gene, and chromosomal disorders, caused by whole-chromosome gain, loss or rearrangement (Down's syndrome, Turner's syndrome — covered in a separate lesson on mutations and chromosomal disorders). This lesson stays inside the Mendelian category: haemophilia, colour blindness, phenylketonuria, thalassemia.

Colour blindness (page 73) is X-linked recessive, from a defective red/green cone gene, occurring in about 8% of males and 0.4% of females — males need only one copy since they carry a single X. Haemophilia (page 74), also X-linked recessive, affects a blood-clotting protein; the text gives no population percentage for it but illustrates it through Queen Victoria's pedigree, where carrier daughters transmitted the disease to grandsons across European royal houses. In both, a carrier mother is phenotypically normal because her one dominant normal allele suppresses the recessive one — she is not "partially affected."

Phenylketonuria (page 75) is autosomal recessive: a missing enzyme fails to convert phenylalanine to tyrosine, so phenylalanine and its derivatives accumulate and cause mental retardation. Thalassemia (page 75) is also autosomal recessive but is a quantitative defect — too few globin molecules made (HBA1/HBA2 on chromosome 16 for α-thalassemia, HBB on chromosome 11 for β-thalassemia) — distinct from sickle-cell anaemia's qualitative defect (a mis-shaped globin from one base substitution).

Watch-out: never classify a chromosomal disorder as Mendelian just because it is "genetic," and never treat a carrier's normal phenotype as evidence she isn't a carrier.

Can you answer these Mendelian Disorders Pedigree MCQs?

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

Pedigree analysis, as described in NCERT, is best defined as:

Show answer and why every option is right or wrong

Answer: C. NCERT Class 12 Biology, Chapter 4, page 72 defines pedigree analysis as analysing the inheritance of a trait across several generations of a family, represented as a family tree, used because controlled crosses are not possible in humans.

Why A is wrong: A is wrong because NCERT explicitly says controlled crosses (as done in pea plants) are NOT possible in humans — that is exactly why pedigree analysis is used instead.

Why B is wrong: B is wrong because allele-frequency statistics belong to population genetics, not the family-tree method the text defines as pedigree analysis.

Why D is wrong: D is wrong because pedigree analysis is a record of family history and standard symbols (Figure 4.13), not a DNA sequencing technique; the text never mentions sequencing.

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MCQ 2Easy RecallPractice

Which of the following is classified as a Mendelian disorder in NCERT's genetic-disorders section?

Show answer and why every option is right or wrong

Answer: A. NCERT Class 12 Biology, Chapter 4, page 73 lists phenylketonuria among the Mendelian disorders (with haemophilia, cystic fibrosis, sickle-cell anaemia, colour blindness, thalassemia), each caused by alteration of a single gene.

Why B is wrong: B is wrong because Down's syndrome is caused by an extra copy of chromosome 21 — a chromosomal disorder (whole-chromosome gain), not a single-gene Mendelian disorder.

Why C is wrong: C is wrong because Turner's syndrome results from loss of an X chromosome, which is a chromosomal disorder, not a single-gene alteration.

Why D is wrong: D is wrong because Klinefelter syndrome (an extra X chromosome) is likewise a chromosomal disorder caused by abnormal chromosome number, not a single altered gene.

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MCQ 3Easy RecallPractice

According to NCERT, colour blindness occurs in approximately what proportion of males and females respectively?

Show answer and why every option is right or wrong

Answer: B. NCERT Class 12 Biology, Chapter 4, page 73 states colour blindness occurs in about 8 per cent of males and only about 0.4 per cent of females, because males have only one X chromosome.

Why A is wrong: A is wrong because it inverts the actual figures — the text gives the higher figure (8%) for males, not females, since males have only one X chromosome to carry the recessive allele.

Why C is wrong: C is wrong because 50% is the transmission probability to a carrier's son in a cross, not the population incidence figure the text reports for colour blindness.

Why D is wrong: D is wrong because it applies the male incidence figure to females too, ignoring that females need two copies of the recessive allele (rarer) while males need only one.

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MCQ 4Direct ApplicationPractice

A woman is a known carrier of haemophilia but shows no bleeding symptoms herself. Which statement correctly explains this, per NCERT?

Show answer and why every option is right or wrong

Answer: D. NCERT Class 12 Biology, Chapter 4, page 74 states the carrier mother is not herself affected because the gene is recessive and its effect is suppressed by her matching dominant normal gene on the other X chromosome.

Why A is wrong: A is wrong because a carrier is, by definition, heterozygous for the disease allele and can still transmit it to sons — the text explicitly calls such women carriers.

Why B is wrong: B is wrong because it garbles the mechanism: females do have two X chromosomes, but that is exactly why one normal dominant allele can mask the recessive one — the text ties this to dominance, not to an automatic protection rule.

