Genetics is one of the top-three NEET weightage topics. A focused study strategy turns it from a grind into a scoring area. This topic is the strategy — what to study, in what order and how to practice.
Core Concepts
Study order
- Mendel’s laws with ratios. 2. Punnett squares monohybrid and dihybrid. 3. Exceptions — incomplete dominance, co-dominance, multiple alleles. 4. Linkage and crossing over. 5. Sex-linked inheritance. 6. Pedigree analysis. 7. Genetic disorders. This order builds complexity gradually.
Why this order matters: Each topic builds on the previous one. You cannot understand modified ratios (like 9:3:4 for epistasis) unless you first understand why the unmodified ratio is 9:3:3:1. You cannot analyse a pedigree unless you can work a Punnett square. Skipping ahead creates gaps that haunt you in exams.
NCERT priority
Read Chapter 5 of Class 12 (Principles of Inheritance) line by line — at least 90% of NEET questions come directly from it. Boxes and figures are frequently the source of one-mark questions.
Specific NCERT gold mines:
- Table 5.2 (deviations from Mendel’s ratios) — memorise every row
- Figure 5.9 (sex determination) — XY in humans, ZW in birds
- The ABO blood group box — genotypes, phenotypes, and antigen-antibody combinations
- Pedigree symbols and conventions (Figure 5.14)
- Genetic disorders table at the end of the chapter
The Mendelian ratios you must own
| Cross type | Ratio | When it appears |
|---|---|---|
| Monohybrid (Aa × Aa) | 3:1 phenotypic, 1:2:1 genotypic | Basic dominance |
| Dihybrid (AaBb × AaBb) | 9:3:3:1 | Independent assortment |
| Test cross (Aa × aa) | 1:1 | Confirming heterozygosity |
| Incomplete dominance (Aa × Aa) | 1:2:1 (phenotypic = genotypic) | Red × white → pink |
| Complementary genes | 9:7 | Both genes needed for trait |
| Epistasis (recessive) | 9:3:4 | One gene masks the other |
| Duplicate genes | 15:1 | Either gene alone gives trait |
| Inhibitory genes | 13:3 | One gene inhibits the other |
All modified dihybrid ratios are rearrangements of 9:3:3:1. If you get 9:7, it means the 3+3+1 categories all look the same. If you get 9:3:4, the 3+1 categories merged. Learn to decompose any modified ratio back to 9:3:3:1.
Practice strategy
Solve PYQs chronologically. Group by topic type — monohybrid, dihybrid, pedigree, blood groups, sex linkage. You will see patterns repeat within weeks.
Structured practice plan (40 hours over 2 weeks):
| Day | Topic | Hours | Activity |
|---|---|---|---|
| 1-2 | Mendel’s laws | 4 | NCERT reading + basic problems |
| 3-4 | Monohybrid crosses | 4 | 20 Punnett square problems |
| 5-6 | Dihybrid crosses | 4 | Modified ratios practice |
| 7-8 | Multiple alleles (ABO) | 4 | Blood group problems |
| 9-10 | Sex linkage | 4 | Haemophilia/colour blindness problems |
| 11-12 | Pedigree analysis | 6 | Pattern recognition drills |
| 13 | Linkage, crossing over | 4 | Recombination frequency problems |
| 14 | Full PYQ sets | 6 | Timed practice, last 5 years |
Common NEET traps
Watch for ‘among the following’ wording — it often tests the exception, not the rule. Codominance vs incomplete dominance is the classic mix-up zone.
Trap 1: Codominance vs Incomplete dominance
- Incomplete dominance: Heterozygote shows a blended phenotype. Red flower × White flower → Pink flower (1 red : 2 pink : 1 white)
- Codominance: Both alleles are fully expressed. Blood group AB = II — both A and B antigens are present, not a blend
NEET loves giving you a scenario and asking which type of dominance it represents. The key question: is the heterozygote a blend (incomplete) or does it show both parental traits simultaneously (codominance)?
Trap 2: Carrier mothers in sex-linked inheritance A colour-blind man (XY) marries a carrier woman (XX). What fraction of their sons will be colour-blind?
