NEET Biology 2027 — Principles of Inheritance and Variation: Complete Notes and MCQs
Principles of Inheritance and Variation is one of the highest-weightage chapters in NEET Biology Unit V. Carrying 5-8 questions per year, mastery of Mendel’s laws, chromosomal theory, genetic disorders and DNA replication is essential for a 600+ NEET score. This guide provides complete notes and 10 practice MCQs.
Mendel’s Laws of Inheritance
Gregor Johann Mendel conducted experiments on garden pea (Pisum sativum) from 1856-1863 using 7 pairs of contrasting characters. His findings are summarised in three laws:
| Law | Statement | Example |
|---|---|---|
| Law of Dominance | In a cross, only one allele (dominant) expresses in F1 | Tall (T) dominates over Dwarf (t) |
| Law of Segregation | Alleles separate during gamete formation | Tt plant makes T and t gametes |
| Law of Independent Assortment | Alleles of different genes assort independently | Dihybrid ratio 9:3:3:1 |
Monohybrid and Dihybrid Cross
Monohybrid cross: TT × tt → Tt (F1, all tall) → F2 ratio 3 Tall:1 Dwarf (phenotype), 1TT:2Tt:1tt (genotype). Dihybrid cross: TTYY × ttyy → F1 TtYy → F2 ratio 9:3:3:1. Test cross: F1 hybrid × homozygous recessive; ratio 1:1 confirms hybrid nature.
Chromosomal Theory of Inheritance
Sutton and Boveri (1902) proposed that chromosomes are carriers of hereditary units (genes). Linkage: genes on the same chromosome tend to be inherited together. Morgan worked on Drosophila melanogaster and demonstrated sex-linkage. Crossing over occurs during prophase I of meiosis, creating recombinant chromosomes. Morgan coined the terms linkage, crossing over, and sex linkage.
Sex Determination
| Organism | Mechanism | Sex of Heterogametic |
|---|---|---|
| Humans, Drosophila | XX-XY | Male (XY) |
| Grasshopper | XX-XO | Male (XO) |
| Birds, Butterflies | ZW-ZZ | Female (ZW) |
| Honeybee | Haplodiploidy | Male = haploid (from unfertilised) |
Genetic Disorders — Key for NEET
| Disorder | Type | Chromosome/Gene | Symptoms |
|---|---|---|---|
| Down Syndrome | Trisomy | Trisomy 21 | Mental retardation, short stature, loose joints |
| Klinefelter Syndrome | Trisomy | XXY (47 chromosomes) | Male, reduced fertility, gynecomastia |
| Turner Syndrome | Monosomy | XO (45 chromosomes) | Female, sterile, short stature |
| Sickle Cell Anaemia | Autosomal recessive | Chromosome 11, HbS | Sickle-shaped RBCs, anaemia |
| Colour Blindness | X-linked recessive | X chromosome | Inability to distinguish red-green |
| Haemophilia A | X-linked recessive | Factor VIII gene | Uncontrolled bleeding |
| Phenylketonuria | Autosomal recessive | PAH gene, Chr 12 | Mental retardation, accumulation of phenylalanine |
Pedigree Analysis — NEET Key Points
- Square = Male, Circle = Female, Filled shape = Affected individual
- Horizontal line between square and circle = mating, Vertical line = offspring
- Autosomal dominant: trait in every generation, both sexes equally affected
- Autosomal recessive: can skip generations, carrier parents (Aa × Aa)
- X-linked recessive: more common in males, carrier females pass to sons
Frequently Asked Questions — NEET Genetics 2027
How many questions come from Genetics in NEET?
Genetics and Evolution (Unit V and VI) together contribute approximately 12-15 questions in NEET Biology. Principles of Inheritance and Variation alone accounts for 5-8 questions, making it one of the highest-weightage chapters.
What is the difference between genotype and phenotype?
Genotype is the genetic constitution of an organism (e.g., TT, Tt, tt). Phenotype is the observable characteristic (e.g., tall or dwarf). Genotypic ratio and phenotypic ratio differ — in monohybrid cross F2, genotypic ratio is 1:2:1 but phenotypic ratio is 3:1.
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