CUET UG Biology Genetics Chromosomal Disorders Down Syndrome Turner Klinefelter Mutations

Must-Know (15–20 detailed bullets)

• Down syndrome is caused by trisomy of chromosome 21, resulting in 47 chromosomes in total.
• The karyotype of Down syndrome is 47, XX or 47, XY, +21 — the extra chromosome arises due to non-disjunction during meiosis.
• Physical features of Down syndrome include a flat facial profile, upslanting palpebral fissures, and a single palmar crease (simian crease).
• Individuals with Down syndrome have an increased risk of congenital heart defects, especially atrioventricular septal defect.
• The incidence of Down syndrome increases with maternal age; risk is significantly higher in mothers over 35 years.
• Turner syndrome results from monosomy of the X chromosome, with a karyotype of 45, X — one X chromosome is missing.
• Turner syndrome affects only females and is characterized by short stature, webbed neck, and underdeveloped ovaries (streak ovaries).
• Females with Turner syndrome are typically sterile and do not undergo normal pubertal development without hormone therapy.
• Klinefelter syndrome is caused by an extra X chromosome in males, with a karyotype of 47, XXY.
• Males with Klinefelter syndrome are often tall, have gynecomastia, small testes, and reduced facial and body hair.
• Klinefelter individuals produce little or no sperm and are usually infertile.
• The presence of two or more Barr bodies in a male cell indicates Klinefelter syndrome (one Barr body per extra X chromosome).
• Mutations are sudden heritable changes in DNA sequence or chromosome structure; they can be point mutations or chromosomal aberrations.
• Chromosomal disorders arise due to aneuploidy — abnormal number of chromosomes caused by non-disjunction during cell division.
• Non-disjunction in meiosis I or II leads to gametes with an extra or missing chromosome, which upon fertilization causes trisomy or monosomy.
• Down syndrome, Turner syndrome, and Klinefelter syndrome are examples of aneuploidy involving autosomes or sex chromosomes.
• Turner syndrome is the only viable monosomy in humans; autosomal monosomies are usually lethal in utero.
• In Klinefelter syndrome, the extra X chromosome undergoes inactivation, forming a Barr body, to maintain X dosage compensation.
• verify from NCERT — exact percentage risk of Down syndrome at maternal age 40.
• verify from NCERT — frequency of Klinefelter syndrome in male births.

Difficulty Level

Intermediate — requires understanding of karyotypes, meiotic errors, and phenotypic effects, but facts are directly from NCERT.

Common CUET Traps (3 bullets)

• Trap: Assuming Down syndrome is caused by a gene mutation.
  Avoid: Down syndrome is due to chromosomal aneuploidy (trisomy 21), not a point mutation.

• Trap: Believing Turner syndrome individuals have Y chromosome material.
  Avoid: Turner syndrome karyotype is 45, X — no Y chromosome is present; presence of Y would lead to different diagnosis.

• Trap: Confusing Barr body count in Klinefelter vs. normal female.
  Avoid: A 47, XXY male has one Barr body (one inactivated X), same as 46, XX female; number of Barr bodies = number of extra X chromosomes.

Practice MCQs (5 questions)

Q1. Which of the following is the karyotype of an individual with Down syndrome?
A. 45, X
B. 47, XXY
C. 47, XY, +21
D. 46, XX

Answer: C
Explanation: Down syndrome is caused by trisomy 21, so the karyotype is 47, XY, +21 or 47, XX, +21.
Why others fail: Option B (47, XXY) is Klinefelter syndrome, commonly confused due to both having 47 chromosomes.

Q2. Which chromosomal disorder is characterized by the presence of a single X chromosome in females?
A. Down syndrome
B. Turner syndrome
C. Klinefelter syndrome
D. Patau syndrome

Answer: B
Explanation: Turner syndrome has karyotype 45, X, with monosomy of the X chromosome.
Why others fail: Klinefelter affects males (XXY), and Down syndrome involves chromosome 21, not sex chromosomes.

Q3. A male individual presents with tall stature, gynecomastia, and small testes. His karyotype is most likely:
A. 47, XYY
B. 47, XXY
C. 45, X
D. 47, XY, +18

Answer: B
Explanation: These are classic symptoms of Klinefelter syndrome (47, XXY).
Why others fail: 47, XYY males are fertile and do not typically show gynecomastia or infertility.

Q4. Which of the following conditions results from non-disjunction during meiosis?
A. Sickle cell anemia
B. Phenylketonuria
C. Down syndrome
D. Thalassemia

Answer: C
Explanation: Down syndrome results from non-disjunction leading to trisomy 21.
Why others fail: Sickle cell, PKU, and thalassemia are autosomal recessive disorders due to gene mutations, not chromosomal non-disjunction.

Q5. How many Barr bodies are present in a somatic cell of a person with Klinefelter syndrome?
A. 0
B. 1
C. 2
D. 3

Answer: B
Explanation: A 47, XXY individual has one inactivated X chromosome, forming one Barr body.
Why others fail: Students often think more X chromosomes mean more Barr bodies, but the rule is: number of Barr bodies = number of X chromosomes minus one.

Last‑Minute Revision (15–20 one‑liners)

• ⚠️ Down syndrome: trisomy 21 → 47 chromosomes.
• ⚠️ Turner syndrome: 45, X — only viable human monosomy.
• ⚠️ Klinefelter syndrome: 47, XXY — male with one Barr body.
• ⚠️ Non-disjunction: failure of chromosome separation in meiosis I or II.
• ⚠️ Down syndrome risk ↑ with maternal age >35.
• ⚠️ Turner syndrome: webbed neck, streak ovaries, no puberty without therapy.
• ⚠️ Klinefelter: tall, gynecomastia, small testes, infertile.
• ⚠️ Barr body = inactivated X chromosome.
• ⚠️ Number of Barr bodies = total X chromosomes minus one.
• ⚠️ verify from NCERT — incidence of Down syndrome in 35-year-old mothers.
• ⚠️ verify from NCERT — frequency of Klinefelter syndrome.
• ⚠️ Autosomal trisomies: 13 (Patau), 18 (Edward), 21 (Down).
• ⚠️ Sex chromosome aneuploidy: less severe than autosomal due to X inactivation.
• ⚠️ Down syndrome: congenital heart defects common.
• ⚠️ verify from NCERT — percentage of Down syndrome cases due to translocation.
• ⚠️ Mutation: heritable change in DNA sequence or chromosome structure.
• ⚠️ Chromosomal disorders: caused by aneuploidy or structural changes.
• ⚠️ Monosomy: missing one chromosome; trisomy: extra chromosome.
• ⚠️ verify from NCERT — name of the syndrome with 47, XYY.
• ⚠️ Klinefelter: extra X inactivated → one Barr body.