Cells, Genetics, and Reproduction
Core concepts
Cells are the basic unit of life. Prokaryotes lack a nucleus; eukaryotes have membrane-bound organelles. Key organelles include the nucleus, mitochondria (energy), ribosomes (protein synthesis), chloroplasts (photosynthesis), and cell membrane (selective transport). Mitosis produces identical body cells; meiosis produces gametes with half the chromosome number. DNA carries genes; alleles are variants. Dominant and recessive alleles determine phenotype; genotype is the genetic makeup. Punnett squares predict offspring ratios. Asexual reproduction produces clones; sexual reproduction creates genetic variation.
Traps to watch
- Confusing mitosis with meiosis.
- Thinking dominant alleles are more common in a population.
- Mixing genotype and phenotype.
- Forgetting that gametes are haploid and zygotes are diploid.
- Assuming all traits follow simple dominance.
Worked example
Cross two heterozygotes (Tt × Tt). Offspring genotypes: TT, Tt, Tt, tt, giving a 1:2:1 genotype ratio and a 3:1 phenotype ratio if T is dominant. The recessive phenotype appears only when both alleles are recessive.
Rule of thumb
Mitosis: two identical diploid cells. Meiosis: four haploid gametes. Dominant allele masks recessive in heterozygotes.