Genetic Disorders 🧬

1. Pedigree Analysis 🧑‍🤝‍🧑

Geneticists draw a family tree (pedigree) to see how a trait travels through the generations. Squares stand for males, circles for females; a filled symbol marks someone who shows the trait. By reading the pattern you can spot whether the trait is dominant / recessive and whether it rides on autosomes or sex chromosomes. Students use pedigrees to predict risks and plan genetic counseling. 😊

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2. Mendelian Disorders 🧪

These disorders spring from a single mutated gene and follow Mendel’s laws.

2.1 Colour Blindness 🎨

  • Inheritance: X-linked recessive.
  • Mutation in red / green cone genes on the X chromosome.
  • Affects ≈ 8 % of males but only ≈ 0.4 % of females.
  • A carrier mom has a 50 % chance of producing a colour-blind son.

2.2 Haemophilia 🩸

  • Inheritance: X-linked recessive.
  • The clotting cascade misses a protein, so even a small cut bleeds continuously.
  • Carrier women may have haemophilic sons; haemophilic daughters are extremely rare.

2.3 Sickle-cell Anaemia 🔬

  • Inheritance: autosomal recessive; alleles \( \text{Hb}^{A} \) (normal) and \( \text{Hb}^{S} \) (mutant).
  • Disease shows only in \( \text{Hb}^{S}\text{Hb}^{S} \); \( \text{Hb}^{A}\text{Hb}^{S} \) carriers stay healthy but can pass the allele.
  • Point mutation: \( \text{GAG} \rightarrow \text{GUG} \) swaps Glu for Val at the 6th spot of β-globin.
  • Low O2 triggers HbS polymerisation → red cells bend into sickles, blocking capillaries.

2.4 Phenylketonuria 🧠

  • Inheritance: autosomal recessive.
  • Missing enzyme that turns phenylalanine into tyrosine.
  • Phenylalanine piles up, converts to phenylpyruvic acid → mental retardation; excess leaves in urine.

2.5 Thalassemia ❤️

  • Inheritance: autosomal recessive.
  • Too few globin chains, causing anaemia.
  • α-thalassemia: genes HBA1 + HBA2 on chromosome 16 (1-4 copies may be faulty).
  • β-thalassemia: gene HBB on chromosome 11 (one or both copies mutated).
  • Key contrast: Thalassemia = quantitative shortfall; sickle-cell = qualitative defect in Hb.

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3. Chromosomal Disorders 📑

Aneuploidy adds or removes a chromosome; polyploidy changes whole sets. The human set is 46 (22 autosome pairs + 1 sex-chromosome pair).

3.1 Down’s Syndrome 😊

  • Extra chromosome 21 → trisomy 21 \((47,\,21^+)\).
  • Short stature, flat facial profile, furrowed tongue, single palm crease, heart defects, intellectual disability.

3.2 Klinefelter’s Syndrome ♂️♀️

  • Karyotype \(47,\,\text{XXY}\).
  • Tall male, weak muscles, breast development (gynaecomastia), sterility.

3.3 Turner’s Syndrome ♀️

  • Karyotype \(45,\,\text{X0}\).
  • Short female, rudimentary ovaries, missing secondary sexual characters, sterility.

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Important Concepts for NEET 🎯

  1. Use pedigree patterns to spot autosomal vs sex-linked and dominant vs recessive traits.
  2. Remember the single-base switch \( \text{GAG} \rightarrow \text{GUG} \) that drives sickle-cell anaemia.
  3. Contrast quantitative (thalassemia) and qualitative (sickle-cell) Hb defects.
  4. Know hallmark aneuploidies: Down’s (trisomy 21), Klinefelter’s (XXY), Turner’s (X0).
  5. Master X-linked recessives like colour blindness and haemophilia, including their 50 % son-risk from carrier moms.