Variation, revision notes

Overview

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.

Types and causes of variation (CS 27.1)

Content Standard 27.1 opens the chapter by defining variation as the differences that exist between members of the same species, then sorting those differences into two types. Discontinuous variation places every individual into one of a small number of sharp, non-overlapping categories, with no intermediate value possible; a person belongs to blood group A, B, AB or O, and there is no fifth in-between group.

This type of variation is usually controlled by one gene or a small number of genes, so the environment has little or no effect on which category an individual falls into.

Continuous variation, by contrast, spreads across an unbroken range from one extreme to the other, so that almost any value between the lowest and highest is possible; human height can take a wide range of values rather than fall into a handful of fixed categories. This type of variation is usually controlled by many genes acting together, often with a substantial contribution from the environment, such as diet and exercise affecting height or body mass.

Because so many genes and factors combine, a continuous character rarely settles into a small number of neat groups.

Recognising which type a described character belongs to is the skill this content standard rewards first, because the correct choice of graph, the correct explanation of its genetic basis and the correct example all follow directly from that classification.

Sorting human examples and reading their graphs (CS 27.2)

Content Standard 27.2 asks you to apply the distinction using familiar human examples and to justify each choice with a reason rather than a guess. Tongue-rolling, blood group and earlobe attachment are the standard discontinuous examples: each person can roll their tongue or cannot, has one of four blood groups, and has earlobes that are either free or attached, with every person fitting cleanly into one category.

Height, body mass and skin colour are the standard continuous examples, because a population sampled at random produces a smooth spread of values with no natural breaks between one person's measurement and the next. Presenting this data correctly matters as much as classifying it: discontinuous data belongs in a bar chart with separate, non-touching bars, one for each category, while continuous data belongs in a histogram whose bars touch and which typically approximates a bell-shaped normal distribution curve, with most individuals clustered near the average and progressively fewer towards each extreme.

A mismatched graph, such as a bar chart drawn for height, is one of the most common ways marks are lost on this content standard, even when the underlying biology has been understood correctly.

Where genetic variation comes from: meiosis, fertilisation and mutation

Two ordinary events in sexual reproduction already generate genetic variation before any mutation occurs. During meiosis, homologous chromosome pairs separate independently of one another, so the combination of maternal and paternal chromosomes that ends up in any one gamete is essentially random, shuffling existing alleles into new combinations.

At fertilisation, which of the many gametes produced by each parent happens to combine is also a matter of chance, adding a second layer of shuffling on top of the first.

These two processes explain why siblings from the same parents differ from one another despite sharing the same two sources of genes: meiosis and fertilisation redistribute existing alleles rather than creating new ones. Mutation is the only process that adds a genuinely new allele to the pool available to a population, which is why it is treated as a separate, third source of genetic variation rather than a variation on shuffling.

Twins illustrate the distinction well. Identical twins arise when a single fertilised egg splits into two embryos, so both share one identical genotype and skip the shuffling step entirely; any difference between them must be environmental.

Non-identical twins arise from two separately fertilised eggs, so they are shuffled and combined independently, making them no more alike genetically than ordinary siblings born at different times.

Gene mutation: substitution, insertion and deletion (CS 27.3)

A gene mutation is a change confined to the base sequence of a single gene, and the syllabus expects three named types. Substitution swaps one base for a different one at a single position, changing only the codon in which it occurs; because the genetic code has more than one codon for many amino acids, a substitution can sometimes be silent, producing a codon that still specifies the same amino acid and leaving the protein unaffected.

Insertion adds an extra base into the sequence, and deletion removes one; because bases are read in fixed groups of three during protein synthesis, either change shifts every codon that follows the mutated site, an effect called a frameshift. A frameshift typically scrambles the entire amino acid sequence downstream of the mutation and usually produces a non-functional protein, which is why insertion and deletion are generally more damaging than a single substitution.

Whether a gene mutation matters at all depends on where it falls and what it changes; a mutation that happens not to alter the amino acid produced, or one that affects a non-critical part of the protein, may have no observable effect on the organism.

Chromosomal mutation and Down syndrome

A chromosomal mutation acts on a much larger scale than a gene mutation, changing the structure of a whole chromosome or the total number of chromosomes in a cell rather than the sequence within one gene. Structural changes include deletion of a chromosome segment, duplication of a segment, inversion in which a segment is reversed in place, and translocation in which a segment moves to a different, non-homologous chromosome.

Numerical change usually arises from nondisjunction, a failure of homologous chromosomes or sister chromatids to separate correctly during meiosis, so that one resulting gamete gains an extra chromosome and another is left short of one. Down syndrome is the standard named example: fertilisation involving a gamete with an extra copy of chromosome 21 produces a zygote with 47 chromosomes instead of the usual 46, a condition that can be detected from a karyotype, an image of a person's chromosomes arranged and matched by size and shape.

Because a chromosomal mutation can involve thousands of genes at once rather than one, its effects are typically far more extensive than those of a single gene mutation, which is one reason exam questions expect the two types to be kept clearly apart.

Mutation, mutagens and natural selection: antibiotic resistance

A mutation can occur spontaneously during DNA replication, simply as a rare copying error, or its rate can be raised by a mutagen, an external agent that damages DNA. Ionising radiation such as X-rays, gamma rays and ultraviolet light is one recognised category of mutagen, and certain chemicals, including some found in tobacco smoke and industrial pollutants, are another.

Most mutations are neutral or harmful to the organism that carries them, but the small number that happen to suit a particular environment can give a survival advantage, and it is this possibility that links mutation to natural selection. Antibiotic resistance in bacteria is the syllabus's standard worked example: a random mutation that happens to protect a bacterium from a particular antibiotic can already exist in a population before that antibiotic is ever used.

When the antibiotic is applied, non-resistant bacteria are killed while the resistant one survives, reproduces and passes the resistance allele to its offspring, so the resistant type comes to dominate the population over successive generations purely because the antibiotic selects for a mutation that was already present, not because bacteria decide to become resistant on demand.

Continuous variation compared with discontinuous variation

Setting the two types of variation side by side is one of the most common ways this content standard is examined, because a single table answers questions on their genetic basis, the effect of the environment, the correct graph and typical examples all at once.

Key concepts to master

• 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.

Quick recall checklist

1. Can you define and explain Continuous variation?
2. Can you define and explain Discontinuous variation?
3. Can you define and explain Genetic and environmental factors?
4. Can you define and explain Sources of genetic variation?
5. Can you define and explain Mutation?
6. Can you define and explain Causes of mutation?
7. Can you define and explain Examples of human variation?
8. Can you define and explain Distribution curves?
9. Can you define and explain Types of gene mutation?
10. Can you define and explain Types of chromosomal mutation?
11. Can you define and explain Effect of a mutation on a protein?
12. Can you define and explain Variation and natural selection?
13. Can you define and explain Twins and variation?
14. Can you define and explain Mutagens in detail?
15. Can you define and explain A mutation can be advantageous?
16. Can you define and explain Karyotyping?

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Source:SRC-DSKP-EN