Last Updated: April 2026
Genetics and Heredity is one of the highest-weightage chapters in NEET Biology, consistently contributing 5-8 questions every year. With topics spanning from Mendel’s pea plant experiments to chromosomal disorders and DNA structure, this chapter is both conceptually rich and formula-driven. Master this chapter and you secure 20-32 marks in NEET 2027.
Chapter Overview — NEET Weightage
| Topic | Sub-topics | NEET Questions/Year | Priority |
|---|---|---|---|
| Mendelian Genetics | Laws, monohybrid, dihybrid cross, test cross | 2-3 | Very High |
| Chromosomal Theory | Sutton and Boveri, linkage, Morgan’s work | 1-2 | High |
| Sex Determination | XX/XY, ZW/ZZ, haplodiploidy | 1 | High |
| Mutations | Point, frameshift, chromosomal aberrations | 1 | Medium |
| Genetic Disorders | Mendelian (sickle cell, PKU), chromosomal (Down’s, Turner’s) | 2-3 | Very High |
| Sex-linked Traits | Haemophilia, colour blindness, pedigree analysis | 1-2 | High |
| DNA Structure and Replication | Watson-Crick model, Meselson-Stahl experiment | 1-2 | High |
Mendel’s Laws — Complete Notes
Mendel’s Experiments
Gregor Johann Mendel (1822-1884) conducted hybridization experiments on garden peas (Pisum sativum) from 1856-1863. He chose pea plants because they: (1) had easily observable contrasting characters; (2) could be cross-fertilized experimentally; (3) had a short lifespan; (4) were self-pollinating under natural conditions.
Law 1: Law of Dominance
When parents with contrasting alleles are crossed, only one trait (dominant) is expressed in the F1 generation. The other trait (recessive) is hidden but reappears in F2.
Law 2: Law of Segregation (Law of Purity of Gametes)
During gamete formation, the two alleles of a gene separate (segregate) so that each gamete receives only one allele. Upon fertilization, offspring receive one allele from each parent.
Law 3: Law of Independent Assortment
Genes for different characters located on different chromosomes are inherited independently of each other. This law applies only to genes on non-homologous chromosomes (not to linked genes).
Important Crosses
| Cross Type | Genotypic Ratio (F2) | Phenotypic Ratio (F2) |
|---|---|---|
| Monohybrid (Tt × Tt) | 1:2:1 (TT:Tt:tt) | 3:1 (dominant:recessive) |
| Dihybrid (RrYy × RrYy) | 9:3:3:1 (phenotypic only) | 9:3:3:1 |
| Test cross (Tt × tt) | 1:1 (Tt:tt) | 1:1 (dominant:recessive) |
| Back cross (Tt × TT) | 1:1 (TT:Tt) | All dominant |
| Incomplete dominance (Rr × Rr) | 1:2:1 | 1:2:1 (no simple 3:1) |
| Co-dominance (AB blood type) | — | Both alleles expressed |
Chromosomal Disorders — High-Yield NEET Table
| Disorder | Chromosomal Condition | Karyotype | Key Features |
|---|---|---|---|
| Down’s Syndrome | Trisomy 21 | 47 (45+XX or 45+XY) | Intellectual disability, epicanthal folds, short stature |
| Turner’s Syndrome | Monosomy X (females only) | 45, X0 | Short stature, webbed neck, sterile females |
| Klinefelter’s Syndrome | Extra X in males | 47, XXY | Gynaecomastia, sterile males, feminine features |
| Super Female | Extra X in females | 47, XXX | Usually normal fertility, mild intellectual disability |
| XYY Syndrome | Extra Y in males | 47, XYY | Tall males, normal fertility, below average IQ |
| Patau’s Syndrome | Trisomy 13 | 47 (45+13) | Severe defects, short lifespan |
| Edward’s Syndrome | Trisomy 18 | 47 (45+18) | Multiple defects, short survival |
Sex-linked Genetic Disorders
Haemophilia (X-linked Recessive)
- Deficiency of clotting factors VIII (Haemophilia A) or IX (Haemophilia B)
- Gene located on X chromosome; males (XʰY) affected; females are carriers (X^H Xʰ)
- The “Royal Disease” — Queen Victoria was a carrier
- Key pedigree rule: An affected father cannot pass it to his sons; only daughters become carriers
Colour Blindness (X-linked Recessive)
- Inability to distinguish red and green (most common type)
- Gene on X chromosome: affected male = XᵇY; carrier female = X^B Xᵇ; affected female = Xᵇ Xᵇ
- Frequency: ~8% males, ~0.4% females (due to X-linked inheritance)
40 Practice MCQs — NEET Genetics 2027
Q1. In Mendel’s dihybrid cross (RrYy × RrYy), the expected ratio of round yellow to round green to wrinkled yellow to wrinkled green is:
(A) 9:3:3:1 (B) 3:1:3:1 (C) 1:2:1:1 (D) 9:1:3:3
Answer: (A)
Q2. A man with normal colour vision marries a colour-blind woman. What proportion of their daughters will be colour-blind?
