Variation
Members of the same species are not identical; the differences between them are called variation. This chapter covers the types of variation, the roles of genes and the environment, and mutation as a source of new variation.
You should be able to classify a variation as continuous or discontinuous and give its cause.
Content standard 27.2 expects you to sort familiar human examples correctly and to explain why each behaves the way it does statistically. Discontinuous traits such as blood group, tongue-rolling and earlobe attachment fall into a small number of sharp categories and are usually controlled by one or a few genes, so a bar chart with separate bars is the correct way to display them.
Continuous traits such as height, mass and skin colour form an unbroken range across a population, are usually controlled by many genes together with the environment, and are best displayed as a histogram that approximates a bell-shaped normal distribution curve.
Mutation, standard 27.3, is examined at two levels. A gene mutation changes the base sequence within a single gene through substitution, insertion or deletion, which may alter the protein made or, if it is a silent change, may have no effect at all.
A chromosomal mutation instead changes the structure or number of whole chromosomes, as in Down syndrome, which results from an extra copy of chromosome 21. Whether a mutation is harmful, neutral or beneficial depends on the environment, and this link to survival is often tested through the classic example of antibiotic resistance evolving in a bacterial population.
One-hour paid trial · Same-day reply
Content standards in this chapter
- 27.1 Types and Factors of Variation
- 27.2 Variations in Humans
- 27.3 Mutation
Key concepts
- Continuous variation
- Variation with a range of values and no clear categories, such as height or mass, usually controlled by many genes and the environment.
- Discontinuous variation
- Variation with distinct categories and no in-between, such as blood group, usually controlled by one or a few genes.
- Genetic and environmental factors
- Variation comes from genes, from the environment, or from both acting together.
- Sources of genetic variation
- Meiosis and random fertilisation shuffle alleles, and mutation creates new alleles.
- Mutation
- A change in the DNA (gene mutation) or in the chromosomes (chromosomal mutation) that can be passed on if it occurs in a gamete.
- Causes of mutation
- Mutations can be spontaneous or caused by mutagens such as radiation and certain chemicals.
- Examples of human variation
- Tongue-rolling, earlobe attachment and blood group are commonly used discontinuous examples because each person fits into one clear category. Height, body mass and skin colour are common continuous examples because they form an unbroken range with no natural break between one value and the next.
- Distribution curves
- Continuous variation, when measured in a large sample and plotted as a histogram, typically produces a bell-shaped normal distribution curve with most individuals near the average and fewer at the extremes. Discontinuous variation is instead shown as a bar chart with separate bars for each distinct category, since there are no intermediate values.
- Types of gene mutation
- A gene mutation changes the base sequence of DNA through substitution, where one base is swapped for another, insertion, where an extra base is added, or deletion, where a base is removed. Insertion and deletion can cause a frameshift, altering every codon that follows and often producing a completely different, usually non-functional, protein.
- Types of chromosomal mutation
- A chromosomal mutation changes the structure of a chromosome through deletion, duplication, inversion or translocation of a segment, or changes the total number of chromosomes through nondisjunction during meiosis. Down syndrome, for example, results from an extra copy of chromosome 21, giving 47 chromosomes instead of the usual 46.
- Effect of a mutation on a protein
- Because the sequence of bases in a gene determines the sequence of amino acids in a protein, a mutation can change, disable or destroy the protein's normal function. Some mutations are silent and cause no noticeable change, especially if they do not alter the amino acid produced or affect a non-critical part of the protein.
- Variation and natural selection
- Variation within a population provides the raw material on which natural selection can act, since individuals with characteristics better suited to the environment are more likely to survive and reproduce. Without genetic variation, an entire population would share the same weaknesses and could be wiped out by a single new disease or environmental change.
- Twins and variation
- Identical twins develop from a single fertilised egg that splits in two, so they share an identical genotype and any differences between them are purely environmental. Non-identical twins develop from two separately fertilised eggs, so they differ genetically as much as ordinary siblings, showing both genetic and environmental variation.
- Mutagens in detail
- Ionising radiation such as X-rays, gamma rays and ultraviolet light can damage DNA directly and increase the mutation rate, which is why protective shielding and limited exposure are used around such sources. Certain chemicals, including some found in cigarette smoke and industrial pollutants, are also mutagens that raise the chance of a gene or chromosomal mutation.
