Hi Tara. Inheritance is the topic that most rewards being organised, and it is the one where the neatest student in the room beats the cleverest one. There are only about a dozen facts in it. What there is instead is a method, and the method has to be so worn-in that you can write it while thinking about something else.
Here is the shape. 17.1 is the vocabulary of the material itself — chromosome, gene, allele, haploid, diploid — plus how a gene actually gets a protein made and how sex is inherited. 17.2 and 17.3 are the two kinds of cell division, and the good news there is enormous: the named stages of both are not on your syllabus, so you never have to learn them. 17.4 is monohybrid crosses and the seven-line layout, which is the heart of the topic. 17.5 is the same layout with harder notation — codominance, blood groups and sex linkage. 17.6 is the checklist for the night before.
Three warnings that will each save you marks. First: an allele is a version of a gene, not a version of a chromosome, and getting that backwards spoils half a dozen answers. Second: mitosis gives genetically identical cells and meiosis gives genetically different ones — that one contrast is worth more marks across the paper than any other single fact. Third, and biggest: a correct ratio with no working scores one mark out of four. Write the layout. Every time. Even when they only ask for the ratio.
Four Words, Four Different Sizes
Almost every mark lost at the start of this topic comes from using one of four words where another belongs. They are not synonyms and they are not interchangeable. They are four different sizes of the same material.
- Chromosome — a structure in the nucleus, made of DNA. DNA carries genetic information in the form of genes.
- Gene — a length of DNA that codes for a protein. Learn that phrase exactly. Not “a piece of DNA that decides a feature”, not “a unit of inheritance”. It codes for a protein.
- Allele — an alternative form of a gene. B and b in Fig. 1.1 are two alleles of one gene, sitting at the same position on a pair of chromosomes.
The commonest wrong sentence in this whole topic is “an allele is a different type of chromosome”. It is not. An allele is a version of a gene. A chromosome carries thousands of genes; a gene has a small number of alleles — usually two.
Say it back to yourself as a size: chromosome > gene > allele. An allele is not smaller than a gene in length — it is the same length. It is smaller in the sense that it is one option out of several for that one gene.
Haploid and Diploid
Chromosomes come in matching pairs. Each pair carries the same genes in the same order — one chromosome of the pair came from the mother and one from the father. That is why you have two alleles of every gene.
a diploid nucleus contains two sets of chromosomes
The Inheritance of Sex
One of those 23 pairs is the pair of sex chromosomes. A female has XX. A male has XY. Everything about human sex inheritance follows from one asymmetry: a female can only put an X into an egg cell, while a male can put either an X or a Y into a sperm.
The ratio is 1 : 1, which means a 50% chance at each fertilisation. It does not mean that a family of four will contain two boys and two girls, and it does not mean that after three boys a girl is “due”. Every fertilisation is a fresh 50-50.
And be careful with the language about the father. It is true that the sex of the child is determined by which sperm fertilises the egg cell — but the father does not choose, decide or control anything. Write “the sperm that fertilises the egg cell carries either an X or a Y chromosome”, which is a mechanism, rather than “the father decides”, which is not.
From a Sequence of Bases to a Working Protein
You met the structure of DNA in Topic 4: two strands coiled into a double helix, held together by pairs of bases, with A always pairing with T and C always pairing with G. Here is what that sequence is for.
The sequence of bases in a gene determines the sequence of amino acids used to make a specific protein. Different sequences of amino acids give proteins different shapes, and the shape is what decides what the protein can do. That single chain — bases → amino acids → shape → function — is the whole of the Supplement content here, and it is worth being able to write out in one breath.
Learn the six steps as a list, because a question asking you to “explain how a protein is made” is marked point by point:
- the gene stays in the nucleus — it never leaves
- mRNA is a copy of a gene
- the mRNA is made in the nucleus and moves to the cytoplasm
- the mRNA passes through a ribosome
- the ribosome assembles amino acids into a protein
- the sequence of amino acids is determined by the sequence of bases in the mRNA
The mechanism that reads the bases and the machinery that carries the amino acids in are not required at IGCSE, and writing about them wins nothing while eating your time. Six steps, then stop.
