Principles of Inheritance and Variation is the one Biology chapter where you must calculate, not just recall. That is why many students fear it, and why it separates a 320 from a 350 in Biology. This genetics NEET PYQ analysis shows which crosses, ratios and disorders NTA repeats, so you can solve these questions in under a minute each.
For concept building, pair this with our genetics preparation strategy guide, and see where the chapter fits in the plan to score 360 in Biology. This article focuses on what past papers actually ask.
Want to test yourself first? Solve the Principles of Inheritance and Variation PYQs for free, then use this guide to fix the gaps.
Table of Contents
Why Genetics Is a Must-Do Chapter
NEET usually asks four to five questions from this chapter, which makes it one of the heaviest chapters in Class 12 Biology. Together with Molecular Basis of Inheritance, the Genetics unit is worth around 12 to 15 questions in some years.
It also ranks high in our list of the most repeated Biology chapters. The chapter builds on meiosis, so revise our Cell Cycle PYQ patterns article first if segregation and independent assortment feel unclear.
Genetics NEET PYQ Analysis: The Repeated Question Types
When you sort a decade of NEET biology PYQs from this chapter, six question types cover almost everything.
| Question type | Frequency | What it tests |
|---|---|---|
| Mendelian crosses and ratios | Most frequent | Monohybrid, dihybrid cross, test cross |
| Deviations from Mendel | Very frequent | Incomplete dominance, codominance, multiple alleles |
| Genetic disorders | Very frequent | Mendelian and chromosomal disorders |
| Pedigree analysis | Regular | Identifying the pattern from a family chart |
| Sex determination | Regular | XX–XY, XO, ZW and honeybee systems |
| Linkage and polygenic traits | Regular | Morgan’s work, pleiotropy, skin colour |
For a quick warm-up before the full set, try these five quick genetics practice questions. You can also keep the Principles of Inheritance class notes open while you read.
Mendelian Ratios You Must Know by Heart
| Cross | Phenotypic ratio | Genotypic ratio |
|---|---|---|
| Monohybrid (Tt × Tt) | 3 : 1 | 1 : 2 : 1 |
| Monohybrid test cross (Tt × tt) | 1 : 1 | 1 : 1 |
| Dihybrid cross (RrYy × RrYy) | 9 : 3 : 3 : 1 | 1:2:1:2:4:2:1:2:1 |
| Dihybrid test cross (RrYy × rryy) | 1 : 1 : 1 : 1 | 1 : 1 : 1 : 1 |
| Incomplete dominance (F₂) | 1 : 2 : 1 | 1 : 2 : 1 |
Quick counting rules for any cross with n heterozygous gene pairs:
- Types of gametes = 2ⁿ
- F₂ phenotypes = 2ⁿ (with complete dominance)
- F₂ genotypes = 3ⁿ
So a dihybrid cross gives 4 gamete types, 4 phenotypes and 9 genotypes. NTA asks this count again and again.
Deviations From Mendel
- Incomplete dominance: the F₁ is intermediate. Example: flower colour in snapdragon (Antirrhinum), where red × white gives pink.
- Codominance: both alleles express fully. Example: the AB blood group, where Iᴬ and Iᴮ are both expressed.
- Multiple alleles: the ABO system has three alleles (Iᴬ, Iᴮ, i), giving six genotypes and four phenotypes.
- Pleiotropy: one gene, many effects. Example: phenylketonuria.
- Polygenic inheritance: many genes, one trait. Example: human skin colour.
Sex-Linked Inheritance and Genetic Disorders
| Disorder | Type | Key fact |
|---|---|---|
| Haemophilia | X-linked recessive | Clotting protein affected; carrier mothers pass it to sons |
| Colour blindness | X-linked recessive | About 8% of males, 0.4% of females |
| Sickle-cell anaemia | Autosomal recessive | Glutamic acid replaced by valine at position 6 of the β-globin chain |
| Phenylketonuria | Autosomal recessive | Phenylalanine is not converted to tyrosine |
| Thalassaemia | Autosomal recessive | Reduced synthesis of globin chains (a quantitative problem) |
| Down’s syndrome | Chromosomal | Trisomy of chromosome 21 |
| Klinefelter’s syndrome | Chromosomal | 47, XXY |
| Turner’s syndrome | Chromosomal | 45, X0 |
In sex-linked inheritance, a father never passes an X-linked trait to his son, because the son receives his Y chromosome. This single fact solves most haemophilia and colour blindness questions.