Students often calculate all children (1/4 colour-blind). But the question asks about sons only. Among sons: 50% are XY (normal) and 50% are XY (colour-blind). So 1/2 of sons are colour-blind — not 1/4 of all children.
Trap 3: Pedigree analysis — autosomal vs X-linked
- If affected father has all normal daughters → likely autosomal recessive (daughters are carriers)
- If affected father has all affected daughters → likely X-linked dominant
- If only sons are affected → likely X-linked recessive
- If affected individuals appear in every generation → likely dominant (autosomal or X-linked)
Memorisation list
ABO genotypes and antigens, Rh system, sex chromosomes in humans, hemophilia and colour-blindness patterns, Klinefelter (XXY) and Turner (XO), sickle-cell and PKU inheritance.
ABO blood group system:
| Blood group | Genotype | Antigens on RBC | Antibodies in plasma | Can donate to | Can receive from |
|---|---|---|---|---|---|
| A | II or Ii | A | Anti-B | A, AB | A, O |
| B | II or Ii | B | Anti-A | B, AB | B, O |
| AB | II | A and B | None | AB only | All (universal recipient) |
| O | ii | None | Anti-A and Anti-B | All (universal donor) | O only |
Key genetic disorders:
| Disorder | Chromosome/gene | Inheritance | Key feature |
|---|---|---|---|
| Sickle cell anaemia | Chromosome 11, HBB gene | Autosomal recessive | Glu → Val at position 6 of -chain |
| Colour blindness | X chromosome | X-linked recessive | More common in males (8% vs 0.5% in females) |
| Haemophilia A | X chromosome (Factor VIII) | X-linked recessive | Delayed blood clotting |
| Down syndrome | Trisomy 21 | Chromosomal (non-disjunction) | Short stature, intellectual disability |
| Klinefelter | XXY (47 chromosomes) | Chromosomal | Male, infertile, gynaecomastia |
| Turner | XO (45 chromosomes) | Chromosomal | Female, infertile, short stature |
| PKU | Chromosome 12 | Autosomal recessive | Cannot metabolise phenylalanine |
Pedigree analysis framework
When you see a pedigree in an exam, follow this systematic approach:
If two unaffected parents produce an affected child → recessive. If an affected parent always has at least some affected children → likely dominant.
If affected females are present in significant numbers → likely autosomal. If only males are affected (or vastly more males) → likely X-linked recessive. An affected father with all carrier daughters but no affected daughters → X-linked recessive.
Start with affected individuals (their genotype is known for sure in recessive conditions). Work outward to parents and siblings using the rules of inheritance.
Use a Punnett square for the specific cross in question. Remember to consider only the relevant subset (e.g., “among sons” or “among daughters”).
Worked Examples
It appears in at least one NEET question every year, often in disguised form — percentage values, fraction values or ratio values. Recognising the pattern instantly saves two minutes.
If you plan 12 hours for genetics revision, spend 3 on laws, 3 on problems, 3 on pedigree practice, 2 on exceptions and 1 on PYQ patterns. Balanced coverage beats deep-diving one subtopic.
In Labrador retrievers, coat colour is controlled by two genes. Gene B (black/brown) and gene E (expression/no expression). Genotype B_E_ is black, bbE_ is brown, and _ _ee is yellow (regardless of B).
Cross BbEe × BbEe:
9 B_E_ (black) : 3 bbE_ (brown) : 3 B_ee (yellow) : 1 bbee (yellow)
The 3 + 1 categories merge → 9 black : 3 brown : 4 yellow (recessive epistasis, 9:3:4).
Carrier mother (XX) × Normal father (XY)
| X | X | |
|---|---|---|
| X | XX (normal girl) | XX (carrier girl) |
| Y | XY (normal boy) | XY (colour-blind boy) |
Probability of colour-blind child = 1/4. Probability of colour-blind son = 1/2 (among sons only).
Common Mistakes
Skipping linkage because it feels advanced. NEET tests it directly.
Memorising ratios without understanding derivation. The derivation is what lets you handle modified ratios like 9:3:4.
