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 those versions produce, and a genetic diagram is how you work out which versions the offspring can get.
• Gene, allele, and the question each answers. Which gene is it? — the gene for flower colour. Which version of it is this one? — the allele for purple. For every gene met in this subtopic a body cell carries two alleles, one on each chromosome of the pair, and they may be the same as each other or different. Everything in subtopic 17.4 is built on that one sentence.
• Say the sentence with all three in it. “DNA controls cell function by controlling which proteins the cell makes — its enzymes, its membrane carriers and its receptors for neurotransmitters — and each of those works by having the right shape.” Three classes, one reason.
• The sentence to carry into the exam. “All the body cells of an organism contain the same genes, but different genes are expressed in different cells, so different proteins are made and the cells do different jobs.” Everything the statement asks for is in that one line.
• Anchor for the four roles: Grow, Repair, Replace, Reproduce — one G and three Rs.
• 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.
• The move, in one line. Two unaffected parents with an affected child ⇒ the characteristic is recessive, and both parents are heterozygous. Two affected parents with an unaffected child ⇒ the characteristic is dominant, and both parents are heterozygous. Look for either of those pairings first; everything else follows from it.
• One habit covers all three. Before you write anything, write the key: what each letter means, what each symbol means, which phenotype is shaded. Every diagram in this topic is an argument, and an argument whose terms are undefined cannot be marked.

• State that chromosomes are made of DNA, which contains genetic information in the form of genes. (17.1.1)
• Define a gene as a length of DNA that codes for a protein. (17.1.2)
• Define an allele as an alternative form of a gene. (17.1.3)
• Describe the inheritance of sex in humans with reference to the X and Y chromosomes. (17.1.4)
• Describe inheritance as the transmission of genetic information from generation to generation. (17.4.1)
• Describe genotype as the genetic make-up of an organism, in terms of the alleles present. (17.4.2)
• Describe phenotype as the observable features of an organism. (17.4.3)
• Describe homozygous as having two identical alleles of a particular gene, and state that two identical homozygous individuals that breed together will be pure-breeding. (17.4.4, 17.4.5)
• Describe heterozygous as having two different alleles of a particular gene, and state that a heterozygous individual will not be pure-breeding. (17.4.6, 17.4.7)
• Describe a dominant allele as one that is expressed if it is present in the genotype, and a recessive allele as one that is only expressed when no dominant allele of the gene is present. (17.4.8, 17.4.9)
• Interpret pedigree diagrams for the inheritance of a given characteristic. (17.4.10)
• Use genetic diagrams to predict the results of monohybrid crosses and calculate phenotypic ratios, limited to 1 : 1 and 3 : 1. (17.4.11)
• Use Punnett squares in crosses which result in more than one genotype, to work out and show the possible different genotypes. (17.4.12)
• State that the sequence of bases in a gene determines the sequence of amino acids used to make a specific protein. (17.1.5)
• Explain that different sequences of amino acids give different shapes to protein molecules. (17.1.6)
• Explain that DNA controls cell function by controlling the production of proteins, including enzymes, membrane carriers and receptors for neurotransmitters. (17.1.7)
• Explain how a protein is made, limited to the six steps from gene to ribosome. (17.1.8)
• Explain that most body cells contain the same genes, but many genes in a particular cell are not expressed. (17.1.9)
• Describe a haploid nucleus as containing a single set of chromosomes and a diploid nucleus as containing two sets. (17.1.10, 17.1.11)
• State that in a diploid cell there is a pair of each type of chromosome, and that a human diploid cell has 23 pairs. (17.1.12)
• Describe mitosis as nuclear division giving rise to genetically identical cells, and state its role in growth, repair of damaged tissues, replacement of cells and asexual reproduction. (17.2.1, 17.2.2)
• State that the exact replication of chromosomes occurs before mitosis, and that during mitosis the copies separate, maintaining the chromosome number in each daughter cell. (17.2.3, 17.2.4)
• Describe stem cells as unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions. (17.2.5)
• State that meiosis is involved in the production of gametes. (17.3.1)
• Describe meiosis as a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells. (17.3.2)
• Explain how to use a test cross to identify an unknown genotype. (17.4.13)
• Describe codominance as a situation in which both alleles in heterozygous organisms contribute to the phenotype. (17.4.14)
• Explain the inheritance of ABO blood groups: phenotypes A, B, AB and O, alleles IA, IB and Io. (17.4.15)
• Describe a sex-linked characteristic as one whose gene is on a sex chromosome, and explain why that makes it more common in one sex. (17.4.16)
• Describe red-green colour blindness as an example of sex linkage. (17.4.17)
• Use genetic diagrams to predict the results of monohybrid crosses involving codominance or sex linkage, and calculate phenotypic ratios. (17.4.18)

Why two ratios come out of one square. The genotype ratio counts what is in each cell. The phenotype ratio counts what each cell would look like. Dominance is the translation between them, and it merges TT with the two Tt cells because all three contain at least one dominant allele. If the gene showed codominance instead — the subject of section Q — no merging would happen and both ratios would be 1 : 2 : 1.

