Human Genetic Disorders

A genetic disorder is a disease or syndrome caused by an abnormality in an individual's DNA, ranging from a single gene mutation to the loss or gain of an entire chromosome or chromosomal set.​

Molecular Basis: How a Gene Defect Causes Disease

The chain of causation is:

Gene mutation / chromosomal error → Abnormal or absent protein → Disrupted metabolic/structural function → Disease phenotype

A normal human cell contains 46 chromosomes (2n = 46) arranged as 23 homologous pairs:

• 22 pairs = Autosomes (chromosomes 1–22)
• 1 pair = Sex chromosomes (XX in females; XY in males)
  Normal female karyotype: 44 + XX | Normal male karyotype: 44 + XY

Human karyotype

Genetic disorders are broadly divided into two main categories:

• Meaning: Genetic disorders are diseases caused by abnormalities in genes or chromosomes.
• Types: They are broadly classified into Mendelian disorders and chromosomal disorders.
• Mendelian Disorders: Caused by a mutation in a single gene; examples include thalassemia, sickle-cell anaemia, colour blindness, haemophilia, and phenylketonuria.
• Chromosomal Disorders: Caused by the absence or excess of chromosomes or structural abnormalities; examples include Down’s syndrome, Turner’s syndrome, and Klinefelter’s syndrome.
• Examples of Effects: Down’s syndrome causes mental retardation; Turner’s syndrome leads to sterile females; Klinefelter’s syndrome causes sterility in males; thalassemia affects haemoglobin production.