Genetics and evolution for NEET: how to study the unit

Summary: Treat Principles of Inheritance as a problem-solving chapter and practise crosses on paper, treat Molecular Basis of Inheritance as a sequence-and-experiment chapter, and treat Evolution as an NCERT-reading chapter. Most lost marks come from ratios done in the head and experiments remembered without their conclusion.

The three chapters and what each asks of you

The unit is in Class 12 NCERT: Principles of Inheritance and Variation, Molecular Basis of Inheritance, and Evolution. They look similar on the contents page, but they need three different ways of studying.

Principles of Inheritance and Variation

Start with Mendel's monohybrid and dihybrid crosses, the laws of dominance, segregation and independent assortment, and the test cross. Then learn the departures from Mendel: incomplete dominance, codominance, multiple alleles (the ABO blood groups), pleiotropy and polygenic inheritance. After that come linkage and recombination, sex determination in different organisms, pedigree analysis, and the Mendelian and chromosomal disorders.

Worked examples you should be able to do in under a minute

Dihybrid cross. Crossing two plants heterozygous for two independent genes (RrYy × RrYy) gives a phenotype ratio of 9:3:3:1. If a question asks what fraction of the offspring are homozygous recessive for both traits, the answer is 1/4 × 1/4 = 1/16. Break a dihybrid question into two monohybrid ones and multiply. That is faster and safer than drawing a 16-box Punnett square.

Test cross. RrYy × rryy gives four phenotypes in a 1:1:1:1 ratio. If the observed numbers are far from equal, with two parental types much more common, the genes are linked.

Incomplete dominance. In the snapdragon example in NCERT, a cross of two pink (Rr) plants gives red, pink and white in a 1:2:1 ratio. Here the phenotype and genotype ratios are the same, which is the giveaway in a question.

ABO blood groups. A father with blood group A (genotype I^Ai) and a mother with blood group B (I^Bi) can have children of all four groups: AB, A, B and O, each with a probability of 1/4. Questions often give the children's groups and ask for the parents' genotypes, so practise working backwards.

Sex-linked recessive. A carrier mother (X^CX^c) and a normal father (X^CY) have, on average, half their sons affected and none of their daughters affected, though half the daughters are carriers. Write the X and Y each time; doing it mentally is where mistakes happen.

Always write the gametes before you write the offspring. In our experience, most wrong answers in genetics tests come from skipping this one line.

Pedigrees and disorders

Learn the pedigree symbols from the NCERT figure, then practise identifying whether a trait is dominant or recessive and autosomal or X-linked. For disorders, make a table with the disorder, whether it is Mendelian or chromosomal, the inheritance pattern or chromosome change, and one feature. Sickle-cell anaemia, phenylketonuria, thalassemia, haemophilia and colour blindness go in the Mendelian half; Down's, Klinefelter's and Turner's syndromes in the chromosomal half.

Molecular Basis of Inheritance

This chapter is long and dense, and it rewards order. Study it in this sequence:

1. Structure of DNA and RNA, the double helix, and packaging of DNA in the nucleosome
2. The search for genetic material: Griffith, Avery, MacLeod and McCarty, and Hershey and Chase
3. Replication, with the Meselson and Stahl experiment and the enzymes involved
4. Transcription in prokaryotes and eukaryotes, including splicing, capping and tailing
5. The genetic code and its features, and the role of tRNA
6. Translation: initiation, elongation, termination
7. Regulation of gene expression through the lac operon
8. The Human Genome Project and DNA fingerprinting

For each experiment, make a small card with four lines: who, what organism or material, what they did, and what they concluded. NEET questions often give the method and ask for the conclusion, or the other way round. For the lac operon, draw the operon twice, once with lactose absent and once with lactose present, and mark what the repressor is doing in each.

Evolution

This chapter is closer to the rest of Biology: read NCERT carefully and remember the examples. Key areas are the origin of life and the Miller experiment, evidences for evolution (homologous and analogous organs, industrial melanism, adaptive radiation with Darwin's finches and Australian marsupials), Darwin's theory and mutation theory, the Hardy-Weinberg principle and the factors that disturb it, and the outline of human evolution.

A Hardy-Weinberg example

Suppose 16% of a population shows a recessive trait. Then q² = 0.16, so q = 0.4 and p = 1 − 0.4 = 0.6. The frequency of heterozygous carriers is 2pq = 2 × 0.6 × 0.4 = 0.48, or 48%. Check your answer by confirming that p² + 2pq + q² = 0.36 + 0.48 + 0.16 = 1.

Make a simple timeline for human evolution with the names and the features NCERT gives for each. Examples such as homologous and analogous organs are easy to swap under pressure, so write them in two columns and test yourself by covering one.

A four-week plan for the unit

Our chapter notes on Principles of Inheritance, Molecular Basis of Inheritance and Evolution follow the same order as this plan.

Common questions