Inheritance, DNA and evolution
Biology · GCSE · The School
The nesting order
DNA is the molecule; a GENE is a section of DNA coding for a characteristic; a CHROMOSOME is a long coiled DNA molecule carrying many genes; humans have 23 pairs. Getting the nesting right — DNA inside genes inside chromosomes inside the nucleus — fixes most confusion in this topic.
Punnett squares
Each parent gives one allele. Cross Bb × Bb: the square gives BB, Bb, Bb, bb — a 3:1 ratio of dominant to recessive appearance. Capital letters are dominant, lower case recessive; a recessive characteristic only shows when BOTH alleles are recessive. Draw the square every time; doing it in your head is where errors come from.
Natural selection, step by step
Variation exists in a population. The environment changes (a predator, a drug, a colder climate). Individuals whose variation suits the new conditions survive and breed more. Their offspring inherit those characteristics. Over generations the population changes. Antibiotic resistance is this happening now, fast enough to measure — which is why finishing a prescribed course matters.
Before industrial soot, pale peppered moths on pale lichen were well camouflaged and dark ones were eaten. Soot blackened the trees. Now the dark moths were hidden and the pale ones were conspicuous, so more dark moths survived to breed and the dark form became common within decades. When the air was cleaned, the proportions swung back. No moth changed colour — the PROPORTIONS did, in both directions.
The vocabulary examiners will not let you blur
Several pairs get confused every year and each has a precise meaning. GENE is a section of DNA coding for a protein; ALLELE is a version of that gene. GENOTYPE is the alleles present, written as letters; PHENOTYPE is the characteristic you can observe. HOMOZYGOUS means two identical alleles, HETEROZYGOUS two different ones. DOMINANT describes an allele expressed even when only one copy is present; RECESSIVE needs two. Answers that say someone 'has the dominant gene' are marked wrong, because dominance describes an allele and its expression, not the gene itself.
Probability is not a prediction about four children
A monohybrid cross giving a three to one ratio means each offspring has a three in four chance, independently. It does not mean that in four children exactly three will show the trait — a family of four can easily have none. This is the single most common misreading of a Punnett square, and questions are written to test it. Say 'probability' or 'expected ratio' rather than 'will be', and if asked about a specific small family, state that each conception is independent of the last. The ratio describes a large population, not a household.