The one error worth naming: the mRNA is a copy, so the gene itself never travels. An answer that says “the gene moves out of the nucleus to the ribosome” contradicts step one and usually loses the whole mark.
What Those Proteins Actually Do
DNA controls how a cell behaves by controlling the production of proteins. Cambridge names three kinds that matter, and each one connects to a topic you have already done:
- enzymes — every reaction rate in the cell, from digestion to respiration, is set by an enzyme, and every enzyme is a protein with a specific shape (Topic 5)
- membrane carriers — the protein carriers that move molecules and ions across a membrane in active transport (Topic 3)
- receptors for neurotransmitters — the proteins at a synapse that a neurotransmitter fits into (Topic 14)
So “a gene codes for a protein” is not a small statement. It means a gene can set an enzyme, a transport route or a nerve connection. When a question asks how a change in DNA could change an organism, that chain is the answer.
Same Genes, Different Cells
Most body cells contain the same genes, because they all came by mitosis from the same zygote. A neurone and a cell in your pancreas hold the identical set. They differ because many genes are not expressed — a cell only makes the proteins it needs.
What Mitosis Is, and What It Is Not
Here is the definition, and then the good news.
genetically identical cells
Two facts do the work in almost every question:
- Exact replication of the chromosomes happens before mitosis. Every chromosome is copied first. That is why there is enough material for two full cells.
- During mitosis the copies separate, maintaining the chromosome number in each daughter cell. A cell with 46 gives two cells with 46. Nothing is halved.
Cambridge marks “genetically identical” and refuses “the same”, “similar” and “alike”. It is one word and it is free. Write it.
The mirror-image word for meiosis is “genetically different”. If you can attach the right one of those two phrases to the right division, you have the single most heavily examined fact in Topic 17.
What Mitosis Is For
Four uses, and you should be able to give an example of each because questions like to put you in an unfamiliar situation and ask which one is happening:
| Role of mitosis | What it looks like in real life |
|---|---|
| Growth | A seedling gets taller; a child gets taller. More cells, all carrying the same genes. |
| Repair of damaged tissues | A cut in the skin closes over. New cells must be identical to the ones that were lost, or the repair would not match. |
| Replacement of cells | Red blood cells last about 120 days and the lining of the small intestine is replaced constantly. This goes on in a healthy adult who is not growing at all. |
| Asexual reproduction | A strawberry runner, a potato tuber, a bacterium dividing. This is the link back to Topic 16 — asexual offspring are genetically identical because they are produced by mitosis. |
They get confused because both make new cells. Growth means the organism gets bigger — a permanent increase in size and dry mass, which is the Topic 1 definition. Replacement means keeping the number the same while worn-out cells are lost. An adult who has stopped growing still runs mitosis constantly, and a question about the lining of the gut or about blood cells is asking about replacement, not growth.
Stem Cells
daughter cells that can become specialised for specific functions
Put this next to what you met in 17.1 and the topic starts to lock together. A stem cell and a fully specialised neurone contain the same genes. They look and behave completely differently because different genes are expressed in each. Specialisation is not a matter of losing genes; it is a matter of switching most of them off.
“Stem cells are only found in embryos.” They are not. Adults carry stem cells in bone marrow, in the base of the skin and in the lining of the gut, which is exactly why those tissues can be replaced throughout life.
“Stem cells divide by meiosis because they can turn into anything.” They divide by mitosis. Meiosis produces gametes and nothing else on this syllabus. If a question describes cells that keep dividing to give more cells of the same kind, it is mitosis, however remarkable the cells are.
The Smallest Sub-Topic on the Syllabus
Cambridge gives meiosis three statements. That is genuinely all of it, and the stages are excluded here exactly as they are for mitosis.
halved from diploid to haploid, resulting in genetically different cells
Why the Halving Has to Happen
This is the question that separates a 2-mark answer from a 4-mark one, so think it through once properly rather than memorising a sentence.
