Inheritance

• The sentence to carry into the exam. Genes are instructions, alleles are versions of an instruction, genotype is which versions you carry, phenotype is what that produces, and selection decides which versions the next generation carries.
• The one-question test. Ask: can I imagine an individual sitting exactly between these two? If yes, the variation is continuous. If the idea is meaningless — half blood group A, a seed that is three-quarters round — it is discontinuous.
• Genotype is what you carry; phenotype is what you show. A recessive allele in a heterozygote is carried but not shown. That single sentence resolves most of the confusion in this topic, including why two tall plants can produce a short one.
• Six words that carry a natural-selection answer. Variation — competition — survival — reproduction — inheritance — frequency. If all six appear in your answer, in that order, it is very hard to miss a marking point.
• If you remember nothing else: genotype is what you carry and phenotype is what you show; mutation is random and selection is not; and every ratio in genetics is an expectation, never a promise.

• Define variation and state that it may be caused by genes, by the environment, or by both acting together.
• Describe continuous variation, give body length and body mass as examples, and plot such data as a frequency distribution or a histogram with touching bars.
• Describe discontinuous variation, give ABO blood group and pea seed shape and colour as examples, and plot such data as a bar chart with separated bars.
• Design an investigation into variation that specifies the variable, the method, the precision, the sample size and how the sample will be chosen without bias.
• Describe DNA as two strands coiled into a double helix, each strand a chain of nucleotides, each nucleotide carrying one base represented A, T, C or G.
• State that bonds hold complementary bases together, that A pairs with T and C pairs with G, and use that rule to complete a strand, count bases and spot an impossible pairing.
• Define a gene as a length of DNA that codes for a protein, and distinguish gene, allele and chromosome without hesitating.
• Explain the chain from base sequence to amino-acid sequence to protein shape to protein function to cell activity to phenotype, using an enzyme as the worked case.
• Define and use allele, genotype, phenotype, dominant, recessive, homozygous, heterozygous and pure-breeding correctly and consistently.
• Construct a complete genetic diagram: symbols, parental phenotypes, parental genotypes, gametes, Punnett square, offspring genotypes, offspring phenotypes, ratio.
• Produce and explain the expected 3:1 phenotype ratio from Tt × Tt and the expected 1:1 ratio from Tt × tt, and say why the genotype ratio differs from the phenotype ratio.
• Convert an expected ratio into a fraction, a percentage and an expected number, and explain why a real family or a small sample often departs from it.
• Define codominance and distinguish it from complete dominance.
• Give all six ABO genotypes with correct notation and work out the possible blood groups of children from any pair of parents.
• Explain chromosomal sex determination from XX and XY, draw the cross, and state that each fertilisation is independent with an approximately equal probability.
• Define mutation, distinguish a gene mutation from a chromosome mutation, and give sickle cell anaemia and Down’s syndrome as the required examples.
• State that ionising radiation and some chemicals increase mutation rate, and that mutation remains random with respect to what the organism needs.
• List the sources of genetic variation: mutation, meiosis, random mating and random fertilisation.
• Set out the full natural selection sequence and define evolution as change in the inherited features of a population over generations.
• Explain the development of antibiotic resistance as natural selection acting on variation that already existed.
• Outline artificial selection, compare it with natural selection, and separate it from genetic modification.

Why this is the example chosen. Antibiotic resistance is natural selection running fast enough to watch. Bacteria divide in minutes, so a change that would take thousands of years in a beetle population takes days in a bacterial one — and the consequence, an infection that no longer responds to treatment, is why completing a prescribed course and not using antibiotics for viral illnesses matter.

Test Cross

A cross between an individual showing a dominant phenotype, whose genotype is unknown, and an individual that is homozygous recessive for the same gene. Because the homozygous recessive parent can only produce gametes carrying the recessive allele, every offspring's phenotype is decided by the allele it receives from the unknown parent. If any offspring shows the recessive phenotype, the unknown parent must be heterozygous; if a large number of offspring all show the dominant phenotype, the unknown parent is probably homozygous dominant, although no finite number of offspring proves this with certainty.

