Genetics is among the most predictable parts of NEET Biology, which makes the marks lost there particularly annoying. The problems follow a small number of forms, and the errors are almost always procedural rather than conceptual.
Write the genotypes before anything else
Every problem. Both parents, using clear symbols, before you draw a square or compute a ratio.
Most confusion in genetics comes from holding the genotypes in your head while manipulating them. Written down, a dihybrid cross is bookkeeping; remembered, it is a guess.
Identify the pattern first
Before solving, name what you are dealing with:
- Monohybrid or dihybrid?
- Complete dominance, incomplete dominance, or codominance?
- Autosomal or sex-linked?
- Any lethal allele, which changes the expected ratio?
Naming it takes ten seconds and determines the entire method. Solving first and identifying afterwards is how a correct calculation produces a wrong answer.
The ratios that tell you what happened
| Ratio seen | Usually means |
|---|---|
| 3:1 | Monohybrid, complete dominance |
| 1:2:1 | Incomplete dominance or codominance |
| 9:3:3:1 | Dihybrid, independent assortment |
| 2:1 | A lethal homozygote |
| 1:1 | A test cross |
Working backwards from the ratio is often faster than working forwards from the cross, and questions are frequently posed that way deliberately.
Pedigree questions have a fixed method
- Is the trait dominant or recessive? Unaffected parents with an affected child means recessive.
- Autosomal or X-linked? An affected father with unaffected daughters points away from X-linked dominant.
- Only now assign genotypes, starting with the individuals you are certain about.
Assigning genotypes before establishing the inheritance pattern is the single most common way to lose a pedigree question.
Practise the arithmetic of probability
Many questions ask for the chance of a specific outcome rather than a ratio. Multiply independent events, add mutually exclusive ones — and be sure which you are facing, because that distinction is usually the question.