Revision/fast recall

1. TITLE

Quick Recall: Mendelian vs Chromosomal Disorders cheat sheet — Everything for NEET on one page

2. QUICK FACTS BOX

MENDELIAN VS CHROMOSOMAL DISORDERS — QUICK FACTS

  • Mendelian disorders result from an alteration or mutation in a single gene.
  • Chromosomal disorders arise from an abnormal chromosome number or arrangement.
  • Pedigree analysis traces Mendelian disorders through family generations.
  • Haemophilia is a sex-linked recessive Mendelian disorder.
  • Sickle-cell anaemia is an autosome-linked recessive genetic trait.
  • Phenylketonuria lacks the enzyme converting phenylalanine to tyrosine.
  • Thalassemia is a quantitative problem of synthesising too few globin molecules.
  • Down's syndrome is caused by an extra copy of chromosome 21.
  • Klinefelter's syndrome affects males with an extra X chromosome (47, XXY).
  • Turner's syndrome affects females missing an X chromosome (45, XO).

3. FORMULA SHEET

KARYOTYPE FORMULAS:

→ Use when identifying Down's Syndrome: 45 Autosomes (Trisomy 21) + XX/XY.
→ Use when identifying Klinefelter's Syndrome: 44 Autosomes + XXY.
→ Use when identifying Turner's Syndrome: 44 Autosomes + XO.

4. MUST-REMEMBER VALUES/CONSTANTS

KEY VALUES:

Normal Human Chromosomes = 46 (23 pairs).
Down's Syndrome Karyotype = 47 chromosomes.
Klinefelter's Karyotype = 47 chromosomes.
Turner's Karyotype = 45 chromosomes.

5. COMPARISON SUMMARY

Feature Mendelian Disorders Chromosomal Disorders
Primary Cause Mutation or alteration in a single gene Absence, excess, or abnormal arrangement of chromosomes
Tracing Method Follows inheritance principles; traced via Pedigree Analysis Usually sporadic errors during cell division (Aneuploidy/Polyploidy)
Key Examples Haemophilia, Sickle-cell anaemia, Thalassemia, PKU Down's Syndrome, Klinefelter's Syndrome, Turner's Syndrome

6. COMMON MISTAKES — QUICK LIST

DON'T:

  • Confuse Thalassemia (quantitative problem synthesising too few globins) with Sickle-cell anaemia (qualitative problem synthesising incorrect globin).
  • Mix up Klinefelter's (XXY) and Turner's (XO) syndrome karyotypes.
  • Assume women are frequently haemophilic (they are usually just carriers because a haemophilic father is unviable in later stages of life).

DO:

  • Remember that Sickle-cell anaemia is an autosome-linked recessive trait.
  • Recall that Aneuploidy is the gain or loss of a chromosome due to failure of chromatid segregation.

7. MNEMONICS & MEMORY TRICKS

REMEMBER:
* Sickle Cell Mutation: GAG to GUG represents Glutamic acid to Valine ("A" becomes "U").
* Turner's Syndrome: "Turned off X" = XO (missing one X chromosome, causing sterility).
* Klinefelter's Syndrome: "Extra feminine" = XXY (masculine development but expresses Gynaecomastia).

8. HIGH-YIELD NEET POINTS

NEET LOVES TO ASK:

  • The exact amino acid substitution in Sickle-cell anaemia: Glutamic acid is replaced by Valine at the sixth position of the beta-globin chain.
  • The chromosomal basis of Down's Syndrome: Trisomy of chromosome number 21.
  • Which disorder shows characteristic transmission from an unaffected carrier female to male progeny: Haemophilia (sex-linked recessive).

9. ONE MCQ — SELF-TEST

Q: Which of the following is an example of a chromosomal disorder resulting in a 45, XO karyotype?
A) Haemophilia
B) Sickle-cell anaemia
C) Turner's Syndrome
D) Klinefelter's Syndrome

Answer: C) Turner's Syndrome.

10. NCERT PAGE REFERENCES

Revise from:
- Page 73: Mendelian Disorders, Pedigree Analysis, and Haemophilia.
- Page 74: Sickle-cell anaemia, Phenylketonuria, and Thalassemia.
- Page 75-76: Chromosomal Disorders (Down's, Klinefelter's, and Turner's Syndromes).

11. CTA

Detailed explanation → Genetics and Evolution Guide
Quick test → Mock exams