Q.Down's syndrome is caused due to

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1. CBSE 2026Set A1 markMCQ
   Q.Which of the following is not a chromosomal disorder?

   (a) Klinefelter's syndrome

   (b) Turner's syndrome

   (c) Down's syndrome

   (d) Haemophilia

   ›Reveal solutionSolution

   Klinefelter's, Turner's and Down's syndromes are chromosomal (numerical) disorders; haemophilia is a gene mutation.

   Chromosomal disorders result from an abnormal number of chromosomes (aneuploidy): Klinefelter's (47, XXY), Turner's (45, X0) and Down's syndrome (trisomy 21). Haemophilia, however, is a Mendelian (single-gene) disorder caused by m …

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2. CBSE 2026Set ZOOLOGY1 markMCQ
   Q.What is the genotype of a person suffering from Klinefelter's syndrome?

   (a) AA + XXY

   (b) AA + XYY

   (c) A + XXY

   (d) A + XYY

   ›Reveal solutionSolution

   Klinefelter's syndrome arises from non-disjunction, giving an individual 44 autosomes plus XXY sex chromosomes (47,XXY).

   Klinefelter's syndrome is a genetic disorder caused by non-disjunction of the X chromosomes during gamete formation, resulting in a zygote with an extra X chromosome. Such an individual has a total of 47 chromosomes: 44 autosomes (written as AA, representing the 22 homologous pairs) plus the sex chromosomes XXY (instead of the normal XY). Affected individuals are …

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3. CBSE 2026Set ANNUAL1 markMCQ
   Q.Genetic disorder which is caused due to the absence of one X-chromosome

   (a) Down's syndrome

   (b) Turner's syndrome

   (c) Klinefelter's syndrome

   (d) Haemophilia

   ›Reveal solutionSolution

   Turner's syndrome is caused by monosomy of the X chromosome (45, X0), i.e. total absence of one X chromosome in females.

   Turner's syndrome (45, X0) arises due to the absence of one X chromosome - women with this syndrome have only 45 chromosomes with a single X (no second X or Y). Affected individuals are sterile, short in stature, and show a webbed neck and rudimentary ovaries. Down's syndrome is trisomy of chromosome 21; Kli …

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4. CBSE 2026Set ANNUAL1 mark
   Q.Write the causes of the genetic disorder Klinefelter's syndrome.

   ›Reveal solutionSolution

   Klinefelter's syndrome results from non-disjunction of the sex chromosomes during meiosis, giving a male an extra X chromosome (47, XXY).

   During gametogenesis, if the sex chromosomes fail to separate properly (non-disjunction) - for example, a gamete carrying XX (instead of a single X) fuses with a normal Y-bearing sperm, or an XY-bearing sperm fuses with a normal X-bearing egg - the resulting zygote has an extra X chromosome, giving the karyotype 47, XXY. Affected individuals are male, but show feminised body features (gynecomastia/breast developme …

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5. CBSE 2026Set ANNUAL1 mark
   Q.What is chromosomal disorder? Give one example.

   ›Reveal solutionSolution

   Chromosomal disorders result from the gain, loss, or structural abnormality of whole chromosomes or chromosome segments, unlike gene/point mutations affecting a single gene.

   A chromosomal disorder arises from the absence, excess, or abnormal arrangement of one or more chromosomes — that is, an error at the level of whole chromosomes (aneuploidy or structural rearrangement), rather than a change within a single gene's DNA sequence (which would be a gene/point mutation). These errors usually result from problems during meiosis, such as non-disjunction (failure of chromosome pairs/chromatids to separate properly).

   …

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6. CBSE 2026Set ANNUAL1 markMCQ
   Q.Down's syndrome is caused due to

   (a) non-disjunction of chromosome number 21

   (b) deletion of chromosome number 21

   (c) non-disjunction of chromosome number 16

   (d) deletion of chromosome number 16

   ›Reveal solutionSolution

   Down's syndrome is caused by trisomy 21, arising from the failure of chromosome 21 to separate properly (non-disjunction) during meiosis.

   Down's syndrome results when chromosome 21 fails to segregate properly during meiosis (a phenomenon called non-disjunction), so that a gamete ends up carrying two copies of chromosome 21 instead of one; when this gamete combines with a normal gamete at fertilization, the resulting individual has three copies of chromosome 21 (trisomy 21) instead of the usual two, giving a total of 47 chromosomes. Deletion o …

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7. CBSE 2026Set ANNUAL1 mark
   Q.A female suffering from Turner's syndrome has 45 chromosomes. She will have—

   (a) how many autosomes;

   (b) how many X-chromosome(s)? ($\tfrac{1}{2}+\tfrac{1}{2}=1$)

   ›Reveal solutionSolution

   Turner's syndrome results from monosomy of the X chromosome, so an affected female has the normal 44 autosomes but only a single X chromosome.

