Types of Variation
Types of Variation
Caused by differences in genes inherited from parents
Examples: Blood type, eye colour, inherited diseases
Caused by conditions in which organism develops
Examples: Scars, language spoken, tanned skin
Most characteristics are influenced by BOTH:
- Height: Genes set potential, but nutrition affects actual height
- Weight: Metabolism is genetic, but diet/exercise affect it
- Intelligence: Genetic component, but education matters too
Biology glossary
- What is variation in biology?
- Differences in characteristics between individuals of the same species. Variation can be due to differences in genes (genetic variation), the environment, or a combination of both.
- What are the two main causes of genetic variation?
- 1. Mutations — random changes to DNA that create new alleles. 2. Sexual reproduction — combines alleles from two parents in unique combinations, producing offspring different from both parents.
Most characteristics are influenced by BOTH:
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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.
Now try it yourself
Quiz · Question 1 of 26
What is the main difference between genetic variation and environmental variation?
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