Variations in Humans

Continuous variation in humans

• Height
• Body mass
• Skin colour
• Intelligence, as measured by test performance

Discontinuous variation in humans

• ABO blood group (A, B, AB, or O)
• Ability to roll the tongue into a U-shape
• Attached or free earlobes
• Presence or absence of dimples

Why variation exists among humans

• Independent assortment during meiosis, homologous chromosomes separate into gametes randomly, giving each gamete a different mix of the mother's and father's chromosomes.
• Crossing over during meiosis, homologous chromosomes exchange segments, creating new allele combinations not present in either parent.
• Random fertilisation, any one of millions of possible sperm can fertilise the egg, adding another layer of chance to the offspring's genotype.
• Mutation, a sudden change in DNA can introduce an entirely new allele into a population.

Why variation matters and how this is examined

Variation gives a species the raw material to adapt to a changing environment: individuals with characteristics better suited to a new condition are more likely to survive and reproduce, passing those characteristics to the next generation. Without variation, a whole population would share the same weaknesses and could be wiped out by a single disease or environmental change.

SPM questions commonly ask you to classify a listed human characteristic as continuous or discontinuous, or to explain how meiosis and fertilisation together produce genetically varied offspring even from the same two parents.

Worked exam-style question

Question. Table 1 lists five human characteristics: (i) height, (ii) ABO blood group, (iii) skin colour, (iv) ability to roll the tongue into a U-shape, (v) body mass. (a) Classify each characteristic in Table 1 as showing continuous variation or discontinuous variation.

(b) State one feature of a graph plotted for a large sample of people that would show continuous variation, and one feature of a graph that would show discontinuous variation. (c) Explain why height shows continuous variation even though it is influenced by genes.

Model answer. (a) Continuous variation: height, skin colour, body mass. Discontinuous variation: ABO blood group, tongue-rolling ability.

(b) A graph of continuous variation is a smooth, bell-shaped curve with no gaps between values, since individuals can take any value across a wide range. A graph of discontinuous variation is a bar chart of distinct, separate categories with no intermediate values.

(c) Height is controlled by many genes acting together (polygenic inheritance) and is also strongly affected by environmental factors such as nutrition, so the combined effect produces a continuous range of values rather than a small number of distinct categories.

Practice question

Try this. A student claims that because ABO blood group is inherited, it must show continuous variation like height does. Explain why the student's reasoning is incorrect.

Exam tip

Key terms

These Chapter 27 terms name the concepts examined in this topic:

• Variation, differences that exist between individuals of the same species.
• Continuous variation, variation in which individuals show a complete range of values with no distinct categories, usually controlled by many genes and the environment.
• Discontinuous variation, variation in which individuals fall into distinct, separate categories with no intermediate forms.
• Mutation, a sudden, random change in the DNA sequence that can introduce a new allele into a population.

Source:SRC-DSKP-EN

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