Continuous Variation
Variation can be discrete or continuous
- Discrete variation is variation that falls into two or more clear-cut categories with no overlap or in-between categories
- Blood group is an example of discrete variation
- All human blood is either group O, A, B or AB, each with a Rhesus factor (+ or -)
- This gives just 8 distinct blood groups:
Pie chart showing global blood group distribution
Worldwide A, B, O blood group distribution by percentage, 2019
(data varies regionally with ethnicity)
Continuous Variation
- Continuous variation occurs when two or more genes affect the final characteristic
- For example, height in humans is determined by many genetic factors:
- Bone length
- Skeletal muscle structure
- Ability to absorb food substances effectively
- Hormone production
- …As well as environmental factors like diet, exercise, prenatal nutrition, lifestyle etc
- Most characteristics are determined by more than one gene - a polygenic characteristic
- Even grouped data like shoe size appears to be discrete but in fact, peoples' feet vary continuously in size
- Shoe size is merely a practicality for shoe manufacturers, who cannot make exactly the right-sized shoes for everybody
- Continuous variation in birth mass results in the population displaying a normal distribution (bell-shaped curve)
- Environmental factors can also affect birth mass, e.g. mother's diet, presence of a twin, smoking etc
- Continuous variation occurs when there are quantitative differences in the phenotypes of individuals within a population for particular characteristics
- Quantitative differences do not fall into discrete categories like in discontinuous variation
- For example, the mass or height of a human is an example of continuous variation
- Instead for these features, a range of values exist between two extremes within which the phenotype will fall
- The lack of categories and the presence of a range of values can be used to identify continuous variation when it is presented in a table or graph
Normal distribution curve
Graph showing population variation in height: an example of continuous variation with quantitative differences
Genetic basis of continuous variation
- This type of variation is caused by an interaction between genetics and the environment
- Phenotype = genotype + environment
- At the genetic level:
- Different alleles at a single locus have a small effect on the phenotype
- Different genes can have the same effect on the phenotype and these add together to have an additive effect
- If a large number of genes have a combined effect on the phenotype they are known as polygenes
- An example of a continuous polygenic trait is skin colour
- Skin colour is determined by several genes that cause the production of a protein called melanin
- The more melanin is produced, the darker the skin pigmentation becomes
- Skin colour is also influenced by environmental factors such as UV exposure, which can cause the skin colour to become darker