For sex determination, remember: humans and Drosophila are XX–XY, grasshoppers are XO, birds are ZW (the female is heterogametic), and honeybees use haplodiploidy (males are haploid).
Pedigree Analysis in Three Checks
- Does the trait skip generations? If unaffected parents have an affected child, the trait is recessive.
- Are mostly males affected? If yes, and it passes from carrier mother to son, it is X-linked recessive.
- Does every affected person have an affected parent? Then it is likely dominant.
Pedigree analysis questions in NEET rarely need more than these three checks.
Worked PYQ-Pattern Questions
Q1. How many genotypes and phenotypes appear in the F₂ of a dihybrid cross?
Answer: 9 genotypes and 4 phenotypes. Use 3ⁿ and 2ⁿ with n = 2.
Q2. A colour-blind man marries a woman with normal vision who is not a carrier. What will their children be?
Answer: All sons normal, all daughters carriers. Sons get the mother’s normal X; daughters get the father’s affected X.
Q3. Parents have blood groups A and B, both heterozygous. Which groups are possible in their children?
Answer: A, B, AB and O. The cross Iᴬi × Iᴮi gives all four groups in equal proportion.
Q4. A heterozygous tall plant is test crossed. What is the ratio of tall to dwarf?
Answer: 1 : 1. Tt × tt gives half Tt and half tt.
Q5. Which of these is a chromosomal disorder? (a) Haemophilia (b) Sickle-cell anaemia (c) Turner’s syndrome (d) Phenylketonuria
Answer: (c) Turner’s syndrome. The other three are Mendelian disorders caused by a single gene.
Common Traps in This Chapter
- Mixing up incomplete dominance and codominance
- Writing the phenotypic ratio when the question asks for the genotypic ratio
- Saying a father can pass haemophilia to his son
- Calling thalassaemia a qualitative defect (that is sickle-cell anaemia)
- Forgetting that the female is heterogametic in birds
Practise Every Genetics PYQ
This genetics NEET PYQ analysis gives you the ratios and rules. Practice makes them automatic. Every previous year question from this chapter is available chapter-wise, free:
👉 Solve all Principles of Inheritance NEET PYQs
For every cross question, write the parental genotypes and the gametes before you look at the options. Most wrong answers come from skipping that step.
Conclusion
Genetics rewards a method more than memory. Use this genetics NEET PYQ analysis to fix the standard ratios, the deviations, the disorder table and the three pedigree checks. Then practise until every cross feels routine.
❓ FAQ
Q: How many questions come from Principles of Inheritance in NEET? A: Usually four to five questions, which makes it one of the highest-weightage chapters in Class 12 Biology.
Q: What is the most important topic in this chapter? A: Mendelian crosses and their ratios are the most tested, followed by genetic disorders and deviations such as codominance and incomplete dominance.
Q: How do I solve pedigree questions quickly? A: Check whether the trait skips generations, whether mostly males are affected, and whether every affected person has an affected parent. These three checks identify the pattern.
Q: What is the difference between incomplete dominance and codominance? A: In incomplete dominance the F₁ shows an intermediate phenotype, such as pink flowers. In codominance both alleles are fully expressed, as in the AB blood group.
Q: How should I revise genetics for NEET? A: Use a genetics NEET PYQ analysis to list the repeated ratios and disorders, learn the NCERT examples, and solve past questions chapter-wise.
Q: Where can I practise Principles of Inheritance NEET PYQs for free? A: You can solve every previous year question from this chapter for free using the practice link in this article.