Class XII Biology: Reproduction, Heredity, Applications and Ecology
Class XII Biology: reproduction, heredity, applications and ecology
The Assam HS Class XII Biology paper covers the NCERT-aligned second-year course: sexual reproduction in flowering plants and humans, genetics and evolution, human welfare, biotechnology and its applications, and ecology. The exam favours mechanism and evidence: how a process works, what cross or treatment proves it, and what follows from it.
Core rules and concepts
- Flowering-plant reproduction: the embryo sac is typically 8-nucleate and 7-celled; double fertilisation produces a diploid zygote and triploid primary endosperm nucleus.
- Human reproduction: spermatogenesis and oogenesis differ in timing and in cytoplasmic division; the menstrual cycle is driven by FSH, LH, oestrogen and progesterone.
- Genetics: Mendel's laws apply to unlinked loci; linked genes show recombination frequencies below 50%; sex-linked recessive traits appear more often in males.
- Evolution: evidence comes from fossils, homologous and analogous structures, embryology, and molecular sequences; natural selection changes allele frequencies.
- Biotechnology: restriction endonucleases cut DNA at specific sequences, DNA ligase joins fragments, and PCR uses denaturation, annealing and extension.
- Ecology: populations interact as competition, predation, parasitism, mutualism and commensalism; energy flow is unidirectional and pyramids can be inverted.
Worked example: X-linked recessive inheritance
A woman whose father has haemophilia is unaffected and marries an unaffected man. What is the chance that their son will be affected?
Reasoning: Haemophilia is X-linked recessive. An affected father is X^hY and passes X^h to every daughter. Because the woman is unaffected, she must be a carrier, X^H X^h. Each son receives Y from the father and either X^H or X^h from the mother, so the chance of an affected son is 1/2. Daughters receive X^H from the father, so none is affected; each has a 1/2 chance of being a carrier. State the X-linked recessive model; examiners award the reasoning, not just the fraction.
Traps that cost marks
- Confusing mitosis with meiosis, or the 9:3:3:1 ratio with a test-cross ratio.
- Treating linked genes as independently assorting.
- Giving the genotype ratio where the phenotype ratio is asked.
- Mixing up restriction enzymes and ligase, or the three PCR steps.
- Describing amniocentesis as a routine sex-determination test; it is for detecting chromosomal and genetic disorders.