Fertilisation is the fusion of the nuclei of two gametes. Whatever each gamete brings, the zygote ends up with the sum of the two. If gametes were diploid — 46 chromosomes each — the zygote would have 92. Its own gametes would have 92, so its children would have 184, and the number would double every single generation. That is impossible, so something has to halve it, and that something is meiosis.
23 + 23 = 46. The gametes are haploid so that fertilisation can restore the diploid number, and the number stays constant from generation to generation. A question worth 3 or 4 marks wants exactly that chain: gametes are haploid → two haploid nuclei fuse at fertilisation → the zygote is diploid → so the chromosome number of the species is kept constant.
Genetically Different — and Why That Matters
The four cells produced by meiosis are genetically different from one another and from the parent cell. That single word is where variation in a species comes from, and it is the reason sexual reproduction produces offspring that are not copies of their parents.
You are not asked how meiosis produces that difference — the mechanisms are not on this syllabus and writing about them earns nothing. You are asked to know that it does, and to use it: two children of the same parents are different because each was made from a different pair of gametes, and every gamete is genetically different from every other.
Papers ask for this again and again, sometimes as a table to complete and sometimes as a “state two differences” question. Give the number of cells, the chromosome number and whether the cells are identical — three differences, three marks.
| Mitosis | Meiosis | |
|---|---|---|
| Cells produced | 2 | 4 |
| Chromosome number | maintained (diploid → diploid) | halved (diploid → haploid) |
| Daughter cells | genetically identical | genetically different |
| Used for | growth, repair, replacement of cells, asexual reproduction | production of gametes |
| Where | throughout the body | in the reproductive organs only |
“Meiosis makes gametes” is worth one mark at most and often none, because it does not say what makes it a reduction division. Add the number: “meiosis halves the chromosome number from diploid to haploid, producing genetically different gametes”. Same length, three times the marks.
Cambridge’s sub-topic 17.4 is by far the biggest thing in this topic, so it is taught here in two sections: this one covers the vocabulary, the layout and monohybrid crosses, and section 17.5 covers codominance, ABO blood groups and sex linkage.
This Section Is Different From Every Other Section on This Site
Nearly all of IGCSE Biology is recall. This is not. A genetic cross is a procedure, and Cambridge awards marks for the layout itself — for writing the parental phenotypes down, for writing the genotypes down, for circling the gametes — before you have got anywhere near an answer. Students who understand genetics perfectly well lose four or five marks a paper by writing the working out in a heap.
So we are going to treat it exactly like a maths method: you will see it worked in full, then with one step missing, then with two steps missing, then you will do one from nothing. Do it in that order. The aim is not to understand it — you probably will after five minutes — the aim is to make the layout automatic, so that under time pressure your hand writes it without asking you.
The Vocabulary First, Because the Layout Uses All of It
| Word | What Cambridge means by it |
|---|---|
| inheritance | the transmission of genetic information from generation to generation |
| genotype | the genetic make-up of an organism, in terms of the alleles present — the letters, e.g. Tt |
| phenotype | the observable features of an organism — the words, e.g. tall |
| homozygous | having two identical alleles of a gene, e.g. TT or tt |
| heterozygous | having two different alleles of a gene, e.g. Tt |
| pure-breeding | two identical homozygous individuals bred together are pure-breeding. A heterozygous individual will not be pure-breeding. |
| dominant | an allele that is expressed if it is present in the genotype |
| recessive | an allele that is only expressed when no dominant allele of that gene is present |
A recessive allele is not weaker, damaged, rarer or worse. It is simply not expressed when a dominant allele is there too. That is the whole of it. Write “expressed”, never “stronger” or “overpowers” — those words are refused by mark schemes because they describe a fight that is not happening.
And plenty of recessive alleles are extremely common. In some populations the recessive allele is the one most people carry. Frequency has nothing to do with dominance.
If you can photograph it, it is the phenotype: tall, black, round, group A. If it is written with letters, it is the genotype: Tt, bb, IAIO.