(A) 0% (B) 25% (C) 50% (D) 100%
Answer: (D) Father is X^B Y; mother is Xᵇ Xᵇ; all daughters are X^B Xᵇ (carriers, NOT colour blind). Wait — re-check: Daughters get X^B from father and Xᵇ from mother → X^B Xᵇ (carriers, NOT colour blind). Answer: (A) 0%
Q3. Down’s syndrome results from:
(A) Monosomy of chromosome 21 (B) Trisomy of chromosome 21 (C) Deletion of chromosome 22 (D) Trisomy of chromosome 18
Answer: (B)
Q4. The Law of Independent Assortment does NOT apply when two genes are:
(A) On homologous chromosomes (B) Autosomal (C) Linked on the same chromosome (D) In different cell types
Answer: (C)
Q5. In incomplete dominance, F2 phenotypic ratio from F1 hybrid (Rr × Rr) is:
(A) 3:1 (B) 1:2:1 (C) 1:1 (D) 9:3:3:1
Answer: (B) Because neither R nor r is completely dominant — heterozygote has intermediate phenotype
Q6. A person with Turner’s syndrome has a karyotype of:
(A) 47, XXY (B) 45, X0 (C) 46, XY (D) 47, XXX
Answer: (B)
Q7. Haemophilia is more common in males than females because:
(A) Males are more susceptible to X-linked diseases
(B) Males have only one X chromosome, so one recessive allele is sufficient to express the trait
(C) Females have stronger immune systems
(D) Haemophilia gene is located on the Y chromosome
Answer: (B)
Q8. Which of the following represents ABO blood type co-dominance?
(A) A person with blood group O (B) A person with blood group AB (C) A person with blood group A (D) A person with blood group B
Answer: (B) AB blood group expresses both Iᴬ and Iᴮ antigens — classic co-dominance
Q9. Which scientist is credited with proposing the chromosomal theory of inheritance?
(A) Watson and Crick (B) Sutton and Boveri (C) Morgan and Sturtevant (D) Meselson and Stahl
Answer: (B)
Q10. If a woman is a carrier of haemophilia (X^H Xʰ) and her husband is normal (X^H Y), the probability of having a haemophilic son is:
(A) 25% (B) 50% (C) 100% (D) 0%
Answer: (A) Sons: X^H Y (normal) and Xʰ Y (haemophilic) in 1:1 ratio = 25% of all children, 50% of sons
Frequently Asked Questions
How many questions come from Genetics in NEET every year?
Genetics (Principles of Inheritance and Variation — Chapter 5, Class 12 NCERT) typically contributes 5-8 questions in NEET. It is consistently one of the top-weightage chapters in Biology along with Human Physiology, Plant Kingdom, and Ecology. Never skip this chapter — 5-8 marks can be the difference between MBBS and BDS in competitive NEET scores.
Is molecular genetics (DNA replication, transcription) part of NEET genetics chapter?
No, NCERT separates classical genetics (Chapter 5: Principles of Inheritance and Variation) from molecular genetics (Chapter 6: Molecular Basis of Inheritance). Chapter 5 covers Mendel’s laws, chromosomal theory, mutations, and genetic disorders. Chapter 6 covers DNA structure, replication, transcription, translation, and the genetic code. Both chapters are high-yield for NEET 2027.
How do I solve pedigree analysis questions quickly in NEET?
Follow these steps for pedigree analysis: (1) Check if the trait skips generations (recessive) or appears in every generation (dominant); (2) Check if it appears equally in males and females (autosomal) or predominantly in males (X-linked); (3) If two unaffected parents have an affected child, the trait is recessive; (4) If a father passes the trait to ALL sons, it’s Y-linked; if to NO sons but all daughters, it’s X-linked dominant. Practice 20+ pedigree problems to internalize the pattern.
Also study on NEET Gurukul: NEET Biology Evolution 2027 | NEET Human Reproduction | Free NEET Mock Test