Monohybrid Cross

A cross that follows the inheritance of a single gene with two alleles from one generation to the next. In the standard example two heterozygous parents, each with the genotype Tt, are crossed. Each parent produces gametes carrying either the dominant allele T or the recessive allele t in equal proportions, and combining them gives an expected genotype ratio of 1 TT to 2 Tt to 1 tt. Because T is dominant, three of those four genotypes give the same phenotype, producing the characteristic expected phenotype ratio of 3 tall to 1 short.

Genetic Diagram

A standard way of setting out a cross so that the possible offspring can be predicted. A complete genetic diagram defines the allele symbols, states the phenotype and genotype of each parent, shows the gametes each parent can produce with one allele in each gamete, combines the gametes in a Punnett square or by combination lines, and then states the genotypes and phenotypes of the possible offspring together with the expected ratio. Marks in an examination are distributed across these rows, so a correct Punnett square with no parental genotypes above it scores only part of the total.

DNA

Deoxyribonucleic acid, the molecule that carries genetic information in living organisms. A DNA molecule consists of two strands coiled together into a double helix. Each strand is a chain of nucleotides, and each nucleotide contains one base, represented by the letters A, T, C and G. The two strands are held together by bonds between complementary pairs of bases, with A always pairing with T and C always pairing with G, so the order of bases along one strand determines the order along the other.

ABO Blood Group System

A human gene with three alleles that determines blood group. The alleles I to the power A and I to the power B are codominant with each other, and both are dominant to the allele I to the power O. A person with genotype I-A I-A or I-A I-O is group A, one with I-B I-B or I-B I-O is group B, one with I-A I-B is group AB because both codominant alleles are expressed, and only the homozygous recessive genotype I-O I-O gives group O. The system is the standard Cambridge O Level Biology example of codominance and of a gene with more than two alleles.

Discontinuous Variation

Variation in which the phenotypes of a species fall into a limited number of distinct categories with no intermediates between them. Each individual belongs to exactly one category, the data are counted rather than measured, and in the standard Cambridge O Level Biology examples the categories are controlled mainly by genes with little or no environmental influence. ABO blood group, pea seed shape and pea seed colour are the required examples, and the correct graph is a bar chart with separated bars and frequency on the vertical axis.

Complementary Base Pairing

The rule that governs which bases can face each other across the two strands of a DNA molecule: adenine always pairs with thymine, and cytosine always pairs with guanine, with bonds holding each pair together. Because the pairing is fixed, the base sequence of one strand completely determines the base sequence of the other, and any complete double-stranded DNA sample must contain equal numbers of A and T bases and equal numbers of C and G bases.

Mutation

A random change in the genetic material of a cell. A gene mutation is a change in the sequence of bases in a length of DNA, which can alter the amino-acid sequence of the protein that gene codes for. A chromosome mutation is a change in the number of chromosomes or in their structure, and therefore affects many genes at once. Mutations occur at random and are not directed by the needs of the organism; they may be harmful, neutral or, occasionally, beneficial, and only mutations in cells that give rise to gametes can normally be passed to offspring.

Antibiotic Resistance

The ability of some bacteria to survive treatment with an antibiotic that kills other members of the same species. Resistance arises from a random mutation, which occurs independently of whether the antibiotic is present. When the antibiotic is used it acts as a selection pressure: bacteria without the resistance allele are killed or reproduce less, while resistant bacteria survive and reproduce, passing the allele to their offspring. The proportion of resistant bacteria in the population therefore rises and the antibiotic becomes less effective against it. This is natural selection acting on a bacterial population, not a change produced in individual bacteria by the antibiotic.

Codominance

A situation in which both alleles of a gene are expressed in the phenotype of a heterozygous individual, so that the heterozygote shows the effects of both alleles rather than the effect of only one. Codominance differs from complete dominance, where the heterozygote shows only the dominant allele's effect, and from incomplete dominance, where the heterozygote shows an intermediate phenotype rather than both. The ABO blood group system is the required Cambridge O Level Biology example: the alleles for A and B are codominant, so a person carrying one of each has blood group AB.

