Principles of Inheritance and Variation is the single highest-weightage chapter in NEET Biology (3–4 questions). Ratios, exceptions and disorders are asked directly.
Mendel’s laws
- Dominance; Segregation (alleles separate during gamete formation); Independent assortment (for genes on different chromosomes).
- Monohybrid F₂: phenotype 3 : 1, genotype 1 : 2 : 1. Test cross ratio 1 : 1.
- Dihybrid F₂: 9 : 3 : 3 : 1; dihybrid test cross 1 : 1 : 1 : 1.
Deviations
| Type | Ratio / Example |
|---|---|
| Incomplete dominance | 1 : 2 : 1, snapdragon (pink) |
| Codominance | ABO blood group (IᴬIᴮ) |
| Multiple alleles | ABO: Iᴬ, Iᴮ, i |
| Pleiotropy | Phenylketonuria, sickle cell |
| Polygenic | Human skin colour (3 genes) |
Linkage and recombination
- Morgan (Drosophila): tightly linked genes show less recombination.
- Recombination frequency ∝ distance; used by Sturtevant for mapping.
Sex determination
- XX–XY (human, Drosophila), XX–XO (grasshopper), ZW–ZZ (birds), haplodiploidy (honeybee: males from unfertilised eggs).
Disorders
| Disorder | Cause |
|---|---|
| Haemophilia, colour blindness | X-linked recessive |
| Sickle cell anaemia | Autosomal recessive, Glu→Val at 6th position of β-globin |
| Phenylketonuria | Autosomal recessive, enzyme deficiency |
| Down syndrome | Trisomy 21 |
| Klinefelter | XXY (male, sterile) |
| Turner | XO (female, sterile) |
Pedigree quick rules
- Autosomal dominant: appears in every generation.
- Autosomal recessive: skips generations, unaffected parents can have affected child.
- X-linked recessive: more males affected, passed by carrier mothers.
Common mistakes
- Writing 9 : 3 : 3 : 1 for a dihybrid test cross
- Calling ABO blood group incomplete dominance
- Mixing Klinefelter (XXY) and Turner (XO)
Related: Cell: The Unit of Life notes.
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Frequently asked questions
Is this Genetics NEET Notes content at JEE/NEET level?
Yes. It covers the formulas, facts and question types that appear in JEE Main, JEE Advanced and NEET, with the NCERT base that these exams follow.
How many questions come from Genetics NEET Notes?
Typically one to three questions per paper, depending on the year. Check the chapter-wise weightage post and give this topic time in proportion.
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