Genetics, populations, evolution and ecosystems: every key term you need (+ practice quiz)
16 flashcard terms for A-Level Biology Topic 7, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 8-question quiz — free, no account needed.
Genotype is the alleles an organism carries; phenotype is the expressed characteristics, shaped by both genotype and environment.
Codominance
Both alleles are expressed in the heterozygote, so the phenotype shows both — as in the AB blood group.
Sex linkage
A gene carried on the X chromosome. Males have only one copy, so recessive conditions such as haemophilia appear far more often in them.
Epistasis
One gene masks or modifies the expression of another at a different locus, distorting the expected dihybrid ratio.
Autosomal linkage
Genes on the same chromosome tend to be inherited together, so parental combinations appear far more often than recombinants.
Chi-squared test
Compares observed offspring numbers with those expected from a genetic ratio, testing whether the difference is due to chance.
Hardy-Weinberg principle
Predicts allele frequencies stay constant across generations, provided the population is large, mating is random and there is no selection or migration.
Directional selection
Favours one extreme phenotype, shifting the mean over time. Antibiotic resistance is the standard example.
Stabilising selection
Favours intermediate phenotypes and removes extremes, reducing variation. Human birth mass is a common illustration.
Genetic drift
Random change in allele frequency between generations. Its effect is far stronger in small populations than in large ones.
Allopatric speciation
A geographical barrier separates populations; independent selection and drift eventually make them reproductively isolated.
Carrying capacity
The maximum population an environment can sustain, set by limiting factors such as food, space, light and disease.
Density-dependent factor
A limiting factor whose effect increases as population density rises — competition, predation and disease transmission.
Succession
Directional change in a community over time. Pioneer species alter conditions, allowing others to outcompete them until a climax community forms.
Mark-release-recapture
Estimates population size from the proportion of marked individuals recaptured. Assumes marks do not affect survival and mixing is complete.
Conservation vs preservation
Conservation actively manages habitats for sustainable use; preservation protects them from human interference altogether.