Artificial Selection

The process in which humans choose which individuals of a species will breed, in order to increase the frequency of a desired inherited feature in later generations. Individuals showing the desired feature are selected and bred together, their offspring are assessed, those showing the feature most strongly are selected as the next parents, and the process is repeated over many generations. Artificial selection is used to produce crop varieties with higher yield or disease resistance and livestock with desired characteristics. Because it repeatedly breeds from a small number of chosen individuals it tends to reduce genetic diversity, which can leave the variety vulnerable to a new disease or a change in conditions. It is distinct from genetic modification, which changes an organism's genetic material directly.

Sources of Genetic Variation

The processes that produce genetic differences between individuals of the same species: mutation, meiosis, random mating and random fertilisation. Mutation is the only one that creates a new allele, by changing the base sequence of DNA. Meiosis puts a different combination of the existing alleles into each gamete. Random mating determines which two individuals breed together, and random fertilisation determines which particular gametes fuse, so that each offspring receives a combination of alleles that is unlikely to have occurred before. Together these processes supply the inherited variation on which natural selection and artificial selection act.

Natural Selection

The process by which individuals whose inherited features make them better suited to their environment survive and reproduce more successfully than others, so that the alleles responsible become more common in the population over generations. Natural selection requires inherited variation within the population, the production of more offspring than can survive, and competition for limited resources. It acts on variation that already exists rather than creating it, and the outcome is measured in reproductive success rather than in survival alone.

Chromosome Mutation

A change in the number of chromosomes in a cell or in the structure of a chromosome. Because a single chromosome carries many genes, a chromosome mutation affects far more genetic material than a gene mutation does. The required Cambridge O Level Biology example is Down's syndrome, in which body cells contain 47 chromosomes rather than the usual 46 because there is an additional copy of chromosome 21. Chromosome mutations arise at random during the formation of gametes and are not caused by anything the organism does or needs.

Sex Determination

In humans, chromosomal sex is determined by one pair of chromosomes called the sex chromosomes. The typical female combination is XX and the typical male combination is XY. Because a female has two X chromosomes, every egg carries an X; because a male has one X and one Y, half of the sperm carry X and half carry Y. The sperm therefore supplies the chromosome that determines whether the offspring is XX or XY, and each fertilisation has an approximately equal probability of each outcome, independently of any previous fertilisation.

Selection Pressure

Any factor that causes some individuals in a population to survive and reproduce more successfully than others, so that the frequency of particular alleles changes over generations. In natural selection the pressure comes from the environment, through factors such as predation, disease, competition for food and climate. In artificial selection the pressure is applied deliberately by people, who choose which individuals are allowed to breed. In both cases the pressure acts on inherited variation that already exists in the population; it does not create new variation.

Allele

An alternative form of a gene. Alleles of the same gene occupy the same position on a pair of homologous chromosomes and differ in their base sequence, so they code for slightly different versions of the same protein. A diploid organism carries two alleles of each gene, one inherited from each parent; if the two are identical the organism is homozygous for that gene, and if they differ it is heterozygous.

Gene

A length of DNA that codes for a protein. The sequence of bases along a gene determines the sequence of amino acids in the protein made from it; that amino-acid sequence determines how the protein chain folds and therefore its three-dimensional shape; and the shape determines what the protein can do. Because enzymes are proteins, and enzymes control the chemical reactions of a cell, a change in the base sequence of a gene can change the function of a protein and so change the phenotype of the organism.

Unbiased Sample

A sample chosen in a way that gives every individual in the defined population an equal chance of being included, so that the sample's spread of phenotypes reflects the population's spread. In an investigation into variation, bias is introduced by choosing individuals that are convenient, that are visibly extreme, or that all come from one small part of the habitat, and any of the three makes the resulting distribution unrepresentative however carefully the measurements themselves were taken. Random selection and systematic selection, such as taking every nth individual from a list, are the two standard ways of avoiding it.

