Chromosomal Disorders in Humans
Definition
Chromosomal disorders are genetic disorders caused by abnormalities in
the number or structure of chromosomes.
Chromosomal disorders are caused due to absence or excess or abnormal
arrangement of one or more chromosomes. These abnormalities usually arise due
to failure of chromosome segregation (nondisjunction) during meiosis or
structural damage to chromosomes.
Human Chromosomes
|
Feature |
Normal Human |
|
Total chromosomes |
46 |
|
Autosomes |
44 (22 pairs) |
|
Sex chromosomes |
XX (female), XY (male) |
|
Diploid number |
2n = 46 |
|
Haploid number |
n = 23 |
Classification of Chromosomal Disorders
Chromosomal Disorders
│
├── Numerical abnormalities
│
│
│
├── Aneuploidy
│
└── Polyploidy
│
└── Structural abnormalities
│
├── Deletion
├── Duplication
├── Inversion
└── Translocation
Numerical Chromosomal Abnormalities
These occur due to gain or loss of one or more chromosomes.
1. Aneuploidy
Loss or gain of individual chromosomes.
Examples
- Down syndrome → 47,+21
- Turner syndrome → 45,X
- Klinefelter syndrome → 47,XXY
Types
Monosomy
One chromosome missing. Example- 45, X (Turner syndrome)
Trisomy
One extra chromosome present.
Example- 47, +21 (Down syndrome)
2. Polyploidy
Entire chromosome sets are increased.
Examples
- 3n
- 4n
Usually lethal in humans. Common in plants.
Structural Chromosomal Abnormalities
Result from chromosome breakage.
Deletion
Loss of chromosome segment.
Example: Cri-du-chat syndrome (5p deletion)
Duplication
Repeated chromosome segment.
Inversion
Broken segment rejoins in reverse direction.
Translocation
Segment shifts to another chromosome.
Example: Robertsonian translocation involving chromosome 21. Can cause
familial Down syndrome.
Nondisjunction
Definition
Failure of homologous chromosomes or sister chromatids to separate
properly during meiosis. Produces abnormal gametes.
Normal Meiosis
2 chromosomes
↓
Separate
↓
Normal gametes
Nondisjunction
2 chromosomes
↓
Fail to separate
↓
n+1 n−1
Abnormal gametes
Consequences of Nondisjunction
Normal fertilization
↓
Abnormal zygote
↓
Aneuploidy
↓
Chromosomal disorder
Causes
- Meiotic errors
- Advanced maternal age
- Radiation
- Chemicals
- Rare spontaneous errors
Maternal Age Effect
Risk of nondisjunction increases with increasing maternal age. Especially
after 35 years. Important for Down syndrome.
Prenatal Diagnosis
Methods include
- Amniocentesis
- Chorionic Villus Sampling (CVS)
- Ultrasonography
- Karyotyping
- Fetal DNA analysis (advanced
testing)
Major Chromosomal Disorders (NCERT)
|
Disorder |
Karyotype |
Sex |
|
Down syndrome |
47,+21 |
Both |
|
Klinefelter syndrome |
47,XXY |
Male |
|
Turner syndrome |
45,X |
Female |
Down Syndrome
Cause
Extra copy of chromosome 21, (Trisomy 21), Karyotype 47, +21
Mechanism
Usually caused by maternal meiotic nondisjunction. Less commonly due to
- Robertsonian translocation
- Mosaicism
Clinical Features
- Intellectual disability
- Flat facial profile
- Slanting eyes
- Epicanthic folds
- Open mouth
- Furrowed tongue
- Broad palm with single palmar
crease
- Short stature
- Congenital heart defects
- Delayed development
Facta
✔ Trisomy 21
✔ First described by Langdon Down
✔ Incidence increases with maternal age
Klinefelter Syndrome
Cause
Extra X chromosome in male, Karyotype 47, XXY
Clinical Features
- Male
- Tall stature
- Small testes
- Sterility
- Sparse body hair
- Poor secondary sexual characters
- Gynecomastia
- Mild learning difficulties
Hormonal Features
↓ Testosterone
↑ FSH
↑ LH
Keywords
Sterile male
Female-like development
One extra X chromosome
Turner Syndrome
Cause
Loss of one X chromosome, Karyotype 45, X
Clinical Features
- Female
- Short stature
- Webbed neck
- Broad shield chest
- Widely spaced nipples
- Rudimentary ovaries
- Sterility
- Poor secondary sexual characters
- Primary amenorrhea
Hormonal Features
↓ Estrogen
↑ FSH
↑ LH
Only viable human monosomy.
Comparison Table
|
Feature |
Down |
Klinefelter |
Turner |
|
Karyotype |
47, +21 |
47,XXY |
45,X |
|
Sex |
Both |
Male |
Female |
|
Type |
Trisomy |
Sex chromosome trisomy |
Monosomy |
|
Intelligence |
Reduced |
Mildly affected |
Usually normal |
|
Fertility |
Variable |
Sterile |
Sterile |
|
Stature |
Short |
Tall |
Short |
|
Gonads |
Normal |
Small testes |
Streak ovaries |
|
Secondary sexual characters |
Delayed |
Poor male development |
Poor female development |
Disorders at a Glance
Chromosomal Disorders
Numerical
│
├── Down → Trisomy 21
├── Turner → XO
└── Klinefelter → XXY
Structural
│
├── Deletion
├── Duplication
├── Inversion
└── Translocation
High-Yield Facts
- Down syndrome = Trisomy 21.
- Klinefelter syndrome = XXY male.
- Turner syndrome = XO female.
- Down syndrome risk increases with
maternal age.
- Nondisjunction is the commonest
cause of aneuploidy.
- Turner syndrome is the only
survivable complete monosomy in humans.
- Polyploidy is generally
incompatible with human life.
- Robertsonian translocation can
produce familial Down syndrome.
Common One-Liners
- Normal human chromosome number = 46
- Haploid chromosome number = 23
- Trisomy means 2n + 1
- Monosomy means 2n − 1
- Down syndrome chromosome = 21
- Klinefelter syndrome = XXY
- Turner syndrome = XO
- Down syndrome incidence increases
after maternal age of 35 years
- Nondisjunction occurs during meiosis
- Chromosomal disorders involve
either numerical or structural abnormalities
Memory Tricks
Down Syndrome
"DOWN = Down to 21" → Trisomy 21
Turner Syndrome
"Turn One X Off" → XO (45, X)
Klinefelter Syndrome
"Male with an eXtra X" → XXY
References
- NCERT Biology, Class XII, Chapter 5: Principles of
Inheritance and Variation (latest edition) – Primary source for NEET
UG.
- National Medical Commission (NMC) recommended MBBS genetics
concepts (for foundational understanding).
- Thompson & Thompson Genetics
in Medicine, 9th Edition.
- Harper's Practical Genetic
Counselling, 8th Edition.
- Emery's Elements of Medical
Genetics, 16th Edition.
- National Human Genome Research
Institute (NHGRI) – Chromosomal abnormalities and genetic disorders.
- World Health Organization (WHO) – Congenital disorders and birth
defects resources.
Summary
Chromosomal disorders arise from abnormalities in chromosome number or
structure. Down syndrome (47, +21) results from trisomy 21 and is
strongly associated with advanced maternal age. Klinefelter syndrome (47, XXY)
affects males and causes hypogonadism and infertility, while Turner syndrome
(45, X) affects females and is the only viable complete monosomy in humans.
Understanding the mechanism of nondisjunction, recognizing
characteristic karyotypes, and distinguishing the clinical features of
these three syndromes are among the highest-yield genetics topics for UG exams.