The link between them runs one way and one way only. From a genotype you can always work out the phenotype. From a phenotype you often cannot work out the genotype — a black rabbit is either BB or Bb and no amount of looking will tell you which. That gap is what the whole of the test cross exists to close.
Use the capital and the lower case of the same letter — Bb, never Bd. The capital is always the dominant allele.
Pick a letter whose two cases look obviously different when you write quickly. Avoid Ss, Cc, Oo, Ww and Zz, because a hurried capital S and a hurried small s are the same shape and an examiner cannot award a mark for a letter they cannot read. B, T, R, G, N, E and H are all safe.
The Layout: Seven Lines, Always the Same Seven
- Parental phenotypes — the words. tall × tall
- Parental genotypes — the letters. Tt × Tt
- Gametes — one allele each, and circle them. Cambridge asks for circles by name.
- The Punnett square — gametes on the outside, offspring inside
- Offspring genotypes — read the four boxes out
- Offspring phenotypes — turn each genotype into a word
- The ratio — in the simplest whole numbers
Both parents are heterozygous tall pea plants. Give the phenotype ratio of the offspring.
T is the allele for tall and t is the allele for dwarf; tall is dominant. Watch what a complete answer looks like before you try one.
Notice two things. First, the gametes come straight off the parental genotype — a Tt parent makes T gametes and t gametes, one allele in each, because meiosis halves the number. Second, the phenotype line is not automatic: TT and Tt are both tall, which is why three boxes out of four give tall.
In rabbits, black coat (B) is dominant to brown coat (b). Two heterozygous black rabbits are crossed. Give the phenotype ratio of the offspring.
Count the phenotype line, not the genotype line. Three of the four boxes contain at least one B, so three are black; one box is bb, so one is brown. The ratio is 3 black : 1 brown.
A heterozygous black rabbit is crossed with a brown rabbit. Give the offspring phenotypes and the ratio.
Careful here. The temptation after two 3 : 1 crosses is to write 3 : 1 again without looking. Read the boxes.
The brown parent is bb, so every gamete it makes carries b. That means every offspring receives a b, and whether it is black or brown depends entirely on what the other parent sent. Two boxes are Bb (black) and two are bb (brown), so the answer is 1 black : 1 brown — a 1 : 1 ratio, not 3 : 1.
With one dominant and one recessive allele, the phenotype ratios you will be asked to calculate are 3 : 1 (two heterozygous parents) and 1 : 1 (a heterozygous parent and a homozygous recessive parent). If one parent is homozygous dominant, every offspring shows the dominant phenotype: write “all black”, because there is no ratio to calculate. A result such as 3 : 2 is impossible from four boxes, so it means a slip in the square.
In pea plants, tall (T) is dominant to dwarf (t). A heterozygous tall plant is crossed with a dwarf plant. Write out all seven lines on paper first, then fill in these five boxes.
Case matters in the first four boxes: a capital T and a small t are different alleles, so Tt and TT are not the same answer.
A 3 : 1 ratio means that each offspring has a 3 in 4 chance of showing the dominant phenotype. It does not mean that a litter of four will contain exactly three black and one brown, and a litter of four that comes out 4 : 0 does not disprove anything.
This is examined directly. A question will give you a real set of offspring — say 43 black and 17 brown — and ask what ratio that suggests. The answer is that it is approximately 3 : 1, and the reason it is not exact is that fertilisation is random, so the actual numbers vary by chance, especially when the sample is small.
The Test Cross
Here is the problem the test cross solves. You have a black rabbit. Black is dominant, so its genotype is either BB or Bb, and looking at it will never tell you which. So you breed it with a homozygous recessive individual — a brown rabbit, bb — and let the offspring answer the question for you.