Continuous Variation

Variation in which the phenotypes of a species form a range between two extremes, with intermediate values possible at any point along that range. Continuous variation is measured rather than counted, is usually influenced by many genes acting together with the environment, and produces a smooth frequency distribution when plotted. Body length and body mass are the standard Cambridge O Level Biology examples, and the correct graph is a histogram with touching bars or a frequency distribution curve.

Evolution

The change in the inherited features of a population over generations, brought about by processes such as natural selection acting on inherited variation. Evolution is a property of populations rather than of individuals: an individual organism never evolves, because the alleles it carries are fixed at fertilisation. What changes is the frequency with which particular alleles occur among the members of the population, and therefore how common the features those alleles produce become. Characteristics acquired during an organism's lifetime are not inherited and do not contribute to evolution.

Variation

Differences between individuals of the same species. Variation may be caused by differences in the genes an individual inherited, by differences in the environment the individual has experienced, or by the two acting together, and most measurable features in living organisms are influenced by both. Variation is the raw material on which natural selection and artificial selection act: without differences between individuals there is nothing for selection to select.

Expected Ratio

The ratio of offspring phenotypes or genotypes predicted by a genetic diagram, understood as a set of probabilities that apply independently to each offspring rather than as a guaranteed count. The expected number of offspring of a given type is the probability of that type multiplied by the total number of offspring. Because each fertilisation is an independent event, small numbers of offspring frequently depart from the expected ratio by chance, while larger numbers tend to approach it more closely.

Mutagen

Any agent that increases the rate at which mutations occur. Ionising radiation, such as X-rays and gamma rays, and certain chemicals are the two categories required by the Cambridge O Level Biology syllabus. A mutagen raises the probability that a mutation happens; it does not guarantee that any particular exposure causes one, and it does not determine which mutation occurs or whether the result is harmful, neutral or beneficial. Mutation remains random with respect to the needs of the organism whether or not a mutagen is present.

Sickle Cell Anaemia

An inherited condition used as the required Cambridge O Level Biology example of a gene mutation. A change in the base sequence of the gene coding for haemoglobin alters the amino-acid sequence of the protein. The altered haemoglobin molecules attach to one another inside the red blood cell, which changes the cell's shape from a rounded disc to a curved sickle. Sickled cells carry less oxygen than normal red blood cells and can obstruct small blood vessels. Because it results from a change within one gene rather than a change to a whole chromosome, it is classified as a gene mutation.

