Class 12 Biology Chapter 4 | Genetic Disorders & Inheritance | CBSE 2025–26 | Full Q&A Revision 💥
🎓 Class 12 Biology – Chapter 4: Principles of Inheritance and Variation (Part IV) 📘 Topic: Genetic Disorders, Types of Inheritance & Important Definitions 💡 All NCERT-based questions with easy and exam-ready answers! --- ✳️ Q1. Define Haemophilia with example. Answer: Haemophilia is a sex-linked recessive disorder where the blood fails to clot due to lack of clotting factors (Factor VIII or IX). 🩸 Example: Haemophilia-A (deficiency of Factor VIII). 👉 It mainly occurs in males and females act as carriers. ✳️ Q2. Define Colour Blindness. Answer: Colour blindness is a sex-linked recessive disorder in which a person cannot distinguish between red and green colours. 👁️ Cause: Defective gene on the X chromosome. 👨🦰 Mostly seen in males, while females are carriers. --- ✳️ Q3. Define Sickle Cell Anaemia. Answer: Sickle Cell Anaemia is a genetic disorder caused by a point mutation in the gene coding for β-chain of haemoglobin. 🔬 Result: Abnormal haemoglobin (HbS) causes red blood cells to become sickle-shaped → leads to poor oxygen transport. 🧬 Genotype: Homozygous recessive (HbS HbS). ✳️ Q4. Define Thalassemia. Answer: Thalassemia is a recessive autosomal disorder where formation of haemoglobin chains (α or β) is reduced or absent. 🩸 Types: Alpha Thalassemia → defect in α-chain Beta Thalassemia → defect in β-chain 👨👩👧👦 Seen in children if both parents are carriers. --- ✳️ Q5. Define Phenylketonuria. Answer: Phenylketonuria (PKU) is an autosomal recessive metabolic disorder where the enzyme phenylalanine hydroxylase is absent. 🍬 Effect: Phenylalanine accumulates → brain damage, mental retardation. 🧬 Inherited from both carrier parents. ✳️ Q6. Difference between Autosomal and Sex-linked Inheritance Feature Autosomal Inheritance Sex-linked Inheritance Chromosomes involved Autosomes (1–22 pairs) Sex chromosomes (X or Y) Occurs in Both males & females equally Mostly males Examples Sickle Cell Anaemia, Thalassemia Haemophilia, Colour Blindness ✳️ Q7. Define Pleiotropy with example. Answer: Pleiotropy is when one gene controls more than one trait (phenotypic effect). 🌿 Example: In pea plant, a single gene affects both seed shape and starch synthesis. In humans → Phenylketonuria (one gene causes multiple symptoms). --- 🎯 ✨ Exam Tip: All these questions are NCERT-based and frequently asked in CBSE Term and Board Exams! Learn the causes, inheritance type, and examples properly.
