Topic 3 covers inheritance: how genetic information is passed from parents to offspring through meiosis and fertilisation, and how Mendelian principles predict the outcomes of genetic crosses.
Gene: a heritable factor that controls a specific characteristic; a section of DNA at a specific locus on a chromosome. Alleles: different forms of a gene. Homozygous: identical alleles (AA or aa). Heterozygous: different alleles (Aa). Genotype: allele combination. Phenotype: observable characteristic (genotype + environment). Dominant allele expressed in heterozygote; recessive only when homozygous.
Two divisions producing 4 genetically unique haploid cells (gametes). Meiosis I: homologous pairs separate (reduction division). Meiosis II: sister chromatids separate (like mitosis). Sources of genetic variation: crossing over (prophase I), independent assortment (metaphase I), random fertilisation. Non-disjunction: chromosomes fail to separate → aneuploidy (e.g., Down syndrome = trisomy 21).
Monohybrid cross: one gene, two alleles. Punnett square to predict offspring ratios. Test cross: cross unknown genotype with homozygous recessive. Dihybrid cross: two unlinked genes → 9:3:3:1 ratio in F₂. Codominance: both alleles expressed (e.g., blood groups — Iᴬ and Iᴮ). Sex linkage: gene on X chromosome (e.g., colour blindness, haemophilia) — males more affected.
Genetic modification (GM): transferring genes between species using restriction enzymes, ligase, and vectors (plasmids). Applications: insulin production by bacteria, Bt crops (insect resistance), golden rice (vitamin A). PCR (polymerase chain reaction) amplifies DNA. Gel electrophoresis separates DNA fragments by size. Ethical considerations: safety, biodiversity, corporate control of food supply.
Follow these steps: (1) Define allele symbols (e.g., A = dominant, a = recessive). (2) Determine parent genotypes from the information given. (3) Draw a Punnett square showing all possible gamete combinations. (4) Read off genotype and phenotype ratios. (5) For sex-linked traits, use X^A and X^a notation. For test crosses, if any recessive offspring appear, the unknown parent must be heterozygous.
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