Why C is wrong: C is wrong because the text attributes the lack of symptoms to gene dominance suppression, not to liver compensation, which NCERT never mentions.

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MCQ 5Direct ApplicationPractice

A pedigree question states that a disorder affects about 8% of males and 0.4% of females in the population, and asks which of these two X-linked recessive disorders it describes. Which is correct?

Show answer and why every option is right or wrong

Answer: A. NCERT Class 12 Biology, Chapter 4, page 73 gives the 8%/0.4% incidence specifically for colour blindness; haemophilia (page 74) is instead illustrated through Queen Victoria's pedigree, with no population percentage stated.

Why B is wrong: B is wrong because 'more commonly discussed' is not evidence — the text attaches the 8%/0.4% figure specifically to colour blindness's cone-gene defect, not to haemophilia's clotting-protein defect.

Why C is wrong: C is wrong because sharing the same inheritance pattern (X-linked recessive, carrier-mother transmission) does not mean the two disorders share the same population incidence; NCERT states the figure only for colour blindness.

Why D is wrong: D is wrong because both are X-linked recessive disorders with son-biased transmission described in the text — colour blindness explicitly gives 8% male vs 0.4% female, so the sexes are not equally affected.

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MCQ 6Direct ApplicationPractice

Phenylketonuria, as described in NCERT, results from:

Show answer and why every option is right or wrong

Answer: D. NCERT Class 12 Biology, Chapter 4, page 75 states phenylketonuria patients lack the enzyme that converts phenylalanine to tyrosine, so phenylalanine accumulates and converts to phenylpyruvic acid and other derivatives, causing mental retardation.

Why A is wrong: A is wrong because the text describes a missing/non-functional enzyme, not an extra gene copy — PKU is a loss-of-function single-gene disorder, not a gene-dosage disorder.

Why B is wrong: B is wrong because that description matches haemophilia's clotting-protein defect, not phenylketonuria's metabolic enzyme defect.

Why C is wrong: C is wrong because reduced globin synthesis describes thalassemia, not phenylketonuria, which involves an amino-acid metabolism enzyme, not haemoglobin.

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MCQ 7Concept TrapPractice

Which statement correctly distinguishes thalassemia from sickle-cell anaemia, per NCERT?

Show answer and why every option is right or wrong

Answer: B. NCERT Class 12 Biology, Chapter 4, page 75 states thalassemia differs from sickle-cell anaemia in that the former is a quantitative problem of synthesising too few globin molecules, while the latter is a qualitative problem of synthesising an incorrectly functioning globin.

Why A is wrong: A is wrong because both disorders are described as autosome-linked recessive traits in the text; neither is X-linked.

Why C is wrong: C is wrong because it is sickle-cell anaemia, not thalassemia, that changes red blood cell shape (biconcave disc to sickle shape); thalassemia is described as reduced globin synthesis, not a shape change.

Why D is wrong: D is wrong because thalassemia can affect either the alpha chain (HBA1/HBA2, chromosome 16) or the beta chain (HBB, chromosome 11) depending on type — it is not limited to alpha, while sickle-cell anaemia specifically affects the beta chain via one base substitution, which this option conflates.

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MCQ 8CalculationPractice

In Queen Victoria's haemophilia pedigree, a carrier daughter (X^H X^h) marries a normal, unaffected man (X^H Y). What proportion of their sons is expected to be haemophilic?

Show answer and why every option is right or wrong

Answer: C. NCERT Class 12 Biology, Chapter 4, page 74 states the heterozygous carrier female may transmit haemophilia to sons; a carrier mother (X^H X^h) crossed with a normal father (X^H Y) gives sons X^H Y or X^h Y in equal proportion, so 50 per cent of sons are expected to be haemophilic.

Why A is wrong: A is wrong because the father's genotype does not determine a son's X-linked allele — sons receive their single X chromosome from the mother, who is a carrier, so the father's normal status does not prevent transmission.

Why B is wrong: B is wrong because 25% is the share of ALL children (sons and daughters together) who are haemophilic sons; the question asks only about the sons, half of whom receive the mother's X^h.

Why D is wrong: D is wrong because a carrier mother passes the disease allele to only half her sons on average — the other half inherit her normal X chromosome — so 100% overstates a heterozygous carrier's transmission.

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How do you solve a Mendelian Disorders Pedigree question? A worked example

  1. 1

    Given

    Queen Victoria was a carrier for haemophilia; her pedigree famously shows haemophilic descendants transmitted via carrier daughters to their sons across European royal houses (NCERT Class 12 Biology, Chapter 4, page 74). A carrier daughter has genotype X^H X^h and marries an unaffected man, genotype X^H Y. Separately, colour blindness — a different X-linked recessive disorder — occurs in about 8% of males and 0.4% of females in the general population (page 73).

