Chromosomes and cell division
The nucleus contains chromosomes, long strands of DNA. A gene is a length of DNA that codes for a specific protein; different versions of a gene are alleles. The base sequence is copied into mRNA, which carries it to ribosomes where amino acids are joined in the matching order. Human body cells are diploid (46 chromosomes in 23 pairs); gametes are haploid (23). Mitosis produces two genetically identical diploid cells for growth, repair and asexual reproduction. Meiosis produces four genetically different haploid gametes.
≈ 45 minMonohybrid inheritance
The genotype is the pair of alleles an organism has (e.g. Bb); the phenotype is the characteristic shown (brown eyes). A dominant allele (capital letter) shows whenever present; a recessive allele (lowercase) shows only when there are two (bb). Homozygous means two identical alleles (BB or bb); heterozygous, two different (Bb). A Punnett square predicts offspring: Bb × Bb gives 1 BB : 2 Bb : 1 bb, a 3 : 1 phenotype ratio. A test cross with a homozygous recessive reveals an unknown genotype. In codominance both alleles show (blood group AB).
≈ 45 minVariation, mutation and selection
Variation is the differences between individuals. Continuous variation has a range of values (height, mass) and is affected by many genes and the environment; discontinuous variation falls into distinct groups (blood group) and is mainly genetic. Mutations are random changes in DNA that create new alleles. In natural selection, individuals with features better suited to the environment survive, reproduce and pass on their alleles, so populations change over time: evolution. Artificial selection (selective breeding) is when humans choose which individuals breed.
≈ 40 min