The logic is worth stating properly, because “you cross it with a recessive one” is worth one mark and the reasoning is worth three:
- the homozygous recessive parent can only produce gametes carrying the recessive allele
- so every offspring receives a recessive allele from that parent
- therefore the phenotype of each offspring is decided entirely by what the unknown parent contributed — nothing can be masked
- any offspring showing the recessive phenotype proves the unknown parent was heterozygous
- if all the offspring show the dominant phenotype, the unknown parent was probably homozygous dominant — and the more offspring there are, the safer that conclusion is
A single brown offspring proves Bb. No brown offspring never proves BB, because a Bb parent could produce four black offspring by chance — that has a probability of one in sixteen, which is not at all unlikely. The word to use is “suggests”, and the improvement to suggest is a larger number of offspring.
Pedigree Diagrams
A pedigree is a family tree with the phenotypes marked on. Cambridge asks you to interpret them, which in practice means three things: say whether the allele is dominant or recessive, work out particular genotypes, and say when you cannot.
Find two unaffected parents with an affected child. If the child shows a feature that neither parent shows, the allele for it must be recessive — both parents were carrying it hidden, so both must be heterozygous. In Fig. 4.6 that happens twice: I-1 and I-2 have a brown daughter, and II-3 and II-4 have a brown son.
The reverse test also works: if an affected child has two affected parents and there is an unaffected child in that family, the allele is dominant. Look for the generation where the pattern breaks.
Then work outwards from the individuals you are certain about:
- anyone showing the recessive phenotype is homozygous recessive — there is no other possibility, so start there
- anyone showing the dominant phenotype who has a recessive-phenotype parent or a recessive-phenotype child must be heterozygous, because they had to pass on or receive a recessive allele
- anyone else showing the dominant phenotype could be homozygous dominant or heterozygous — and the right answer is to write “BB or Bb”, not to guess one
Students lose marks by picking one when the pedigree does not decide it. “BB or Bb” is a complete, creditworthy answer — the mark scheme prints exactly that. Guessing BB when it could be either is simply wrong, and it is wrong in a way that suggests you did not understand the question.
This is the second half of Cambridge’s sub-topic 17.4, and all of it is Supplement material. The layout is exactly the same seven lines you drilled in 17.4 — only the notation and the rules for reading the boxes change.
Codominance
heterozygous organism contribute to the phenotype
The standard example is coat colour in cattle. A homozygous red bull crossed with a homozygous white cow gives calves that are roan — and if you look closely at a roan animal you find it is covered in red hairs and white hairs mixed together. It is not pink. That is the picture to hold: both alleles are being expressed, in different hairs, side by side.
With ordinary dominance you write B and b, and the case tells you which is dominant. With codominance neither allele is recessive, so writing one in lower case would be a lie. Instead you use one capital letter for the gene with a superscript for each allele: CR for red and CW for white.
Write the superscripts properly and small. CRCW is a genotype; CRCW is four alleles and means nothing.
Two roan cattle are crossed. Give the phenotype ratio of the offspring.
This is the thing to take away from codominance: with ordinary dominance a 1 : 2 : 1 genotype ratio collapses into a 3 : 1 phenotype ratio, because the heterozygote looks like the homozygous dominant. With codominance it does not collapse, because the heterozygote looks like neither. The genotype ratio and the phenotype ratio are both 1 : 2 : 1.
A homozygous red bull is crossed with a homozygous white cow. Give the phenotypes of the calves.
Every box is CRCW, so every calf is roan — there is no ratio to give beyond “all roan”. Each parent is homozygous, so each can only make one kind of gamete, and there is only one possible combination.
This is the cross that makes people reach for the wrong word. The calves are not a blend and they are not an average. Both alleles are expressed — there are red hairs and there are white hairs.
ABO Blood Groups
ABO is codominance with a third allele added, and it is examined more often than any other named cross on this syllabus. Three alleles exist — IA, IB and IO — but any one person still has only two of them, one on each chromosome of the pair.
The rules are short:
- IA and IB are codominant with each other — if you have both, you express both, and your blood group is AB
- IO is recessive to both of them — so it only shows when there is nothing else
| Phenotype (blood group) | Possible genotypes |
|---|---|
| A | IAIA or IAIO |
| B | IBIB or IBIO |
| AB | IAIB — only one possibility |
| O | IOIO — only one possibility |
Group AB and group O each have exactly one possible genotype, so if a question tells you someone is AB or O you know their genotype instantly and for certain. Groups A and B are the ambiguous ones — each has two possibilities, and you must write both unless the question gives you something that rules one out.