  2. 2

    Required

    What fraction of the carrier daughter's sons is expected to be haemophilic, and how does this per-cross probability differ in kind from the 8%/0.4% figure NCERT states for colour blindness?

  3. 3

    Concept

    Haemophilia is X-linked recessive: a carrier mother transmits the allele to 50% of her sons regardless of the father's phenotype, because sons receive their only X chromosome from the mother. Colour blindness follows the same inheritance logic (X-linked recessive, carrier-to-son) but NCERT states it as a population incidence (8% of males, 0.4% of females), not as a per-cross probability — the two numbers answer different questions.

  4. 4

    Formula

    Cross: X^H X^h (carrier mother) × X^H Y (unaffected father). Sons: 1/2 X^H Y (normal) : 1/2 X^h Y (haemophilic). Daughters: 1/2 X^H X^H (normal) : 1/2 X^H X^h (carrier, unaffected).

  5. 5

    Substitution

    Mother's gametes: X^H, X^h, each with probability 1/2. Father's gametes: X^H, Y, each with probability 1/2 (these are exact Mendelian segregation probabilities, not measured quantities, so they carry no significant-figure count). Sons arise from a father's Y gamete combined with either maternal gamete.

  6. 6

    Calculation

    Among sons only: P(X^H Y) = 1/2 (normal), P(X^h Y) = 1/2 (haemophilic). Of all four offspring classes, X^h Y sons make up 1/4 of total offspring, which is 1/2 of the sons specifically: 1/2 × 100 = 50% of sons.

  7. 7

    Final answer

    50% (1/2) of the carrier daughter's sons are expected to be haemophilic. This is a per-cross Mendelian probability, unlike the 8%/0.4% figure NCERT gives for colour blindness, which is a population-level incidence and not a cross probability — the two numbers must never be substituted for each other.

  8. 8

    Common trap

    Mixing up "population incidence" (colour blindness's 8%/0.4%, a survey-style statistic) with "cross probability" (haemophilia's 50% son-transmission from a known carrier). NCERT gives one figure type per disorder here, not both for each, so a question naming one disorder's number cannot be answered using the other disorder's logic without redoing the cross.

  9. 9

    Similar NEET-style question

    A carrier woman for colour blindness (X^C X^c) marries a colour-blind man (X^c Y). What fraction of their daughters is expected to be colour blind? (Answer: mother's gametes are X^C and X^c, each 1/2; the father contributes X^c to every daughter. Daughters are 1/2 X^C X^c (carrier, unaffected) : 1/2 X^c X^c (colour blind). So 1/2 = 50% of daughters are expected to be colour blind.)

What to remember before solving Mendelian Disorders Pedigree questions

Definition

Pedigree Analysis

Since control crosses possible in a pea plant cannot be performed in human beings, the study of family history about the inheritance of a particular trait provides an alternative; such an analysis of traits over several generations of a family is called pedigree analysis. In pedigree analysis, the inheritance of a particular trait is represented in a family tree over generations. In human genetics, pedigree study is a strong tool used to trace the inheritance of a specific trait, abnormality or disease, using standard symbols shown in Figure 4.13.

-- NCERT Class 12 Biology, Ch. 4, p. 72
Key Fact

Mendelian Disorders

Genetic disorders may be broadly grouped into Mendelian disorders and Chromosomal disorders. Mendelian disorders are mainly determined by alteration or mutation in a single gene and are transmitted to offspring on the same lines as the principles of inheritance; their pattern of inheritance can be traced by pedigree analysis. The most common and prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sickle-cell anaemia, Colour blindness, Phenylketonuria and Thalassemia. Such disorders may be dominant or recessive, and pedigree analysis reveals which; the trait may also be linked to the sex chromosome, as in haemophilia, which is an X-linked recessive trait transmitted from carrier female to male progeny.

-- NCERT Class 12 Biology, Ch. 4, p. 73
Key Fact

Colour Blindness

Colour blindness is a sex-linked recessive disorder due to a defect in either the red or green cone of the eye, resulting in failure to discriminate between red and green colour, caused by mutation in genes present on the X chromosome. It occurs in about 8 per cent of males and only about 0.4 per cent of females, because males have only one X chromosome and females have two.

-- NCERT Class 12 Biology, Ch. 4, p. 73
Key Fact

Haemophilia

Haemophilia is a sex-linked recessive disease that shows transmission from unaffected carrier female to some of the male progeny; a single protein that is part of the cascade of proteins involved in blood clotting is affected, so a simple cut results in non-stop bleeding in an affected individual. The heterozygous female (carrier) for haemophilia may transmit the disease to sons. A female becoming haemophilic is extremely rare because her mother must be at least a carrier and her father must be haemophilic (unviable in the later stages of life). The family pedigree of Queen Victoria shows a number of haemophilic descendants, as she was a carrier of the disease.