   Turner's syndrome is a sex-chromosomal monosomy in females, arising from the absence of one X chromosome (karyotype 45,X or 44+XO), giving a total of 45 chromosomes instead of the normal 46. Since the loss involves a sex chromosome and not an autosome, the number of autosomes remains at the …

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8. CBSE 2025Set 57/5/11 markMCQ
   Q.Which of the following is not an example of aneuploidy ? (A) Turner's syndrome (B) Down's syndrome (C) Phenylketonuria (D) Klinefelter's syndrome

   ›Reveal solutionSolution

   Aneuploidy means an abnormal number of chromosomes (not a complete set). Phenylketonuria is a gene mutation, not a chromosomal number change, so (C) is the answer.

   The concept: What is aneuploidy?

   Aneuploidy refers to a condition where an organism has a chromosome number that is not an exact multiple of the haploid number. In simpler terms, instead of having the normal diploid set (46 chromosomes in humans, arranged as 23 pairs), an aneuploid individual has one or more extra chromosomes or is missing one or more chromosomes.

   The key is that aneuploidy involves whole chromosome gain or loss, not changes within a gene on a chromosome. Common types include:

   • Monosomy: loss of one chromosome (2n - 1 = 45 chromosomes)
   • Trisomy: gain of one chromosome (2n + 1 = 47 chromosomes)

   This typically arises from nondisjunction during meiosis, when chromosomes fail to separate properly.

   Examining each option

   Let's identify which conditions involve chromosome number abnormalities and which do not.

   1. Turner's syndrome (Option A)

      This is a monosomy of the sex chromosomes, denoted as 45,X. Affected individuals (phenotypically female) have only one X chromosome instead of the normal XX pair. Total chromosome count: 45.

      This is aneuploidy.

   2. Down's syndrome (Option B)

      Also called Trisomy 21, this condition results from three copies of chromosome 21 instead of two. The karyotype is 47,XX,+21 or 47,XY,+21. Total chromosome count: 47.

      This is aneuploidy.

   3. Phenylketonuria (Option C) …

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9. CBSE 2025Set F1 markMCQ
   Q.In which of the following is an extra pair of chromosomes found?

   (a) Trisomic

   (b) Double trisomic

   (c) Tetrasomic

   (d) Nullisomic

   ›Reveal solutionSolution

   Tetrasomy (2n+2) adds an extra pair of one chromosome.

   Aneuploidy involves gain or loss of chromosomes. A trisomic (2n+1) has one extra chromosome; a double trisomic (2n+1+1) has one extra copy each of two different chromosomes; a nullisomic (2n-2) has a whole pair missing. A tetrasomic (2n+2) has two extra copies of the same chr …

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10. CBSE 2025Set F1 markMCQ
    Q.Which of the following represents the basic number of chromosomes?

    (a) n

    (b) 2n

    (c) 4n

    (d) n/2

    ›Reveal solutionSolution

    The basic chromosome number is one complete set, the haploid number, written as n.

    A gamete carries a single complete set of chromosomes — the basic or haploid number, represented by n. The zygote formed by fusion of two gametes is diploid (2n), carrying two sets. Polyploids …

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11. CBSE 2025Set ANNUAL1 markMCQ
    Q.Genetic disorder which is caused due to presence of an additional copy of X-chromosome -

    (a) Down's syndrome

    (b) Turner's syndrome

    (c) Klinefelter's syndrome

    (d) Haemophilia

    ›Reveal solutionSolution

    Klinefelter's syndrome arises from non-disjunction giving an individual an additional X chromosome, karyotype 47, XXY.

    Due to non-disjunction of sex chromosomes during gamete formation, some individuals receive an extra X chromosome, resulting in a male with karyotype 44+XXY (47 chromosomes total). Such individuals have male sexual characteristics but with feminising features (gynaecomastia, sterility). Down's syndrome instead involves trisomy of an autoso …

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12. CBSE 2025Set ANNUAL1 markMCQ
    Q.Gynaecomastia in males is observed in –

    (a) Down's syndrome

    (b) Klinefelter's syndrome

    (c) Turner's syndrome

    (d) Phenylketonuria

    ›Reveal solutionSolution

    Gynaecomastia (breast development in males) is a diagnostic feature of Klinefelter's syndrome, caused by an extra X chromosome (47, XXY).

    Klinefelter's syndrome results from non-disjunction of the X chromosomes during gamete formation, giving a zygote with karyotype 47, XXY (an extra X chromosome). Affected individuals are male in gross external appearance but show a mix of male and female features: tall stature, feminised body contours, development of breasts (gynaecomastia), sparse facial/body hair, and sterility due to underdeveloped testes; mild intellectual disability may also occur.

    Why the other options are wrong:

    • Down's syndrome (option a) — trisomy of chromosome 21 (47 chromosomes, an extra chromosome 21); features include short stature, small round head, furrowed tongue, partially open mouth, palm crease, and mental retardation — gynaecomastia is not a feature. …

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