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 89 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?
What is the total chromosome number in a person with Turner syndrome?
Klinefelter syndrome has which of the following karyotypes?
A deletion of 5 nucleotides occurs in a coding region. Which of the following best describes the effect on the reading frame?
An insertion of 6 nucleotides occurs in a gene's coding sequence. What happens to the reading frame downstream of the insertion?
Which of the following correctly distinguishes Down syndrome from Turner syndrome?
A point mutation replaces one purine with another purine in a gene. This type of substitution is called:
A child is born with 47 chromosomes and exhibits gynecomastia, tall stature, and small testes. The underlying chromosomal abnormality is most likely:
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 89 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. 89F2 ratio in monohybrid cross — products of independent assortment of two alleles per locus.
| Symbol | Quantity | SI Unit |
|---|---|---|
| ratio | F2 progeny ratio | - |
F2 ratio in dihybrid cross with two independently-segregating loci, complete dominance, no linkage.
| Symbol | Quantity | SI Unit |
|---|---|---|
| ratio | F2 phenotype ratio | - |
In an idealised population (no mutation, drift, selection, gene flow, random mating), allele and genotype frequencies remain constant.
| Symbol | Quantity | SI Unit |
|---|---|---|
| p | freq of dominant allele A | - |
| q | freq of recessive allele a | - |
Proportion of recombinant offspring measures genetic distance between linked loci. Capped at 50% (independent assortment).
| Symbol | Quantity | SI Unit |
|---|---|---|
| RF | recombination frequency | % |
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Sign Convention
DNA polymerase synthesises only 5'→3'. Leading strand: continuous, same direction as fork. Lagging: discontinuous (Okazaki), opposite to fork.
Question on Okazaki, leading vs lagging, primer direction.
Reading template 3'→5'; synthesising 5'→3'. Lagging strand needs short fragments because it can't run continuously against fork direction.
Category: Negative Marking
Five forces disturb HW: mutation, gene flow, drift, selection, non-random mating. ANY of these violates equilibrium.
Question asks which factor maintains/disturbs HW.
Random mating + no other forces → equilibrium. ANY of mutation/migration/drift/selection/assortative mating → disequilibrium.
Category: Similar Terms
Monohybrid: genotype 1:2:1 (AA:Aa:aa); phenotype 3:1.
Question asks for one ratio while presenting cross details.
Always note dominance: phenotype merges Aa + AA; genotype keeps them separate.
Category: Similar Terms
X-linked recessive (haemophilia, colour-blindness): affects males predominantly; carrier mother → 50% sons affected; affected father → all daughters carriers but not affected.
Pedigree question; carrier vs affected.
Sex chromosomes: XX vs XY. Recessive on X needs both copies (XaXa) in female, only one (XaY) in male.
Category: Similar Terms
Allopatric: geographic isolation. Sympatric: same area, no physical barrier (e.g. polyploidy in plants, host-shift).
Question gives speciation scenario and asks which type.
If geographic barrier mentioned → allopatric. If population overlaps → sympatric.
Root cause: formula misuse
Genotype frequencies: p² (AA) + 2pq (Aa) + q² (aa) = 1. Allele frequencies: p + q = 1. Don't drop 2pq.
Root cause: concept gap
ALL DNA pol synthesises 5'→3'. Lagging strand uses Okazaki fragments to APPEAR to extend toward fork while each fragment grows 5'→3'.
Root cause: concept gap
Frameshift = ANY indel NOT divisible by 3 (since codons are triplets). Indels of 3, 6, 9... preserve reading frame.
Root cause: term confusion
Start codon: AUG (Met / fMet). Stop codons: UAA (ochre), UAG (amber), UGA (opal/umber).
51 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
Identify the statement that is NOT correct.
Which factor is important for termination of transcription?
What is the pattern of inheritance for polygenic trait?
Which of the following statement is correct regarding the process of replication in E.coli?
Which one of the following factors will not affect the Hardy-Weinberg equilibrium?
The phenomenon of pleiotropism refers to
What is the role of RNA polymerase III in the process of transcription in Eukaryotes?
Among eukaryotes, replication of DNA takes place in :
Expressed Sequence Tags (ESTs) refers to
Unequivocal proof that DNA is the genetic material was first proposed by
Broad palm with single palm crease is visible in a person suffering from-
Select the correct group/set of Australian Marsupials exhibiting adaptive radiation.
XO type of sex determination can be found in :
The process of translation of mRNA to proteins begins as soon as :
Which of the following occurs due to the presence of autosome linked dominant trait ?
Detritivores breakdown detritus into smaller particles. This process is called:
If the length of a DNA molecule is 1.1 metres, what will be the approximate number of base pairs?
Which of the following is not a desirable feature of a cloning vector?
Mutations in plant cells can be induced by: 130. Match List-I with List-II.
Which of the following RNAs is not required for the synthesis of protein?
•ŸÈ‹ πŸ ∑ § ‚◊ÿ «UË.∞Ÿ.∞. ∑§Ë ∑È §«U‹Ë ∑§Ê πÊ ‹Ÿ ◊ ∑§ÊÒŸ‚Ê
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
similar term confusion
Biology relies on precise terminology; close terms tempt selection.
Test yourself on this topic with real past-paper questions:
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