-- NCERT Class 12 Biology, Ch. 4, p. 74
Key Fact

Thalassemia

Thalassemia is an autosome-linked recessive blood disease transmitted from parents to offspring when both partners are unaffected carriers (heterozygous). The defect, due to mutation or deletion, results in reduced synthesis of one of the globin chains (alpha and beta chains) of haemoglobin, causing abnormal haemoglobin and resulting anaemia. In alpha Thalassemia, production of the alpha globin chain is affected, controlled by two closely linked genes HBA1 and HBA2 on chromosome 16 of each parent, with more affected genes producing less alpha globin. Beta Thalassemia is controlled by a single gene HBB on chromosome 11 of each parent. Thalassemia is a quantitative problem of synthesising too few globin molecules, unlike sickle-cell anaemia which is a qualitative problem of an incorrectly functioning globin.

-- NCERT Class 12 Biology, Ch. 4, p. 75
Key Fact

Colour Blindness (continued)

The son of a woman who carries the gene has a 50 per cent chance of being colour blind; the mother is not herself colour blind because the gene is recessive and its effect is suppressed by her matching dominant normal gene. A daughter will not normally be colour blind unless her mother is a carrier and her father is colour blind.

-- NCERT Class 12 Biology, Ch. 4, p. 74

Where do students lose marks on Mendelian Disorders Pedigree?

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

Haemophilia vs colour-blindness pedigree pattern mixup

Category: Similar Terms

Both haemophilia and colour blindness are X-linked recessive with carrier-mother transmission, so their pedigree descriptions and statistics get swapped.

When it triggers

A question gives the pedigree of one disorder (e.g., 8% of males affected, 0.4% of females) and asks which disease it is, with haemophilia offered as a distractor to the correct answer, colour blindness, or vice versa.

How to avoid

Colour blindness occurs in about 8% of males and 0.4% of females due to defective red/green cone genes on the X chromosome; haemophilia is a distinct X-linked recessive clotting-protein disorder, famously traced through Queen Victoria's pedigree, with no incidence percentage given in the text.

Trap

Thalassemia (quantitative) vs sickle-cell anaemia (qualitative) mixup

Category: Similar Terms

Students confuse thalassemia's quantitative globin-synthesis defect with sickle-cell anaemia's qualitative structural defect, or mix up which is X-linked vs autosomal.

When it triggers

A question asks which disorder involves 'too little normal globin produced' vs 'an incorrectly functioning globin protein' and swaps thalassemia and sickle-cell anaemia, or claims either is sex-linked.

How to avoid

Thalassemia is an autosome-linked recessive disease that is a quantitative problem — too few globin molecules synthesised (alpha chain via HBA1/HBA2 on chromosome 16, beta chain via HBB on chromosome 11); sickle-cell anaemia is also autosome-linked recessive but is a qualitative problem — an incorrectly functioning (sickle-shaped) globin from a Glu-to-Val substitution at the sixth position of the beta chain.

Mistake

Students classify Down's syndrome or Turner's syndrome as a Mendelian disorder because it is 'genetic', instead of recognising it as a chromosomal disorder.

Root cause: concept gap

Correction

Mendelian disorders (e.g. haemophilia, cystic fibrosis, sickle-cell anaemia, colour blindness, phenylketonuria, thalassemia) are caused by alteration/mutation in a single gene and are traced by pedigree analysis, whereas chromosomal disorders are caused by absence, excess or abnormal arrangement of whole chromosomes — this lesson covers only the Mendelian category.

Wrong option pattern

lists Down's syndrome or Turner's syndrome as an example of a Mendelian disorder

Mistake

Students assume a carrier mother of haemophilia or colour blindness must show some symptoms, missing that the recessive gene's effect is fully suppressed by the matching dominant normal gene.

Root cause: concept gap

Correction

The mother is not herself colour blind (or haemophilic) because the gene is recessive and its effect is suppressed by her matching dominant normal gene on the other X chromosome; she can still transmit the trait to sons with 50 per cent probability.

Wrong option pattern

claims a heterozygous carrier mother shows partial or mild symptoms of the disorder

More in Genetics and Evolution: 17 exam traps and mistakes · 4 formulas · 1 question pattern from its other lessons.

Mendelian Disorders Pedigree questions from past NEET papers

4 questions from NEET 2021, 2022, 2025. Answers verified against NTA official keys.

All 82 past-paper questions from Genetics and Evolution →

Sources

NCERT refs: Class 12 Biology Chapter 4, p.72

Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.

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