Treating pedigree and Punnett as separate skills. Pedigree questions often need Punnett logic inside them.
Confusing phenotypic and genotypic ratios. In complete dominance, the monohybrid phenotypic ratio is 3:1 but the genotypic ratio is 1:2:1. In incomplete dominance, both ratios are the same (1:2:1) because the heterozygote is distinguishable.
Forgetting that blood group AB shows codominance (II), not incomplete dominance. Both A and B antigens are fully expressed, not a blend.
Exam Weightage and Revision
This topic is a repeat performer in board papers and entrance exams. NEET typically asks one to two questions on the core mechanisms, CBSE boards give three to six marks, and state PMT papers often include a diagram-based long answer. The PYQs cluster around a small set of facts — lock those and you clear the topic.
NEET 2023 had two questions on genetics — one on pedigree analysis and one on modified dihybrid ratios. NEET 2022 tested ABO blood groups. CBSE boards ask five-mark questions on Mendel’s laws with diagrams. Genetics carries 10-15 marks in NEET every year — the highest of any single topic.
When a question gives a scenario, identify the core mechanism first, then match it to the concepts above. Most wrong answers come from reading the scenario too quickly.
The 70-20-10 rule for genetics — 70% on Mendelian basics, 20% on extensions, 10% on disorders and deviations.
Practice Questions
Q1. In a cross between AaBb and aabb, what is the expected phenotypic ratio?
This is a test cross. AaBb × aabb produces: AaBb, Aabb, aaBb, aabb in equal proportions. Phenotypic ratio = 1:1:1:1 (four phenotypes in equal numbers). This confirms independent assortment.
Q2. A couple where both are carriers for sickle cell anaemia want to know the probability of having an affected child.
Both parents are HbHb (carriers). Cross: HbHb × HbHb → 1 HbHb : 2 HbHb : 1 HbHb. Probability of affected child (HbHb) = 1/4 or 25%. Probability of carrier child = 2/4 = 50%.
Q3. Why is haemophilia more common in males than females?
Haemophilia is X-linked recessive. Males have only one X chromosome (XY), so a single recessive allele on X causes the disease. Females have two X chromosomes (XX) — they need two copies of the recessive allele to be affected. A female can be a carrier (XX) without showing symptoms. Males cannot be carriers.
Q4. What phenotypic ratio would you expect if two genes show complementary interaction?
9:7. Both dominant alleles are needed to produce the trait. Only the 9/16 class (A_B_) shows the trait. The remaining 3+3+1 = 7/16 all lack the trait because at least one gene is homozygous recessive.
Q5. In a pedigree, two normal parents have an affected son. What can you deduce?
The trait is recessive (both parents carry the allele but do not express it). If only sons are affected, it could be X-linked recessive (mother is a carrier, XX). If daughters are also affected in other families, it is autosomal recessive. More family data would confirm the exact mode.
FAQs
What is the difference between a gene and an allele? A gene is a segment of DNA that codes for a trait (e.g., the gene for flower colour). An allele is a specific version of that gene (e.g., the allele for red or white flowers). At any locus, a diploid organism has two alleles — one from each parent.
Why did Mendel choose pea plants? Peas have several advantages: (1) Many clearly contrasting traits (tall/short, round/wrinkled). (2) Self-pollination is natural, giving pure lines. (3) Cross-pollination is easy to perform artificially. (4) Short generation time. (5) Large number of offspring for statistical analysis.
What is the difference between linkage and crossing over? Linkage means genes on the same chromosome tend to be inherited together (they do not assort independently). Crossing over breaks linkage — during meiosis I, homologous chromosomes exchange segments, creating new allele combinations (recombinants). The recombination frequency is used to map gene positions on chromosomes.
Can a child have a blood group that neither parent has? Yes. If both parents are blood group A (genotype Ii), their child can be blood group O (genotype ii) — with probability 1/4. Neither parent is O, but both carry the recessive i allele.
Genetics strategy is not about studying more — it is about studying in the order that builds confidence. Finish the easy topics first, then extend.