Biology - PSEB Senior Secondary (Class 12 Science)
The Biology domain of the PSEB Class 12 science paper samples the full senior-secondary syllabus: reproduction, genetics and evolution, biology and human welfare, biotechnology, and ecology. Most marks sit on process questions - can you narrate a pathway, name structures in the correct order, and explain why a particular ratio or result appears?
Rules that carry marks
- Double fertilisation in angiosperms. The pollen tube usually enters the ovule through the micropyle and discharges two male gametes into the embryo sac. One fuses with the egg (syngamy, giving a diploid zygote); the other fuses with the two polar nuclei (triple fusion, giving a triploid primary endosperm nucleus). Endosperm is therefore 3n, not 2n.
- Segregation versus independent assortment. Segregation concerns one pair of alleles at one locus. Independent assortment applies only to genes on non-homologous chromosomes; linked genes deviate from the 9:3:3:1 dihybrid ratio.
- Humoral versus cell-mediated immunity. B cells produce antibodies against extracellular pathogens; cytotoxic T cells destroy infected cells. Vaccination works by generating memory cells, so the secondary response is faster and larger.
- Energy flow. Only about 10% of energy passes to the next trophic level, so food chains are short and top carnivores are few.
Worked example
A woman of blood group AB marries a man of blood group O. What groups can their children have? Reasoning: AB is I^A I^B, so she makes I^A and I^B gametes in equal numbers. O is ii, so he makes only i gametes. Offspring genotypes are I^A i (group A) and I^B i (group B). Groups AB and O are impossible here, because neither parent supplies both A and B, and the father supplies no I allele at all.
Traps that cost marks
- Calling the endosperm diploid, or placing triple fusion before syngamy.
- Treating codominance (AB blood group) as incomplete dominance (pink snapdragon).
- Assuming a 3:1 ratio in a test cross; the tester is homozygous recessive, so the ratio is 1:1.
- Using "dominant" to mean "common" in a population - dominance describes expression in a heterozygote, not frequency.
- Omitting units and direction in ecology answers, e.g. kcal m⁻² yr⁻¹.