IISER IAT Biology: Mechanism, Data, and Pedigrees
What the section rewards
IISER IAT Biology questions are usually short data or scenario stems: a pedigree, a table of enzyme rates, a gel, an ecological census. You need to convert the stem into a mechanism. Core areas are molecular biology (replication, transcription, translation, lac operon logic), genetics (Mendelian ratios, linkage, pedigree analysis, chi-square), physiology (feedback, nephron, gas exchange, plant water relations), and ecology/evolution (Hardy–Weinberg, population growth, energy flow).
Worked example
Two unaffected parents have an affected daughter. Does this fit X-linked recessive? No: an affected daughter would need an affected father (she inherits his X) and a carrier mother; the father is unaffected, so rule it out. Autosomal recessive fits: both parents Aa, daughter aa. For their next child, P(affected)=1/4; the previous unaffected child does not change that. If the question asks for the probability that their next child is an unaffected son, compute P(unaffected)=3/4 times P(male)=1/2 = 3/8, and keep the two conditions separate.
Traps that cost marks
- Treating “dominant” as “common”; rare traits are often recessive.
- Forgetting that X-linked recessive cannot pass father-to-son, while X-linked dominant does.
- Using chi-square with percentages instead of expected counts.
- Assuming a frameshift and a substitution have the same protein-level effect.
- Confusing independent assortment with linkage; recombinants are not always 50%.