• Changing symbols part-way through. Defining T and t and then writing S for short in the square. Use one letter for the gene, capital for the dominant allele, lower case for the recessive, from the first line to the last. Fix Write the symbol definition first and never look away from it.
• Putting two alleles in one gamete. A gamete row reading “Tt and Tt”. Gametes are made by meiosis and are haploid: one allele each. Fix Circle each gamete as you write it — the circle only fits one letter.
• Omitting the parental genotypes. Jumping from “two tall plants were crossed” straight to the square. The genotypes are a marking point in their own right, and without them the gametes are unjustified. Fix Write every row, even the ones that feel obvious.
• Assuming a genotype from a dominant phenotype. A tall plant is TT or Tt and you usually cannot tell which from its appearance alone. If the question does not tell you, and does not say pure-breeding, say so and work through both possibilities. Fix Ask “what evidence do I have for this genotype?” before you write it.
• “The beetles became darker to avoid predators.” This says individuals changed, and that they changed on purpose. Neither happened. Fix “Dark beetles were less often eaten, so more of them survived to reproduce, and the proportion of dark beetles increased.”
• “The environment caused a mutation for dark colour.” Mutation is random with respect to need; the environment selects among variants that already exist. Fix “Dark and pale forms were already present in the population through earlier mutation.”
• “Only the strongest survive.” Strength is usually irrelevant. What matters is which inherited features suit this environment, and the outcome is measured in offspring, not in fights. Fix “Individuals better suited to the environment reproduce more successfully.”
• “Every pale beetle died.” Selection is a difference in average success, not a rule with no exceptions. Pale beetles are still present at generation 50. Fix “A smaller proportion of pale beetles survived to reproduce.”
• “The bacteria became immune to the antibiotic.” Immunity is a property of a host’s lymphocytes and antibodies (Chapter 12). Bacteria are not immune to antibiotics; they are resistant. Fix Use the word resistant, every time.
• “The person became resistant to the antibiotic.” The population that changes is the bacterial one. The patient’s own cells are not involved. Fix “The bacteria infecting the patient became resistant.”
• “The antibiotic made the bacteria mutate.” This is the whole misconception the topic exists to correct. The mutation came first, at random; the antibiotic then selected the bacteria that already had it. Fix “A resistant bacterium was already present because of an earlier random mutation.”
• “Continuous variation is controlled only by genes.” It is usually influenced by many genes and by the environment, and that combination is what produces a smooth range rather than distinct groups. Write “Continuous variation is usually caused by many genes acting together with the environment.”
• “Discontinuous variation cannot be inherited.” The reverse: the required examples are controlled mainly by genes and are inherited very reliably. Your blood group was fixed at fertilisation. Write “Discontinuous variation in these examples is controlled mainly by genes, with little environmental effect.”
• “All the differences you can see are inherited.” Scars, tans, muscle built by training and a plant kept small by shade are all genuine variation and none of them is passed on. Write “This variation is environmental, so it is not inherited.”
• “A gene is a chromosome.” A chromosome is the whole structure; a gene is a length of the DNA in it, and one chromosome carries many genes. Write “A gene is a length of DNA that codes for a protein.”
• “DNA is made of amino acids.” DNA is made of nucleotides; proteins are made of amino acids. The two are connected by the coding relationship, not by being the same material. Write “Each strand of DNA is a chain of nucleotides.”
• “A pairs with C.” A pairs with T; C pairs with G. There are exactly two permitted pairs and no others, in either order. Write “A pairs with T and C pairs with G.”
• “The gene makes the characteristic.” It codes for a protein; the protein does something in a cell; the effect of that is the characteristic. Skipping the middle costs most of the marks in an explain question. Write the whole chain: base sequence, amino-acid sequence, protein shape, protein function, cell activity, phenotype.
• “Dominant means common.” Dominant describes what happens in a heterozygote, nothing else. Polydactyly is dominant and rare; blood group O is recessive and common. Write “A dominant allele is expressed in the heterozygote.”
• “Dominant means stronger or better.” There is no contest. A dominant allele produces its effect with one copy present; a recessive one needs two. Neither is superior. Write “The recessive allele is not expressed when a dominant allele is present.”
• “Recessive means weak or rare.” A recessive allele can be extremely common in a population, and is passed on just as faithfully as a dominant one. Write “A recessive allele is expressed only when no dominant allele is present.”
• “Genotype and phenotype mean the same thing.” Genotype is the alleles carried; phenotype is what is observed. They have different ratios from the same cross, which is why questions ask for one specifically. Write genotypes as symbols, phenotypes as words.
• “A heterozygote has two identical alleles.” That is homozygous. Heterozygous means different: one of each. Write “Tt is heterozygous; TT and tt are homozygous.”
• “A gamete can carry both alleles.” Gametes are haploid: meiosis separates the pair, so each gamete gets one allele of the gene. Write gametes as single circled letters.
• “A 3:1 ratio means three out of every four offspring.” It is an expected probability applying to each offspring independently. Four offspring can easily come out 4:0. Write “the expected ratio is 3 : 1”, and add that observed numbers vary by chance, especially in small samples.
• “Codominance means the alleles blend.” Blending is incomplete dominance. Codominance means both alleles are expressed: a person with genotype IAIB has both A and B antigens, not something halfway. Write “Both alleles are expressed in the heterozygote.”
• “Blood group O has two possible genotypes.” Only IOIO, because any dominant allele present would give group A, B or AB. Groups A and B are the ones with two genotypes each. Write the four-row genotype table from 17.4 B.
• “They have had three girls, so the next is more likely to be a boy.” Each fertilisation is independent. The probability is approximately \(\tfrac{1}{2}\) every time, regardless of what came before. Write “Each fertilisation has an approximately equal probability of XX or XY, independently of previous children.”
• “All mutations are harmful.” Most are neutral, some are harmful, and occasionally one is beneficial in a particular environment. Without that occasional useful one there would be no new variation at all. Write “Mutations may be harmful, neutral or beneficial.”
• “The organism mutated because it needed to.” Mutation is random with respect to need. Variation arises first; the environment then selects among it. Write “The mutation occurred at random, and the environment then favoured the individuals carrying it.”
• “Individuals evolve.” Evolution is a change in the inherited features of a population over generations. An individual’s genotype is fixed at fertilisation. Write “The population evolved: the proportion of individuals with that feature increased.”
• “Natural selection is survival of the strongest.” Strength is usually irrelevant, and survival is only half of it — what matters is reproducing. Write “Individuals with inherited features that suit the environment survive and reproduce more successfully.”
• “The antibiotic made the bacteria resistant.” The resistance allele arose earlier, by random mutation. The antibiotic selected the bacteria that already had it. Write “A resistant bacterium was already present; the antibiotic killed the others, so the resistant one survived and reproduced.”
• “Humans become resistant to antibiotics.” The bacterial population becomes resistant, not the patient. Resistance is also not the same as immunity, which is a property of the host’s lymphocytes. Write “The bacteria became resistant.”
• “Artificial selection is genetic modification.” Artificial selection chooses which existing organisms breed. Genetic modification changes genetic material directly. Write “Artificial selection selects parents; it does not alter their genes.”

