Genetic disorders are diseases or conditions that occur due to abnormalities in genes or chromosomes. These abnormalities may be inherited from parents or may arise due to mutations during the formation of gametes or early development.
Understanding genetic disorders helps us:
β’ Explain inherited diseases
β’ Identify patterns of inheritance
β’ Improve diagnosis and treatment
β’ Understand human genetics and evolution
A genetic disorder is defined as:
A disorder caused by changes or defects in genes or chromosomes that affect normal body functions.
Key Features:
β’ May be inherited or arise spontaneously
β’ Can affect physical, physiological, or biochemical functions
β’ Often follow Mendelian inheritance patterns
According to NCERT, genetic disorders are broadly classified into:
1. Mendelian Disorders
2. Chromosomal Disorders
Mendelian disorders are caused by mutations in a single gene and follow Mendelian inheritance patterns (dominant or recessive).
Characteristics:
β’ Controlled by a single gene
β’ Can be autosomal or sex-linked
β’ Show predictable inheritance patterns
Types of Mendelian Disorders
A. Autosomal Recessive Disorders
These disorders appear only when both alleles are defective.
1. Sickle Cell Anaemia
β’ Caused by point mutation in Ξ²-globin gene
β’ Codon change: GAG β GTG
β’ Amino acid change: Glutamic acid β Valine
β’ RBCs become sickle-shaped under low oxygen
π Effects:
β’ Reduced oxygen transport
β’ Anaemia, pain, fatigue
2. Phenylketonuria (PKU)
β’ Caused by absence of enzyme phenylalanine hydroxylase
β’ Phenylalanine accumulates in blood
β’ Leads to mental retardation if untreated
π Inheritance: Autosomal recessive
B. Autosomal Dominant Disorders
Only one defective allele is enough to cause the disorder.
3. Huntingtonβs Disease
β’ Progressive neurodegenerative disorder
β’ Appears in adulthood (delayed onset)
β’ Affects nervous system
π Inheritance: Autosomal dominant
C. Sex-Linked Recessive Disorders
These disorders are caused by defective genes on the X chromosome.
4. Haemophilia
β’ Blood clotting disorder
β’ Due to deficiency of clotting factors
β’ Affected individuals bleed excessively
π Inheritance: X-linked recessive
π Mostly affects males
5. Colour Blindness
β’ Inability to distinguish certain colours (especially red and green)
β’ Caused by mutation on X chromosome
π Inheritance: X-linked recessive
β’ Males have only one X chromosome
β’ A single defective gene is expressed
β’ Females require two defective alleles
Chromosomal disorders are caused by abnormalities in chromosome number or structure.
Types of Chromosomal Disorders
A. Aneuploidy
Gain or loss of one or more chromosomes due to nondisjunction.
1. Down Syndrome
β’ Trisomy of chromosome 21
β’ Karyotype: 47, +21
π Symptoms:
β’ Mental retardation
β’ Broad face
β’ Protruding tongue
2. Turner Syndrome
β’ Monosomy of X chromosome
β’ Karyotype: XO
π Symptoms:
β’ Short stature
β’ Underdeveloped ovaries
β’ Infertility
3. Klinefelter Syndrome
β’ Extra X chromosome in males
β’ Karyotype: XXY
π Symptoms:
β’ Sterile males
β’ Poor development of secondary sexual characters
B. Structural Chromosomal Disorders
Result from deletion, duplication, inversion, or translocation.
4. Cri-du-chat Syndrome
β’ Deletion of short arm of chromosome 5
β’ Characteristic cat-like cry in infants
Common diagnostic methods include:
β’ Karyotyping
β’ Molecular diagnosis (DNA analysis)
β’ Enzyme assays
β’ Prenatal diagnosis
β’ Genetic counseling
β’ Prenatal screening
β’ Dietary control (e.g., PKU)
β’ Hormone therapy
β’ Awareness reduces stigma
β’ Early diagnosis improves quality of life
β’ Ethical use of genetic technologies is essential
β Mendelian disorders are single-gene disorders
β Sickle cell anaemia = point mutation
β Haemophilia is X-linked recessive
β Down syndrome = trisomy 21
β Chromosomal disorders result from nondisjunction