This memory aid covers Memory Aids within Variation for GCSE Biology. Genetic and environmental variation in organisms It is section 7 of 10 in this topic. Use it for quick recall, then test yourself straight afterwards so the memory aid becomes usable in an answer.
Topic position
Section 7 of 10
Practice
25 questions
Recall
20 flashcards
Memory Aids
GEM causes variation: Genes, Environment, Mutations — the three sources of variation. "GEM" is the gem of a mnemonic for this topic.
Continuous vs Discontinuous — "C for Curve, D for Distinct": Continuous variation produces a smooth normal distribution curve on a graph (e.g., a bell curve of heights). Discontinuous variation produces distinct, separate bars on a bar chart (e.g., blood group A, B, AB, O — no in-between).
Examples to remember for each type: Continuous: height, mass, foot size, IQ test score. Discontinuous: blood group (ABO), eye colour (categorical), tongue rolling (yes/no), sex (male/female). When in doubt, ask: "Could there be a value in between?" If yes, it is continuous.
Mutations are rare, random, and mostly neutral: The 3 Rs of mutations. They are not directed towards a particular outcome — a mutation cannot "decide" to be helpful.
Quick Check: A student measures the heights of 100 students in Year 11. Explain why a graph of height would show a normal distribution, and identify one genetic and one environmental factor that contributes to height variation.
Height is an example of continuous variation — it is influenced by many genes (polygenic inheritance) and environmental factors, so individuals show a range of values from shortest to tallest, with most people clustered around the mean. This produces a bell-shaped normal distribution curve. Genetic factor: the genes inherited from parents set the potential height range (tall parents tend to have taller children). Environmental factor: nutrition during childhood — adequate calcium and protein intake supports bone and muscle growth, so better-nourished individuals tend to reach more of their genetic potential height.
Quick Check: Explain how sexual reproduction and meiosis together increase genetic variation in a population compared to asexual reproduction.
During meiosis, homologous chromosomes pair up and crossing over occurs — segments of chromosomes are exchanged, creating new combinations of alleles on each chromosome. Chromosomes then separate randomly into gametes, so each gamete has a unique combination. At fertilisation, gametes from two different parents combine, mixing alleles from four grandparents. This means every offspring (except identical twins) has a genotype different from any other individual. Asexual reproduction produces genetically identical offspring (clones) by mitosis, so no new combinations arise. Sexual reproduction therefore generates much greater genetic diversity within a population.
Quick Check: A new mutation arises in a gene coding for a digestive enzyme. Evaluate the likely impact of this mutation on the organism.
The impact depends on the type of mutation. Most mutations are neutral: if the mutation occurs in a non-coding region, or produces a codon that codes for the same amino acid (silent mutation), the enzyme structure and function are unchanged. A harmful mutation that changes the amino acid sequence could alter the active site shape, reducing or eliminating enzyme function and impairing digestion — this could be fatal if the enzyme is essential. A beneficial mutation is very rare but possible — the enzyme could have an improved active site or greater stability. Overall, neutral or harmful outcomes are far more likely than beneficial ones, but mutations are not always damaging.