Genetic Disorders Notes with PDF
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Overview of Genetic Disorders
Definition:
Genetic disorders are diseases caused by abnormalities in an individual’s genes or chromosomes. A key characteristic is that they are heritable, meaning they can be passed from parents to their offspring.
Categorization:
- Chromosomal Disorders: Caused by changes in the number or structure of chromosomes.
- Example: Down Syndrome (Trisomy 21).
- Single-Gene Disorders: Caused by mutations or defects in a single gene.
- Examples: Haemophilia, Albinism, Phenylketonuria (PKU), Colour blindness, Sickle Cell Anaemia, Thalassemia.
Distinction from Non-Genetic Disorders:
Genetic disorders are distinct from other disease categories.
- Deficiency Diseases: Caused by a lack of essential nutrients (e.g., Rickets from Vitamin D deficiency, Night Blindness from Vitamin A deficiency).
- Hormonal Disorders: Caused by imbalances in hormones (e.g., Diabetes).
- Infections: Caused by pathogens like bacteria, viruses, or fungi (e.g., Tuberculosis, Ringworm).
- Other: Conditions like Irritable Bowel Syndrome (IBS) or diseases caused by pollutants are not primarily genetic.
Specific Genetic Disorders
- Phenylketonuria (PKU)
- An inborn error of metabolism.
- Caused by an inherited lack of a specific enzyme.
- If untreated, the metabolic error leads to mental impairment.
- Haemophilia
- A hereditary genetic disease.
- It is a sex-linked (X-linked recessive) disorder.
- Caused by a defect in the blood clotting mechanism, making it absent or inefficient.
- Inheritance: The gene is carried by women but primarily affects men.
- Down Syndrome
- Caused by a change in chromosome number.
- Specifically, Trisomy 21 (three copies of chromosome 21).
- Colour Blindness
- A sex-linked (X-linked recessive) genetic disorder.
- Inheritance Pattern:
- More common in men because they have only one X chromosome.
- If a colour-blind man marries a normal woman (with no family history):
- Sons: Normal.
- Daughters: Carriers (not colour blind themselves).
- In this scenario, no children are colour blind, but daughters can pass the gene to their sons.
- Thalassemia
- A hereditary disease that affects the blood.
- The body is unable to synthesize normal haemoglobin.
- Other Notable Disorders
- Bubble Baby Disease (Severe Combined Immunodeficiency – SCID): Named because the baby must be treated in a germ-free plastic bubble.
- Muscular Dystrophy: Can be caused by the expression of a recessive gene on a sex chromosome.
Key Concepts in Genetics
- Sex-Linked Inheritance
- Disorders where the defective gene is located on a sex chromosome (usually the X chromosome).
- Examples: Haemophilia, Colour blindness, Muscular Dystrophy.
- This explains why some diseases are much more common in one gender.
- Chromosomal Abnormalities
- Non-disjunction: The failure of chromosomes to separate properly during cell division.
- Trisomy: The presence of an extra chromosome (e.g., Trisomy 21 in Down Syndrome).
- Correctly Matched Disorder Pairs
- Haemophilia – Genetic disease
- Diabetes – Hormonal disorder
- Rickets – Deficiency disease (Vitamin D)
- Ringworm – Fungal infection
- Colour blindness, Haemophilia, Sickle cell anaemia – Hereditary diseases
- Thiamine (B1) – Beriberi
- Ascorbic Acid (C) – Scurvy
- Vitamin K – Blood clotting
- Important Clarification:
- Incorrectly Matched: Vitamin A is linked to Night Blindness, not Colour blindness. Colour blindness is a genetic disorder.
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