A common trap on this topic: confusing when an insertion or deletion causes a frameshift versus when it does not. The rule is simple — any indel NOT divisible by 3 shifts the reading frame. Indels of 3, 6, or 9 bases add or remove whole codons without disrupting the downstream sequence. NEET distractors exploit this by offering "frameshift occurs when 3 bases are deleted" as a plausible-sounding option.
Mutations are heritable changes in genetic material. NCERT Class 12 Biology Chapter 4, page 72 classifies them broadly as gene (point) mutations and chromosomal aberrations. Point mutations include substitution (one base replaced — transitions or transversions), insertion, and deletion. Chromosomal mutations involve larger structural changes: deletion, duplication, inversion, and translocation of chromosome segments. Aneuploidy — gain or loss of individual chromosomes — underlies the three disorders this lesson targets.
Down syndrome (trisomy 21): Three copies of chromosome 21 due to non-disjunction. Karyotype: 47 chromosomes (2n + 1). Features include intellectual disability, short stature, characteristic facial features, and a single palmar crease.
Turner syndrome (45, X0): Monosomy of the X chromosome in females. Karyotype: 45 chromosomes (2n − 1). Features include short stature, webbed neck, infertility due to rudimentary ovaries.
Klinefelter syndrome (47, XXY): An extra X chromosome in males. Karyotype: 47 chromosomes (2n + 1). Features include tall stature, gynecomastia, reduced fertility, and small testes.
The key distinction NEET tests: Down syndrome is autosomal aneuploidy; Turner and Klinefelter are sex-chromosome aneuploidies. A second frequent test point: whether the disorder adds or loses a chromosome — Turner is monosomy (one fewer), while Down and Klinefelter are trisomy (one extra).
Watch out for distractors that swap chromosome numbers (e.g., claiming Turner has 47 chromosomes) or assign the wrong type of aneuploidy to a disorder.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Down syndrome is caused by trisomy of which chromosome?
Answer: D. Down syndrome results from trisomy of chromosome 21, giving a total of 47 chromosomes. This is described in NCERT Class 12 Biology Chapter 4, page 76.
Why A is wrong: A is wrong because trisomy of chromosome 18 causes Edwards syndrome, not Down syndrome.
Why B is wrong: B is wrong because trisomy of the X chromosome leads to conditions like Triple X syndrome (XXX) or Klinefelter syndrome (XXY), not Down syndrome.
Why C is wrong: C is wrong because trisomy of chromosome 13 causes Patau syndrome, not Down syndrome.
What is the total chromosome number in a person with Turner syndrome?
Answer: A. Turner syndrome is monosomy of the X chromosome (45, X0), giving a total of 45 chromosomes instead of the normal 46. Described in NCERT Class 12 Biology Chapter 4, page 76.
Why B is wrong: B is wrong because 44 would represent the loss of two chromosomes, which does not correspond to Turner syndrome.
Why C is wrong: C is wrong because 46 is the normal diploid number; Turner syndrome involves loss of one sex chromosome.
Why D is wrong: D is wrong because 47 chromosomes indicate trisomy (as in Down or Klinefelter syndrome), not the monosomy seen in Turner syndrome.
Klinefelter syndrome has which of the following karyotypes?
Answer: B. Klinefelter syndrome results from an extra X chromosome in a male, giving the karyotype 47, XXY. Described in NCERT Class 12 Biology Chapter 4, page 76.
Why A is wrong: A is wrong because 45, X0 is the karyotype of Turner syndrome, which affects females with monosomy of X.
Why C is wrong: C is wrong because 47, XYY is a distinct sex-chromosome aneuploidy (Jacob syndrome) — the extra chromosome is Y, not X.
Why D is wrong: D is wrong because 46, XX is the normal female karyotype.
A deletion of 5 nucleotides occurs in a coding region. Which of the following best describes the effect on the reading frame?
Answer: C. Codons are triplets. A deletion of 5 bases is not divisible by 3, so the reading frame shifts for all downstream codons. This is the definition of a frameshift mutation — the trap is confusing "multiple of 3" with any number larger than 3 (mistake: mistake: mutation types correction).
Why A is wrong: A is wrong because the reading frame is disrupted whenever the indel is not a multiple of 3. Being greater than 3 is irrelevant — only divisibility by 3 matters (trap: frameshift definition confusion).
Why B is wrong: B is wrong because deletions absolutely affect coding regions. This distractor conflates introns (non-coding, spliced out) with all non-coding sequences.
Why D is wrong: D is wrong because deletions remove a stretch of bases, not necessarily whole codons. Only deletions of 3, 6, 9… bases remove whole codons and preserve the frame.
An insertion of 6 nucleotides occurs in a gene's coding sequence. What happens to the reading frame downstream of the insertion?
Answer: C. Since 6 is divisible by 3, the insertion adds two complete codons without disrupting the reading frame of downstream codons. The rest of the protein sequence remains unaltered. This directly applies the frameshift rule: only indels NOT divisible by 3 cause frameshifts.
Why A is wrong: A is wrong because 6 bases = 2 complete codons, so the downstream reading frame is preserved. Frameshifts occur only when the indel is not a multiple of 3 (trap: frameshift definition confusion).
Why B is wrong: B is wrong because preserving the reading frame means downstream codons are normal. A premature stop codon is possible but not guaranteed — the two new codons could code for any amino acids.
Why D is wrong: D is wrong because mRNA degradation (nonsense-mediated decay) is triggered by premature stop codons, not by in-frame insertions.
