Form 5 · Common mistakes

Variation, common mistakes

The mistakes SPM students make on Variation, why each one loses marks, and the correct version.

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The mistakes, why they lose marks, and the fix

Common mistake	Why it loses marks	What earns the mark
Saying blood group is continuous variation.	Blood group falls into a small number of sharp categories controlled by a single gene, which is the defining feature of discontinuous variation, not a spread of values.	State that blood group is discontinuous variation because every person falls into one of four categories, A, B, AB or O.
Writing that continuous variation is caused only by the environment.	A continuous character such as height is controlled mainly by many genes acting together, with the environment adding further variation on top rather than causing it alone.	State that continuous variation is usually caused by many genes together with the environment.
Confusing a gene mutation with a chromosomal mutation.	A gene mutation changes bases within one gene, while a chromosomal mutation changes the structure or number of whole chromosomes, a difference in scale that affects how each is examined.	State that a gene mutation changes the DNA of one gene, while a chromosomal mutation changes the structure or number of chromosomes.
Saying all mutations are harmful.	Most mutations are neutral or harmful, but a small number are beneficial, and it is exactly this minority that supplies the raw material for natural selection.	State that most mutations are neutral or harmful, though some are beneficial and provide variation for natural selection.
Saying earlobe attachment is continuous variation.	Earlobe attachment has only two possible states, free or attached, with no intermediate form, which is the hallmark of discontinuous variation.	State that earlobe attachment is discontinuous variation because there is no in-between category.
Drawing a bar chart for a continuous variation such as height.	A bar chart with separated bars suits a small number of distinct categories; height forms an unbroken range, which a histogram represents correctly.	Draw a histogram approximating a bell-shaped curve for a continuous variation such as height.
Saying a substitution mutation always changes the protein produced.	Because several codons can specify the same amino acid, a substitution can sometimes produce a codon for the same amino acid and leave the protein unchanged.	State that a substitution can be silent if the new codon still codes for the same amino acid.
Confusing insertion or deletion with substitution.	Insertion and deletion change the total number of bases and shift every codon after the mutation, while substitution swaps one base for another without shifting anything.	State that insertion and deletion cause a frameshift, while substitution changes only a single codon.
Saying Down syndrome is caused by a gene mutation.	Down syndrome results from an extra whole chromosome, chromosome 21, following nondisjunction during meiosis, which is a change in chromosome number, not a change within a single gene.	State that Down syndrome is a chromosomal mutation caused by an extra copy of chromosome 21.
Saying identical twins can show genetic variation between them.	Identical twins develop from a single fertilised egg that splits in two, so they start with one identical genotype and cannot differ genetically from that point onward.	State that any difference between identical twins is environmental, since their genotype is identical.
Saying mutation is the only source of genetic variation.	Meiosis and random fertilisation shuffle alleles that already exist into new combinations; only mutation creates an allele that did not exist before.	State that meiosis and fertilisation also produce genetic variation by shuffling existing alleles, while mutation creates new alleles.
Assuming every mutation is passed on to the next generation.	A mutation is inherited only if it occurs in a cell that goes on to form a gamete; a mutation in an ordinary body cell is not passed on.	State that only a mutation occurring in a cell that produces gametes can be passed to offspring.
Saying bacteria decide to become resistant to an antibiotic.	Resistance comes from a random mutation that already existed in the population before the antibiotic was used; the antibiotic then selects survivors, it does not cause the mutation to appear on demand.	State that resistance arises from a pre-existing random mutation that the antibiotic then selects for.
Saying mutations only occur when a mutagen is present.	Mutations can arise spontaneously as copying errors during DNA replication even with no mutagen present; a mutagen only raises the rate at which they occur.	State that mutations can occur spontaneously, and that a mutagen increases, rather than causes, their occurrence.

How to avoid these mistakes

Sort several examples into continuous and discontinuous variation.
Explain how meiosis, fertilisation and mutation create variation.
Learn the types and causes of mutation with one example each.
Sort five human examples into continuous or discontinuous and sketch the correct type of graph for each.
Make a table comparing gene mutation and chromosomal mutation, with one type and one example of each.
Explain the antibiotic resistance example to link mutation with natural selection in your own words.

Frequently asked questions

What is the difference between continuous and discontinuous variation?

Continuous variation shows a smooth range of values with no distinct groups, such as human height or body mass, and is usually controlled by many genes together with the environment. Discontinuous variation shows distinct categories with nothing in between, such as blood group or tongue-rolling, and is usually controlled by one or a few genes with little environmental effect.

Where does variation come from?

Variation comes from genetic and environmental sources. Genetic variation arises during meiosis, when alleles are shuffled, and at random fertilisation, when different gametes combine; mutation adds entirely new alleles. Environmental variation comes from factors such as diet, climate and lifestyle. Many characteristics result from both genes and the environment acting together.

What is a mutation?

A mutation is a change in the genetic material. A gene mutation is a change in the DNA sequence of a single gene, while a chromosomal mutation changes the structure or number of whole chromosomes. Mutations happen spontaneously or are caused by mutagens such as radiation and certain chemicals. If a mutation occurs in a gamete it can be passed to offspring and become a source of new variation.