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अभ्यासक्रम

Human Genetic Disorders - Chromosomal Disorders or Abnormalities

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Estimated time: 22 minutes
CBSE: Class 12

Introduction

Chromosomal disorders arise from changes in the number or structure of chromosomes, rather than mutations in individual genes. They are classified into Numerical Abnormalities (change in chromosome count) and Structural Abnormalities (change in chromosome architecture). Both types are caused either by errors during cell division or by exposure to mutagenic agents.

CBSE: Class 12

Classification

CBSE: Class 12

Numerical Abnormalities: Aneuploidy

Type Formula Total Chromosomes (human) Viability
Nullisomy 2n − 2 44 Usually lethal
Monosomy 2n − 1 45 Mostly lethal except Turner's (XO)​
Trisomy 2n + 1 47 Variable — depends on chromosome involved
Tetrasomy 2n + 2 48 Rare; some viable sex-chromosome tetrasomies
CBSE: Class 12

Numerical Abnormalities: Autosomal Trisomies

1. Down's Syndrome (Trisomy 21)

  • Karyotype: 47, +21 (extra chromosome 21); also written as 44A + 2sex chromosomes + 1 extra chromosome at 21​
  • Incidence: 1 in 660–700 live births — most common autosomal aneuploidy​
  • Cause: Nondisjunction during oogenesis (maternal nondisjunction in ~95% cases); risk increases with maternal age​

Clinical Features:

  • Short stature, broad hands with short fingers
  • Flat facial profile with broad skull and flat nasal bridge
  • Epicanthal folds (skin fold at inner corner of eye)
  • Large, furrowed tongue; open mouth appearance
  • Intellectual disability (mild to moderate)
  • Congenital heart defects (present in ~50% of cases)
  • Increased risk of acute leukaemia and Alzheimer's disease

2. Patau Syndrome (Trisomy 13):

  • Karyotype: 47, +13 (extra chromosome 13)​
  • Incidence: 1 in 5,000 live births​

Clinical Features:

  • Severe intellectual disability
  • Congenital heart defects
  • Extra fingers (polydactyly)
  • Cleft palate and lip
  • Small eyes (microphthalmia) or absent eyes (anophthalmia)
  • Most affected infants die within the first year of life

3. Edwards Syndrome (Trisomy 18):

  • Karyotype: 47, +18 (extra chromosome 18)​
  • Incidence: Second most common trisomy in live births​

Clinical Features:

  • Severe intellectual disability
  • Characteristic "rocker-bottom" feet
  • Overlapping fingers with clenched fists
  • Congenital heart, kidney, and lung defects
  • Very high neonatal mortality; most survive less than 1 year
CBSE: Class 12

Numerical Abnormalities: Sex Chromosome Aneuploidies

1. Klinefelter's Syndrome (47, XXY):

  • Karyotype: 47, XXY - extra X chromosome in genetic males​
  • Gender: Affects males only
  • Incidence: ~1 in 650 male newborns - most common sex chromosome disorder​
  • Cause: Nondisjunction in Meiosis I or II during gametogenesis in either parent​

Clinical Features:

  • Tall stature with long limbs
  • Small, underdeveloped testes (hypogonadism/micro-orchidism)
  • Gynecomastia (breast tissue development) in late puberty
  • Sparse body and facial hair
  • Infertility (azoospermia/oligospermia) — most common cause of male hypogonadism
  • Mild intellectual or learning difficulties in some patients
  • Elevated urinary gonadotropin levels

2. Turner's Syndrome (45, XO):

  • Karyotype: 45, XO (monosomy X) - only one functional X chromosome
  • Gender: Affects females only
  • Incidence: ~1 in 2,000–2,500 live female births
  • Cause: Nondisjunction resulting in loss of one sex chromosome; missing chromosome is usually the paternal X in ~2/3 of cases​

Clinical Features:

  • Short stature (average adult height ~145 cm)
  • Webbed neck (pterygium colli)
  • Broad chest with widely spaced nipples
  • Rudimentary / streak ovaries → absent secondary sexual characteristics
  • Primary amenorrhoea (no menstruation)
  • Infertility (primary ovarian insufficiency)
  • Normal intelligence in most cases
  • Risk of cardiac abnormalities (coarctation of aorta)

3. Triple X Syndrome (47, XXX):

  • Karyotype: 47, XXX - extra X chromosome in females​
  • Incidence: ~1 in 1,000 female newborns​

Features: Usually phenotypically normal females; taller than average; mild learning difficulties possible; often fertile

4. XYY Syndrome (47, XYY):

  • Karyotype: 47, XYY - extra Y chromosome in males​
  • Incidence: ~1 in 1,000 male newborns​