That asymmetry is what nearly every ABO question is built on. Start with the AB and the O individuals; they are your fixed points, and everyone else is worked out from them.
A man of blood group A, genotype IAIO, and a woman of blood group B, genotype IBIO, have children. What blood groups are possible?
Read the four boxes as phenotypes and the surprise appears: two parents of groups A and B can have a child of group O, and a child of group AB. Neither parent has either of those groups. This is the single most examined consequence of ABO inheritance, so make sure it does not surprise you in an exam.
A group AB parent and a group O parent have children. Give the offspring phenotypes and the ratio.
Two boxes are IAIO, which is group A, and two are IBIO, which is group B. The ratio is 1 A : 1 B.
Look at what has happened: neither child can be group AB like one parent, and none can be group O like the other. Every child is a group the parents do not have. Questions love this cross because the intuitive answer — “half AB and half O” — is completely wrong.
A woman of blood group AB has a child with a man of blood group O. Their first child is group A. They are expecting a second child. Answer these four questions about the second child.
The first child being group A tells you nothing at all about the second: each fertilisation is independent.
Sex Linkage
responsible is located on a sex chromosome
The gene for red-green colour vision sits on the X chromosome. The Y chromosome is much smaller and carries no allele for this gene at all. Everything else follows from that one fact.
Write the sex chromosome as a capital X and hang the allele on it as a superscript: XB for the normal allele and Xb for the colour-blind allele. The Y is written on its own — just Y, with no superscript, because there is nothing there.
Get this right and the marks follow. Writing Bb for a sex-linked cross throws away the whole point, because it hides which chromosome each allele is on, and a mark scheme that asks for XBXb will not accept Bb. Writing XBYb is worse: it puts an allele on the Y chromosome, which is exactly the thing that is not there.
| Genotype | Phenotype |
|---|---|
| XBXB | female, normal vision |
| XBXb | female, normal vision — a carrier |
| XbXb | female, colour blind |
| XBY | male, normal vision |
| XbY | male, colour blind — one allele is enough |
Now look down that table and the reason the condition is commoner in males is sitting there. A female needs two copies of the colour-blind allele to be colour blind, because a single XB on her other X chromosome is enough to give her normal vision. A male has only one X, so a single Xb is expressed — there is no second X to carry a dominant allele that could mask it, and there is nothing on his Y.
A male is either XBY or XbY. He cannot be a carrier, because he has no second X to hide the allele on. If he has the allele, he is colour blind (XbY); if he does not, he has normal vision (XBY). “Carrier” is a word for females only (XBXb).
And the second half of the same idea: a father gives his Y to his sons and his X to his daughters. So a son never gets a sex-linked allele from his father. A colour-blind son always got the allele from his mother. That is the sentence to write.
A woman who is a carrier for red-green colour blindness has children with a man who has normal vision. What proportion of their children will be colour blind?
Read it carefully: none of the daughters is colour blind, though half of them are carriers, and half of the sons are colour blind. The question asks about all the children, so the answer is 1 in 4 (25%). If a question asks about the sons only, count only the two male boxes: 1 in 2. Answer exactly what was asked.
A carrier woman has children with a colour-blind man. Give the ratio of the offspring phenotypes.
1 carrier daughter : 1 colour-blind daughter : 1 normal son : 1 colour-blind son — a 1 : 1 : 1 : 1 ratio. Half the children of each sex are affected.
This is the cross that shows a colour-blind daughter is perfectly possible. It needs an Xb from each parent, which means a colour-blind father and a mother who is at least a carrier. If a pedigree shows a colour-blind female, check her father: he must be colour blind too.
A colour-blind man has children with a woman who is homozygous for normal vision. Write the seven-line layout on paper, then answer these.
If you answered that the sons are colour blind, you have inherited the allele down the wrong line: a son gets his father’s Y, never his X.
Haemophilia: a carrier woman has children with a man who has haemophilia
Exam papers often use a sex-linked condition you have not studied. You do not need to know anything about it: the question tells you it is sex-linked and recessive, and the method is exactly the one you used for colour blindness. Haemophilia is a common one: the blood does not clot properly. The allele is on the X chromosome and the Y carries no allele for it. Use the letter the question gives you, as a superscript on the X: XH (normal clotting) and Xh (haemophilia).
Now read the probability off the square, and read the question twice. Count only the boxes the question is about.
- “Probability that their next child has haemophilia”: 2 of the 4 boxes, so 1/2 (0.5, 50%).
- “Probability that their next child is a son with haemophilia”: 1 of the 4 boxes, so 1/4 (25%). Count all four boxes, because a daughter is one of the possible outcomes.
- “Probability that a son of theirs has haemophilia” (the child is already known to be a boy): look only at the 2 male boxes; 1 is affected, so 1/2.
- “Probability that a daughter is a carrier”: 1 of the 2 female boxes, so 1/2. (In this cross the other daughter has haemophilia, because her father gave her his Xh.)
Every fertilisation is independent: the answer for the next child does not change because of the children already born.
Working backwards from a family
Two parents with normal clotting have a son with haemophilia. The son is XhY. His Y came from his father, so his Xh came from his mother. She has normal clotting, so she is XHXh, a carrier. The father is XHY.
So the chance that their next son has haemophilia is 1/2; the chance that their next child is a son with haemophilia is 1/4; none of their daughters can have haemophilia, but each daughter has a 1/2 chance of being a carrier.
Three signs, and you want at least two of them before you commit: many more affected males than females; the condition skips a generation, passing through an unaffected mother; and no affected father passes it to a son.
The clinching argument runs the other way. If a pedigree shows an affected female whose father is unaffected, the feature cannot be X-linked recessive — she would have had to receive an Xb from him. That single observation rules sex linkage out, and it is worth two marks when you can say why.
The Marks Are in the Working
Say this to yourself once more before the exam, because it is worth more marks than anything else on this page: in a genetics question, the layout is the answer. A correct ratio with no working scores one mark out of four. An incorrect ratio with all seven lines correctly set out usually scores three.
So even when a question only asks for the ratio, write the layout. It takes forty seconds and it is insurance: if you slip in the last line, the marker can still see the parental genotypes, the gametes and the square, and can still award them.
Before you leave any cross question, run down this list. 1. Are the parental phenotypes there, in words? 2. Are the parental genotypes there, two letters each? 3. Are the gametes circled? 4. Does the Punnett square have the gametes on the outside and offspring inside? 5. Are the offspring genotypes written out? 6. Are the offspring phenotypes written as words? 7. Is the ratio in the simplest whole numbers?
If you have a spare minute at the end of the paper, go back and check number 3 and number 6 first — the circles and the phenotype words are the two lines students leave out most often, and they are two of the easiest marks on the paper.
Reading the Question: Which Kind of Cross Is This?
You can tell within ten seconds which of five kinds of cross you are being asked for, and the kind tells you the notation. Get this wrong and every line afterwards is wrong.
| What the question mentions | What kind of cross | Notation to use |
|---|---|---|
| one gene, one feature, a dominant and a recessive | monohybrid | B and b |
| an individual of unknown genotype, and you are asked how to find it out | test cross | B? × bb |
| a third phenotype that is neither parent, such as roan | codominance | CR and CW |
| blood groups A, B, AB, O | ABO — codominance with three alleles | IA, IB, IO |
| a condition that affects mostly males, or the word carrier | sex linkage | XB, Xb and Y |
A monohybrid cross with one dominant and one recessive allele gives 3 : 1 (two heterozygous parents) or 1 : 1 (a heterozygous parent and a homozygous recessive parent). If one parent is homozygous dominant, every offspring shows the dominant phenotype: write “all tall”, because there is no ratio to calculate. If your arithmetic produces 2 : 2, simplify it: that is 1 : 1. If it produces something like 3 : 2, you have made a mistake in the square, so go back and check that each gamete appears in exactly two boxes.