- A mutation can be advantageous
- Although most mutations are neutral or harmful, a mutation that happens to suit a particular environment can give an organism a survival advantage. Antibiotic resistance in bacteria is a well-known example: a random mutation that protects a bacterium from an antibiotic allows it to survive and reproduce while non-resistant bacteria are killed.
- Karyotyping
- A karyotype is an image of a person's chromosomes arranged in matching pairs by size and shape, prepared from cells such as white blood cells or fetal cells. Doctors examine a karyotype to detect a chromosomal mutation, such as an extra chromosome 21 in Down syndrome or a missing or rearranged chromosome segment.
How this chapter is examined
SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.
Common exam angles
- Classifying a given variation as continuous or discontinuous.
- Explaining how meiosis and fertilisation produce variation.
- Describing a cause of mutation and its possible effect.
- Sketching and labelling a distribution curve for a continuous variation and a bar chart for a discontinuous variation.
- Distinguishing gene mutation from chromosomal mutation, with one named example of each.
- Explaining, using antibiotic resistance, how a mutation can give a survival advantage.
Common mistakes
What students write: Saying blood group is continuous variation.
What earns the mark: Blood group is discontinuous, it falls into distinct categories (A, B, AB, O).
What students write: Writing that continuous variation is caused only by the environment.
What earns the mark: Continuous variation is usually caused by many genes together with the environment.
What students write: Confusing a gene mutation and a chromosomal mutation.
What earns the mark: A gene mutation changes the DNA of one gene; a chromosomal mutation changes the structure or number of chromosomes.
What students write: Saying all mutations are harmful.
What earns the mark: Most mutations are neutral or harmful, but some are beneficial and provide the variation for natural selection.
What students write: Saying earlobe attachment is a continuous variation.
What earns the mark: Earlobe attachment is discontinuous, a person either has free or attached earlobes, with no in-between category.
What students write: Drawing a bar chart for a continuous variation such as height.
What earns the mark: Continuous variation is best shown as a histogram or line graph forming a bell-shaped curve; a bar chart with separate bars suits discontinuous variation.
What students write: Saying a substitution mutation always changes the protein produced.
What earns the mark: A substitution can be silent if the new codon still codes for the same amino acid, so not every substitution changes the protein.
What students write: Confusing an insertion or deletion mutation with a substitution.
What earns the mark: Insertion and deletion add or remove a base and can shift the reading frame, changing every codon after the mutation; substitution swaps one base for another without shifting the frame.
What students write: Saying Down syndrome is caused by a gene mutation.
What earns the mark: Down syndrome is caused by a chromosomal mutation, specifically an extra copy of chromosome 21 from nondisjunction, not by a change within a single gene.
What students write: Saying identical twins can show genetic variation between them.
What earns the mark: Identical twins share an identical genotype from a single fertilised egg, so any difference between them is due to the environment, not genetics.
What students write: Saying mutation is the only source of variation.
What earns the mark: Meiosis and random fertilisation also produce genetic variation by shuffling existing alleles; mutation is the source of entirely new alleles.
What students write: Assuming every mutation is passed on to offspring.
What earns the mark: A mutation is only inherited if it occurs in a cell that produces gametes; a mutation in a normal body cell is not passed to the next generation.
What students write: Saying antibiotic resistance means bacteria decide to become resistant.
What earns the mark: Resistance arises from a random mutation that already exists before the antibiotic is used; the antibiotic then selects for bacteria that happen to carry it, it does not cause the mutation to appear on demand.
What students write: Saying mutations only happen when a mutagen is present.
What earns the mark: Mutations can occur spontaneously during DNA replication even without a mutagen, though mutagens such as radiation and certain chemicals increase the rate at which they occur.
Study this chapter
Experiments in this chapter
Frequently asked questions
What is the difference between continuous and discontinuous variation?
Where does variation come from?
What is a mutation?
What are some common examples of human variation used in exam questions?
What is the difference between a gene mutation and a chromosomal mutation?
Why do identical twins look almost the same but not exactly the same?
How does antibiotic resistance show variation and mutation at work?
Source:SRC-DSKP-EN, SRC-FORMAT
Related
One-hour paid trial · Same-day reply