• The cheapest mark in the topic. Writing the word expected in front of every ratio. It costs one word and it is frequently a marking point in its own right, because it is the difference between understanding genetics as probability and treating it as a promise.
• Three ways to lose the graph mark. Leaving gaps between the bars (that makes it a bar chart, and says your data are categories). Omitting the unit on the x-axis — “body length” is not a label, “body length / cm” is. And using unequal class widths without saying so, which distorts the shape the whole graph exists to show.
• The one word that identifies a genotype for you. Pure-breeding means homozygous. So “a pure-breeding tall plant” is TT, with no ambiguity, and “a pure-breeding short plant” is tt. When a question wants you to know a genotype, it will usually either use that word or tell you the phenotype of a parent that must be homozygous recessive.
• Notation matters here. With complete dominance, T and t are ideal, because the case of the letter records which allele is dominant. With codominance there is no dominant partner to record, so alleles are written as the same base letter with different superscripts — IA and IB in the ABO system. Using A and a for two codominant alleles states something false about them before you have written a single line of the cross.

• Inheritance appears in both written papers, and it appears differently in each. Knowing which shape of question you are looking at tells you how much to write before you have read a single word of the stem.
• Usually one or two items. The favourites are: classify a named feature as continuous or discontinuous; pick the correct complementary base sequence; read a genotype off a Punnett square; identify the possible blood groups of a child; and choose the correct order of the natural-selection steps. Distractors are built from the exact confusions in the mistake clinic, so read that section as if it were a list of wrong answers you are about to be offered.
• The topic’s natural home. Expect a short definition (1–2 marks), a full genetic diagram (4–5 marks), a probability or ratio calculation (1–2 marks), and one extended explanation of natural selection, antibiotic resistance or artificial selection (4–6 marks). Data-response versions supply a table of measurements or a family record and ask you to classify the variation, complete the cross, or account for an observed departure from an expected ratio.
• State and name want a phrase, not a sentence. Describe wants what happens. Explain wants why, and an explanation of natural selection with no mention of reproduction cannot score full marks. Suggest means the answer is not in the syllabus — apply a principle to an unfamiliar case. Complete the genetic diagram means every row of the protocol, not just the square.
• When a question says “complete the genetic diagram” or “use a genetic diagram to show…”, the marks are distributed across the rows, not concentrated in the square. A perfectly drawn Punnett square with no parental genotypes above it typically loses half the marks available. The rows are:
• offspring genotypes and phenotypes, then the ratio, described as expected.