Which of the following correctly distinguishes Down syndrome from Turner syndrome?
Answer: D. Down syndrome involves trisomy of autosome 21 (47 chromosomes), while Turner syndrome involves monosomy of the X chromosome (45 chromosomes). One is autosomal, the other is sex-chromosomal; one is trisomy, the other is monosomy. Described in NCERT Class 12 Biology Chapter 4, page 76.
Why A is wrong: A is wrong because Down syndrome involves chromosome 21, an autosome, not a sex chromosome.
Why B is wrong: B is wrong because Turner syndrome is a sex-chromosome disorder (45, X0), not an autosomal trisomy. Confusing autosomal and sex-chromosome aneuploidies is a common distractor pattern.
Why C is wrong: C is wrong because it reverses both disorders. Down syndrome is autosomal trisomy (not sex-chromosome monosomy), and Turner syndrome is sex-chromosome monosomy (not autosomal trisomy).
A point mutation replaces one purine with another purine in a gene. This type of substitution is called:
Answer: A. A transition is a substitution of a purine for a purine (A ↔ G) or a pyrimidine for a pyrimidine (C ↔ T). Replacing one purine with another purine fits the definition of a transition exactly.
Why B is wrong: B is wrong because a transversion is the substitution of a purine for a pyrimidine (or vice versa), not purine for purine. This is a common term-confusion trap.
Why C is wrong: C is wrong because frameshift mutations result from insertions or deletions that alter the reading frame, not from single-base substitutions.
Why D is wrong: D is wrong because chromosomal translocation involves transfer of a segment between non-homologous chromosomes — a structural chromosomal aberration, not a point mutation.
A child is born with 47 chromosomes and exhibits gynecomastia, tall stature, and small testes. The underlying chromosomal abnormality is most likely:
Answer: B. The described features — gynecomastia, tall stature, small testes, and 47 chromosomes — are characteristic of Klinefelter syndrome (47, XXY). Described in NCERT Class 12 Biology Chapter 4, page 76.
Why A is wrong: A is wrong because trisomy 21 (Down syndrome) presents with intellectual disability, short stature, and characteristic facial features — not gynecomastia or small testes.
Why C is wrong: C is wrong because 45, X0 (Turner syndrome) has only 45 chromosomes and affects females. It cannot present with testes.
Why D is wrong: D is wrong because 47, XYY (Jacob syndrome) presents with tall stature but typically does NOT cause gynecomastia or significantly reduced testicular size. The extra chromosome is Y, not X.
Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.
Given
A woman who is phenotypically normal gives birth to a child with Down syndrome. Karyotyping reveals the child has 47 chromosomes with three copies of chromosome 21.
Required
Identify the type of mutation, the mechanism that caused it, and the stage of cell division where the error most likely occurred.
Concept
Down syndrome is caused by aneuploidy — specifically trisomy 21. Aneuploidy results from non-disjunction, the failure of homologous chromosomes (meiosis I) or sister chromatids (meiosis II) to separate properly during cell division. NCERT Class 12 Biology Chapter 4, page 75 documents this mechanism.
Formula / Rule
Normal gamete: n = 23 chromosomes. Non-disjunction produces gametes with n + 1 = 24 or n − 1 = 22. Fertilisation of an n + 1 gamete by a normal n gamete yields 2n + 1 = 47.
Substitution
The child received a gamete carrying 24 chromosomes (with two copies of chromosome 21) from one parent and a normal gamete with 23 chromosomes from the other parent. Total: 24 + 23 = 47.
Calculation
47 chromosomes total. Three copies of chromosome 21 confirmed by karyotype. This is trisomy (2n + 1).
Final answer
The child has trisomy 21 (Down syndrome), caused by non-disjunction during meiosis in one of the parents. The most common origin is non-disjunction during maternal meiosis I, though it can also occur at meiosis II or (rarely) during paternal meiosis.
Common trap
Confusing trisomy (2n + 1 = 47) with triploidy (3n = 69). Triploidy means an entire extra set of chromosomes, which is almost always lethal. Trisomy is the gain of just one extra chromosome. NEET distractors sometimes offer "triploidy" as an option when the question describes a single extra chromosome.
Similar NEET-style question
A male infant is karyotyped and found to have 47 chromosomes with the sex-chromosome complement XXY. Identify the syndrome, state whether it is autosomal or sex-chromosomal aneuploidy, and name the mechanism responsible.
*(Answer: Klinefelter syndrome; sex-chromosome aneuploidy; caused by non-disjunction producing an XX gamete that was fertilised by a Y-bearing gamete, or an XY gamete fertilised by an X-bearing gamete.)*
---
Mendelian: sickle cell anaemia (autosomal recessive), thalassaemia, phenylketonuria, haemophilia (X-linked recessive). Chromosomal: Down syndrome (trisomy 21), Klinefelter (XXY), Turner (XO).
-- NCERT Class 12 Biology, Ch. 4, p. 73These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Root cause: concept gap
Frameshift = ANY indel NOT divisible by 3 (since codons are triplets). Indels of 3, 6, 9... preserve reading frame.
More in Genetics and Evolution: 20 exam traps and mistakes · 4 formulas · 1 question pattern from its other lessons.
5 questions from NEET 2021, 2023, 2024, 2026. Answers verified against NTA official keys.
Broad palm with single palm crease is visible in a person suffering from-
Mutations in plant cells can be induced by:
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.
Test yourself on this topic with real past-paper questions:
Practice this topic →