• “Genes contain chromosomes.” Why wrong It inverts the containment chain. The gene is the smallest unit named here, not the largest. Say instead Chromosomes carry genes; a gene is a length of the DNA that makes up a chromosome.
• “Each chromosome carries one gene.” Why wrong A human chromosome carries many hundreds of genes along its length. Say instead A chromosome carries many genes, each at its own position along the DNA.
• “An allele is a different thing from a gene.” Why wrong It treats the two words as two objects. An allele is not beside a gene or inside one — it is a gene, in one of the forms that gene comes in. Say instead There is one gene for this feature, and it exists in two alternative forms, called alleles.
• “The gene travels to the ribosome.” It does not. That is the one thing bullet 1 exists to rule out.
• “mRNA makes the protein.” The ribosome assembles the protein; the mRNA carries the instructions that set the order.
• “The ribosome decides which amino acids to use.” It does not decide anything. The sequence of bases in the mRNA determines the sequence of amino acids, and the mRNA got that sequence from the gene.
• “Every cell in the body has a different genetic code.” Why wrong All the body cells of one organism come from one zygote by mitosis, which produces genetically identical cells. The genes are the same. Say instead Body cells are genetically identical; they differ in which genes are expressed.
• “A specialised cell has lost the genes it does not need.” Why wrong Nothing is lost. The unused genes are still there, and still pass to the daughter cells if that cell divides. Say instead The genes it does not need are present but not expressed.
• 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”. A parent has two alleles of the gene; each gamete carries one of them, which is why the offspring ends up with two again, one from each parent. 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.
• Replacing the counter is not what a parent does. Replacement keeps the proportions constant, which is right for a parent producing millions of gametes and wrong as a picture of any single gamete being used up.
• Counters are drawn by hand, not by chance alone. A counter can be recognised by touch, or the bag can be poorly shaken, so the draws stop being random — which is exactly the assumption the cross depends on. Identical counters and a shake between draws are the matched improvement.
• The model shows one gene. Real offspring differ in thousands of genes at once, and the counters cannot show that.
• “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.”
• “A gene codes for a characteristic.” The syllabus definition stops at protein, and deliberately: the protein has to do something in a cell before any characteristic appears. Write “A gene is a length of DNA that codes for a protein.”
• “An allele and a gene are two different things.” An allele is a gene, in one of its alternative forms. Two alleles of one gene sit at the same position on the two chromosomes of a pair and differ in their base sequence. Write “An allele is an alternative form of a gene.”
• “The shaded symbols must be the dominant phenotype.” Shading means whatever the key says it means, and a question is free to shade either phenotype. Write nothing until you have read the key.
• “The commoner phenotype in the family is the dominant one.” Counting symbols proves nothing — a family is far too small a sample. Dominance is settled by the pattern: unaffected parents with an affected child means the characteristic is recessive. Write the pattern argument, not a tally.
• “Every individual on the chart has a genotype you can work out.” An individual showing the dominant phenotype, with no affected parent and no offspring shown, could be homozygous or heterozygous. Write “DD or Dd — the pedigree does not show which”, and say why.
• “Dominant means common.” Dominant describes whether the allele is expressed when it is present, nothing else. How common a phenotype is depends on how common its allele is in the population, which is a separate question. Write “A dominant allele is expressed if it is present in the genotype.”
• “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.” The two chromosomes of a pair go into different gametes, so each gamete gets one allele of the gene and the offspring ends up with two again, one from each parent. 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.
• “The gene travels to the ribosome.” The gene stays in the nucleus — that is the first of the six bullets in statement 17.1.8, and it is the reason a messenger is needed at all. Write “An mRNA copy of the gene is made in the nucleus and moves to the cytoplasm.”
• “mRNA makes the protein.” The ribosome assembles the amino acids; the mRNA carries the sequence information that sets their order. Write “The mRNA passes through the ribosome, which assembles amino acids in the order the bases specify.”
