External Examination: Cellular Processes, Genetics and Gene Expression
What the external domain assesses
The externally assessed Level 2 Biology standards are 2.4 (life processes at the cellular level, 4 credits), 2.5 (genetic variation and change, 4 credits) and 2.7 (gene expression, 4 credits). They are assessed in a written examination where you interpret diagrams, explain processes in sequence and apply genetics to unfamiliar organisms. Answers need cause-and-effect reasoning, not definitions alone.
Core rules
- 2.4 covers photosynthesis, aerobic respiration, enzyme action, mitosis and transport across membranes. Link each process to its organelle, inputs, outputs and a limiting factor.
- 2.5 covers meiosis, crossing over, independent assortment, mutation, monohybrid and dihybrid inheritance, and changes in allele frequency through natural selection, genetic drift and migration.
- 2.7 covers DNA structure and replication, transcription and translation, mutations, and how gene expression can be influenced by the environment.
Worked example
In pea plants, tall (T) is dominant to short (t). A tall plant is crossed with a short plant and produces 48 tall and 52 short offspring. Reason: the short parent must be tt, so it contributes t to every offspring. The tall parent must be Tt, because a TT parent would give all tall offspring. The cross Tt × tt gives gametes T and t from the tall parent and t from the short parent, so offspring are Tt (tall) and tt (short) in a 1:1 ratio. The observed 48:52 is close to this expected ratio.
Traps
Calling a dominant allele 'stronger' or 'more common'; confusing mitosis with meiosis; writing mRNA as identical to the DNA template strand instead of complementary; describing translation as occurring in the nucleus; and explaining an adaptation or process without naming the mechanism.