Codominance adds 1 : 2 : 1, and sex linkage and ABO often give 1 : 1 : 1 : 1. The numbers from a 2 × 2 square always add up to 4 before you simplify, so a ratio such as 3 : 2 is a signal to check, not a discovery.
The Words That Score, and the Words That Do Not
| Do not write | Write | Why it matters |
|---|---|---|
| “the dominant allele is stronger” | “the dominant allele is expressed when it is present” | Mark schemes reject “stronger”, “overpowers” and “beats”. Nothing is competing. |
| “an allele is a type of chromosome” | “an allele is an alternative form of a gene” | The commonest definition error in the topic. |
| “a gene decides a feature” | “a gene is a length of DNA that codes for a protein” | The syllabus definition is about proteins, not features. |
| “mitosis makes identical cells” | “mitosis makes genetically identical cells” | One missing word, one lost mark. Every time. |
| “meiosis makes gametes” | “meiosis halves the chromosome number from diploid to haploid, producing genetically different gametes” | The short version gives one mark out of three. |
| “the sperm is half a cell” | “the sperm has a haploid nucleus” | Haploid and diploid describe nuclei, not cells. |
| “3 : 1 means three out of every four” | “each offspring has a 3 in 4 chance” | A ratio is a probability; small samples vary because fertilisation is random. |
| “he is a carrier” (of a sex-linked feature) | “there is no such thing as a male carrier” | A male has one X, so he expresses whatever is on it. |
| “heterozygous means you carry a disease” | “heterozygous means two different alleles” | Heterozygous is a statement about letters, not about health. |
| “the father decides the sex” | “the sex depends on whether the sperm that fertilises the egg cell carries X or Y” | Describe the mechanism, not an intention. |
Command Words in This Topic
- State / name — one word or one phrase. Do not explain. “Name the type of cell division that produces gametes.” → Meiosis. Full stop.
- Describe — say what happens, in order, without saying why. “Describe mitosis” wants the definition plus the replication and the maintained number.
- Explain — say why. Every marking point needs a because. “Explain why colour blindness is more common in males” needs the one-X reason, not the observation.
- Use a genetic diagram to… — this is an instruction, and the diagram itself carries most of the marks. Draw all seven lines even if the question only asks for the probability at the end.
- Suggest — you are being asked to apply what you know to something unfamiliar. There is often more than one creditworthy answer; give the one that uses the biology in the stem.
Give it as a fraction, a percentage or a ratio — all three are accepted — but make sure you are answering the right question. “What proportion of the children?” and “what proportion of the sons?” have different answers in every sex-linked cross, and the difference is a factor of two.
Read the question twice on this one specifically. In a carrier mother × normal father cross, 1 in 4 of the children are colour blind but 1 in 2 of the sons are. Both statements are true and only one of them answers what was asked.
Where the Time Goes
Paper 4 gives you 75 minutes for 80 marks, so roughly a minute a mark. A full seven-line cross takes about ninety seconds once it is automatic, which is why the drilling in 17.4 and 17.5 matters: if you have to think about the layout, the same cross takes four minutes and you have lost three somewhere else.
Two habits worth having. First, when a question gives you a pedigree, write the genotypes onto the diagram itself as you deduce them, starting with the individuals showing the recessive phenotype. It stops you holding four things in your head at once. Second, when a question gives you real offspring numbers, divide by the smallest to see the ratio — 43 and 17 divided by 17 gives about 2.5 and 1, which is near enough to 3 : 1 to say so.
Go back to the gametes. Almost every wrong answer in this topic comes from a gamete line that does not match the parental genotype. A homozygous parent makes only one kind of gamete; a heterozygous parent makes two. If you have written two different gametes under a parent whose genotype is bb, that is your mistake, and fixing it usually fixes everything below it.