• “Specialised cells have different genes.” They have the same genes, because they all descend from one zygote by mitosis. What differs is which genes are expressed. Write “All body cells contain the same genes; different genes are expressed in different cells.”
• “Meiosis is when the sperm and egg join.” That is fertilisation. Meiosis produces gametes; it does not join them. Meiosis halves the chromosome number and fertilisation restores it. Write “Meiosis is a reduction division involved in the production of gametes.”
• “Mitosis halves the chromosome number.” Mitosis maintains it: the chromosomes are replicated exactly beforehand, and the copies separate. Halving is what meiosis does. Write “Mitosis maintains the chromosome number and gives genetically identical cells.”
• “You should name the stages of mitosis.” Both 17.2.1 and 17.3.2 say in so many words that details of the stages are not required. Naming them earns nothing and uses the time the roles needed. Write the description and the four roles.
• “The Y chromosome carries the other allele.” It does not carry an allele of this gene at all, which is exactly why a male shows the recessive phenotype with only one recessive allele. Write the Y bare: XbY, never XbYB.
• “Colour blindness is passed from father to son.” A father gives his son a Y, not his X, so an X-linked allele cannot reach a son from his father. It reaches a son through his mother. Write “A father passes his X to his daughters and his Y to his sons.”
• “Females cannot be red-green colour blind.” They can — they need the recessive allele twice, which means a carrier or affected mother and an affected father. It is uncommon, not impossible. Write “A female needs two recessive alleles, so it is much less common in females than in males.”
• “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 in the heterozygous organism contribute to the phenotype.”
• “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 six-genotype table from section R.
• “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.” The first two entries in this group are Supplement; this one is Core (17.1.4).

• Where the rest of the old “inheritance” material went. If you are looking for variation, mutation, natural selection or artificial selection, they are not in this topic: 0610 places them in Topic 18, Variation and selection. The structure of the DNA molecule — the double helix and the base pairs — is in Topic 4, Biological molecules. Neither is taught here, and this chapter does not assume you have met them.
• What is not in Topic 17, although you may expect it to be. Variation, mutation, natural selection and artificial selection are Topic 18. The structure of DNA — the double helix and complementary base pairing — is Topic 4. Antibiotic resistance is Topic 15. None of them is taught here, and a Topic 17 answer that reaches for them has usually drifted away from the question.
• Core candidates: two places where the temptation is to read on. Sections F (haploid and diploid) and I (meiosis) feel foundational, because the words turn up everywhere in biology. In this syllabus they are Supplement, and you will not be asked for them on Paper 1 or Paper 3. Reading them does no harm and costs time you have not got; the Core account of a cross is complete without either.
• Paper 1 and Paper 3 cannot ask you about mitosis, meiosis, protein synthesis, ploidy, test crosses, codominance, ABO or sex linkage. Every one of those is a Supplement statement. If you are on the Core route and a past paper you are practising contains one, check which paper it came from — it will be Paper 2 or Paper 4.
• The cheapest correction in the topic. Writing the word expected in front of every ratio. It costs one word, and it is the difference between understanding genetics as probability and treating it as a promise — which is the difference the whole of 17.4.11 turns on.
• Two more for Extended candidates, both about stopping in the right place. Statement 17.1.8 says that details of transcription or translation are not required, and statements 17.2.1 and 17.3.2 say that details of the stages are not required. Writing about codons, tRNA, prophase or metaphase does not earn extra credit; it uses the time and the space that the required points needed.
• Where the DNA molecule itself is taught. This chapter needs only the sentence above: chromosomes are made of DNA, and DNA carries genes. The structure of the molecule — two strands, the double helix, and the pairing of the bases A with T and C with G — belongs to Chapter 4, Biological molecules, where it is Supplement material. Do not bring nucleotide chemistry into this chapter: statement 17.1.5 says in so many words that knowledge of the details of nucleotide structure is not required.
• Detailed stages are not required. Statement 17.2.1 carries the limit details of the stages of mitosis are not required in so many words. Prophase, metaphase, anaphase and telophase are not on this syllabus and naming them earns nothing. What is asked for is the description, the four roles, and the two statements about the chromosomes below. Spend your time on those.
• What meiosis does not do. Meiosis is not fertilisation. It produces gametes; it does not join them. Writing “meiosis is when the sperm and egg join” loses every mark in the question, because it names the wrong process entirely. Meiosis halves; fertilisation restores.
• Core candidates: this is where your route restarts. If you came straight here from section B, you have missed nothing you need. Sections C to J are all Supplement, and nothing below depends on any of them.
