Variation, practice questions

C, Blood group falls into four sharp categories, A, B, AB and O, controlled by a single gene, which is the pattern of discontinuous variation.

C, Continuous variation forms an unbroken range, best shown as a histogram whose bars touch and approximate a bell-shaped distribution.

B, Insertion is the addition of an extra base into the sequence, which can cause a frameshift.

C, Nondisjunction is the failure of chromosomes to separate correctly during meiosis, producing gametes with an abnormal chromosome number.

A, Down syndrome is caused by nondisjunction producing a gamete with an extra copy of chromosome 21, giving 47 chromosomes in the zygote.

B, Ultraviolet light is a form of ionising radiation that damages DNA directly and raises the mutation rate.

B, Insertion or deletion changes the total number of bases, shifting every codon that follows and causing a frameshift.

B, A random mutation for resistance already exists before the antibiotic is used; the antibiotic then selects for the bacteria that carry it.

B, Identical twins arise when one fertilised egg splits into two embryos, so both share exactly the same genes.

C, A karyotype is an image of a person's chromosomes arranged by size and shape, used to detect a chromosomal mutation such as an extra chromosome.

• (a) Tongue-rolling ability and blood group are discontinuous variation; height and body mass are continuous variation.
• (b) A continuous example such as height is displayed as a histogram approximating a bell-shaped curve.
• A discontinuous example such as blood group is displayed as a bar chart with separate, non-touching bars.
• (c) Both types of variation are ultimately controlled by genes, whether by one or a few genes for a discontinuous character or by many genes acting together for a continuous one.
• The environment adds a further contribution to a continuous character but has little or no effect on a discontinuous one.

• (a) The three types of gene mutation are substitution, insertion and deletion.
• (b) Insertion adds an extra base and shifts every codon after that point, an effect called a frameshift, scrambling the amino acid sequence and usually producing a non-functional protein.
• A substitution changes only one codon and can sometimes be silent, so its effect is generally more limited.
• (c) A substitution that happens to produce a codon coding for the same amino acid is an example of a silent mutation with no observable effect.
• The protein produced remains unchanged because the same amino acid is still specified.

• (a) A random mutation can, by chance, occur in a bacterium's DNA before the antibiotic is ever used, giving that bacterium a gene that protects it from the antibiotic.
• (b) When the antibiotic is applied, non-resistant bacteria are killed, while the resistant bacterium survives and reproduces.
• The resistant bacterium passes the resistance allele to its offspring, so the proportion of resistant bacteria increases.
• (c) This is natural selection because the antibiotic acts as a selection pressure that favours the pre-existing resistant variant.
• The resistant type increases in frequency across generations because the antibiotic selects for a mutation that already existed, rather than causing it to appear.

Variation with a range of values and no clear categories, such as height or mass, usually controlled by many genes and the environment.

Variation with distinct categories and no in-between, such as blood group, usually controlled by one or a few genes.

Variation comes from genes, from the environment, or from both acting together.

Meiosis and random fertilisation shuffle alleles, and mutation creates new alleles.

A change in the DNA (gene mutation) or in the chromosomes (chromosomal mutation) that can be passed on if it occurs in a gamete.

Mutations can be spontaneous or caused by mutagens such as radiation and certain chemicals.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.