Features: Phenotypically normal males; tall stature; fertile in most cases; associated with behavioural tendencies in older literature (no longer considered clinically significant)

CBSE: Class 12

Numerical Abnormalities: Polyploidy (Euploidy)

  • Increase in the number of entire chromosome sets (3n, 4n, 6n…)​
  • Examples: Triploid (3n), Tetraploid (4n)
  • In Plants: Very common - wheat (Triticum) is hexaploid (6n = 42); banana is triploid (3n); polyploid crops often show increased size and vigour​
  • In Animals: Generally lethal - very rare in mammals as dosage compensation mechanisms cannot accommodate multiple X chromosomes
CBSE: Class 12

Structural Abnormalities

Structural chromosomal abnormalities arise when a chromosome segment breaks and rejoins abnormally. This may occur spontaneously or be induced by radiation, chemicals, or viruses.

Types of Structural Abnormalities:

Type What Happens Example Effect
Deletion A chromosomal segment is lost permanently​ Cri-du-chat (deletion on chr. 5p) Loss of genes; severe developmental effects
Duplication A chromosomal segment is copied and repeated​ Fragile X syndrome (X chromosome repeat expansion at Xq27.3)​ Gene overdose; variable effects
Inversion A segment detaches and reattaches in reverse orientation​ Inversion on chromosome 9 Often silent; may affect gene expression
Translocation A segment from one chromosome attaches to a non-homologous chromosome​ Philadelphia chromosome (chr. 9 → chr. 22) → Chronic Myeloid Leukaemia Cancer risk; carrier may appear normal
CBSE: Class 12
Maharashtra State Board: Class 12

Key Points: Chromosomal Disorders or Abnormalities

Chromosomal Disorder Chromosomal Abnormality Karyotype Major Features Remarks
Down’s Syndrome Extra copy of chromosome 21 (Trisomy 21) 47, +21 Short stature, small round head, furrowed tongue, partially open mouth, broad palm with single crease, mental and physical retardation Most common autosomal aneuploidy
Klinefelter’s Syndrome Extra X chromosome in males 47, XXY Masculine body with feminine traits (gynecomastia), underdeveloped testes, sterile Sex-chromosome aneuploidy
Turner’s Syndrome Absence of one X chromosome 45, XO Short stature, rudimentary ovaries, absence of secondary sexual characters, sterile female Only monosomy compatible with life
Aneuploidy Gain or loss of one or more chromosomes due to non-disjunction 2n ± 1 Severe developmental abnormalities Includes trisomy and monosomy
Polyploidy Increase in whole set of chromosomes due to failure of cytokinesis 3n, 4n, etc. Common in plants; increases size and vigor Rare and usually lethal in animals

Related QuestionsVIEW ALL [9]

The chromosome number is fixed for all normal organisms leading to species specification whereas any abnormality in the chromosome number of an organism results into abnormal individuals. For example, in humans, 46 is the fixed number of chromosomes both male and female. In males it is '44 + XY' and in females, it is '44 +XX'. Thus the human male is heterogametic, in other words produces two different types of gametes one with '22 + X' chromosomes and the other with '22 + Y' chromosomes respectively. Human female, on the other hand, is homo gametic i.e. produce only one type of gamete with '22 + X' chromosomes only.

Sometimes an error may occur during the meiosis of the cell cycle, where the sister chromatids fail to segregate called nondisjunction, leading to the production of abnormal gametes with altered chromosome numbers. On fertilisation, such gametes develop into abnormal individuals.

(a) State what is aneuploidy. (1)

(b) If during spermatogenesis, the chromatids of sex chromosomes fail to segregate during meiosis, write only the different types of gametes with altered chromosome numbers that could possibly be produced. (1)

(c) A normal human sperm (22 + Y) fertilises an ovum with karyotype '22 +XX'. Name the disorder the offspring thus produced would suffer from and write any two symptoms of the disorder. (2)

OR

(c) Name the best-known and most common autosomal aneuploid abnormality in humans and write any two symptoms. (2)

Pataus’ syndrome is also referred to as ____________.

Klinefelters’ syndrome is characterized by a karyotype of ____________.

What is Down’s syndrome? Give its symptoms and cause. Why is it that the chances of having a child with Down’s syndrome increases if the age of the mother exceeds forty years?

Which of the following is not correct?

Define aneuploidy. How is it different from polyploidy? Describe the individuals having following chromosomal abnormalities.

  1. Trisomy of 21st Chromosome
  2. XXY
  3. XO

How was it concluded that genes are located on chromosomes?

Females with Turners’ syndrome have

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