• Two definitions that look almost the same and are not. Compare 17.4.8 with 17.4.9 word by word. The dominant allele is expressed if it is present — one copy is enough. The recessive allele is expressed only if no dominant allele of the same gene is present — which, for a gene with two alleles, means two copies of the recessive one. Note the phrase of the gene: it is a dominant allele of the same gene that masks it, not a dominant allele of some other gene elsewhere.
• 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 without telling you outright, it will usually either use that word or describe a parent that must be homozygous recessive.
• Two statements, and they are not the same one. 17.4.11 asks you to use genetic diagrams to predict the results of monohybrid crosses and calculate phenotypic ratios, and it carries the limit limited to 1 : 1 and 3 : 1 ratios. 17.4.12 asks you to use Punnett squares in crosses which result in more than one genotype to work out and show the possible different genotypes. The first is about the ratio you end up with; the second is about showing the genotypes on the way. A question can ask for either, and an answer that gives only the phenotype ratio has not shown the genotypes.
• 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.
• The second of the two permitted ratios. Statement 17.4.11 is limited to 1 : 1 and 3 : 1 ratios. Section M produced the 3 : 1; this section produces the 1 : 1. Between them they are the only two monohybrid ratios a Core question can require, which is worth knowing when a cross you have worked comes out as something else and you start doubting the arithmetic. Not every cross gives a ratio at all — TT × tt gives one genotype and one phenotype, and that is a perfectly good answer.
• 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 the one thing to avoid is a capital paired with its own lower case: using A and a for two codominant alleles states something false about them before you have written a single line of the cross. Two forms avoid that, and both are accepted: one base letter with different superscripts, which is what the syllabus itself uses for ABO — IA and IB — or two different capitals, as in the figure above. Reproduce the syllabus's form for ABO, define whichever form you use at the top of the answer, and keep to it.
• Give the ratio by sex, not as a bare 3 : 1. Counting the four cells gives three offspring with normal colour vision to one who is colour blind, and that is arithmetically correct — but it hides the point of the question. The expected outcome is no colour-blind daughters and half the sons colour blind, and a sex-linkage question is asking for exactly that distinction. Say which sex, every time.
• Read this before you go looking for an experiment. Topic 17 contributes no experiment to the practical papers. The genetics entry on the official practical list — investigating continuous and discontinuous variation — belongs to Topic 18, not here. You cannot set up a monohybrid cross in a 75-minute examination, and Cambridge does not pretend otherwise. What Topic 17 does contribute is AO3 skill: handling results, judging evidence, and completing diagrams. That is what Paper 6 in particular asks of this topic, and it is what this section trains. None of it is a subject-content statement, and it is labelled as a practical skill rather than tagged as one.
• Supporting context, and why it is here. None of this is a numbered syllabus statement — you will not be asked to “define probability”. It is here because Core statement 17.4.11 asks you to predict the results of a cross, and every prediction in this chapter is a probability. Without this section, a 3 : 1 ratio reads as a promise about four offspring, and a data-response question of the kind “the student expected 3 : 1 but got 7 : 1; suggest why” has no answer. The mathematics is Core-level and both routes need it.
• The marks are spread down the rows, not concentrated in the square. A perfect Punnett square with no parental genotypes above it has skipped the steps that justify it. Section L sets the protocol out in full, row by row, on the figure.

• Inheritance can be assessed in any of the written papers, and each one puts a different shape of question on it. Knowing which shape you are looking at tells you how much to write before you have read a single word of the stem.
• Paper 1 and Paper 3 cannot ask you about mitosis, meiosis, protein synthesis, ploidy, test crosses, codominance, ABO or sex linkage. Every one of those is a Supplement statement. If you are on the Core route and a past paper you are practising contains one, check which paper it came from — it will be Paper 2 or Paper 4.
• A phrase, not a sentence. “State the genotype” wants Tt and nothing else. Writing a paragraph here costs time and earns nothing extra.
• Describe wants what happens. Interpret — the verb in 17.4.10 — wants you to read a supplied pedigree and say what it shows. Use a genetic diagram means every row of the protocol, not just the square.
• Explain wants why, link by link; an explanation of protein synthesis with no mention of the ribosome cannot score full marks. Suggest means the answer is not in the syllabus — apply a principle to an unfamiliar case. Calculate wants the working as well as the figure.
• When a question says “complete the genetic diagram” or “use a genetic diagram to show…”, it is asking for the whole layout, not for the square on its own. Each row is a separate step of the reasoning, so a Punnett square presented with no parental genotypes above it has left several of those steps unanswered, however neatly it is drawn. The rows are: