Biology · 17 Inheritance

17.1 Chromosomes, genes and proteins

Before you start

Animal and plant cells have a nucleus, cytoplasm and cell membrane, and ribosomes are found in the cytoplasm.
Enzymes are proteins; each has an active site with a shape complementary to its substrate.
Active transport moves substances across a membrane using protein carriers.
Mitosis produces genetically identical cells, and gametes are the sex cells that fuse at fertilisation.
Probability written as a fraction, a ratio or a percentage — 1 in 2, 1:1, 50%.

Syllabus objectives covered (12)

State that chromosomes are made of DNA, which contains genetic information in the form of genes
Define a gene as a length of DNA that codes for a protein
Define an allele as an alternative form of a gene
Describe the inheritance of sex in humans with reference to X and Y chromosomes
State that the sequence of bases in a gene determines the sequence of amino acids used to make a specific protein (knowledge of the details of nucleotide structure is not required)
Explain that different sequences of amino acids give different shapes to protein molecules
Explain that DNA controls cell function by controlling the production of proteins, including enzymes, membrane carriers and receptors for neurotransmitters
Explain how a protein is made, limited to: • the gene coding for the protein remains in the nucleus • messenger RNA (mRNA) is a copy of a gene • mRNA molecules are made in the nucleus and move to the cytoplasm • the mRNA passes through ribosomes • the ribosome assembles amino acids into protein molecules • the specific sequence of amino acids is determined by the sequence of bases in the mRNA (knowledge of the details of transcription or translation is not required)
Explain that most body cells in an organism contain the same genes, but many genes in a particular cell are not expressed because the cell only makes the specific proteins it needs
Describe a haploid nucleus as a nucleus containing a single set of chromosomes
Describe a diploid nucleus as a nucleus containing two sets of chromosomes
State that in a diploid cell, there is a pair of each type of chromosome and in a human diploid cell there are 23 pairs

Part 1 of 8

The order of the words: nucleus, chromosome, DNA, gene

Everything in the rest of this unit — mitosis, meiosis, monohybrid crosses, variation — is written in four words. If those four are in the wrong order in your head, correct biology comes out as a wrong sentence. So get the order first.
Start at the top. The nucleus of a cell contains chromosomes.
A chromosome is made of DNA. That is the whole of the first half of the first objective: chromosomes are made of DNA.
The DNA carries genetic information — the instructions for building and running the organism — and that information is stored in the form of genes.
A gene is a length of DNA that codes for a protein. Learn that sentence word for word; it is a one-mark Define and the mark is for those words.
Now hold the whole thing as one picture of decreasing size: a nucleus contains chromosomes; a chromosome is one very long DNA molecule; a gene is a short length of that same molecule; and each gene codes for one protein.
Notice the word 'length'. A gene is not a separate object attached to a chromosome, like a bead on a string. It is a section of the DNA itself — which is why one chromosome carries many genes.
Two wrong versions to strike out now. 'Chromosomes are made of genes' — no, they are made of DNA, and genes are lengths of that DNA. 'DNA is made of genes' — that reverses the relationship: genes are made of DNA, not the other way round.
And 'codes for' is doing real work. It means the information in that length of DNA determines which protein gets made. A gene does not contain a protein, and it is not made of protein.

Part 2 of 8

Gene and allele: the distinction that decides your marks in 17.4

An allele is an alternative form of a gene. Again, learn the phrase: alternative form. Not a different gene — a different version of the same gene.
Here is the shape of it. The gene is the question the DNA asks: 'which protein is made at this position?' The alleles are the different available answers to that one question.
Where the versions come from: a gene sits at the same position on the same chromosome in every member of a species, but the sequence of bases inside it can differ slightly from person to person. A slightly different sequence codes for a slightly different protein — and that is an allele.
Now the structural fact that makes alleles matter. Your chromosomes come in pairs, so for most genes you carry two copies — one on each chromosome of the pair. Those two copies may be the same allele, or two different alleles.
One gene, one position, two chromosomes — so two alleles, which may match or differ.
That single sentence is the foundation of the whole of monohybrid inheritance: dominant and recessive, homozygous and heterozygous, genotype and phenotype all describe which two alleles of a gene an organism has.
So build the habit now. When you mean the length of DNA that codes for a protein, say gene. When you mean one particular version of it, say allele.
Test yourself on this sentence: 'he inherited the gene for cystic fibrosis'. It is wrong. Everybody has that gene — it codes for a membrane transport protein. What he inherited was a particular allele of it.
This is not pedantry, it is arithmetic on your marks. Mark schemes in 17.4 print 'allele' as the required word, and 'the dominant gene' does not score where 'the dominant allele' was wanted. Candidates lose marks on this every session. Fix the habit here, six weeks before you need it.
One piece of notation, so it is not a surprise later. Alleles are written as letters — you will use T and t for tall and short. A gene can have more than two alleles: in 17.4 you meet the ABO blood group gene, whose three alleles are written $I^{A}$ , $I^{B}$ and $I^{o}$ . The A, B and o are written raised, and that last one is a lower-case letter o, not a zero and not a capital O. You do not need it yet; you do need to write it correctly when you get there.

Part 3 of 8

One set, two sets: haploid, diploid and 23 pairs

A set of chromosomes means one of each type. Get that idea straight and both definitions follow from it.
A haploid nucleus is a nucleus containing a single set of chromosomes. A diploid nucleus is a nucleus containing two sets of chromosomes.
Put human numbers on it. One set is 23 chromosomes. A diploid human nucleus contains two sets, so 46 chromosomes — and because the two sets match up type by type, that is 23 pairs.
The pairing is the useful idea, not the number 46. In a diploid cell there is a pair of each type of chromosome: one of each pair came from the mother, one from the father, and the two carry the same genes in the same order.
Which is exactly why you have two copies of every gene, and therefore two alleles of it. The last section and this one are the same fact seen from two directions.
Which cells are which: body cells are diploid, gametes (sperm and egg) are haploid with 23 chromosomes each. At fertilisation two haploid nuclei fuse, restoring the diploid number of 46.
Now a word to be careful with. Both objectives say nucleus, not cell — deliberately. A mature red blood cell has no nucleus at all, so it cannot be called diploid. Saying 'nucleus' is never wrong; saying 'cell' occasionally is.
And do not define haploid as 'half the number of chromosomes' and stop. Half of what? The definition that scores names the set: a single set of chromosomes. Keep 'half the diploid number' as a second sentence, not the first.

Part 4 of 8

The inheritance of sex: X and Y

Of the 23 pairs in a human diploid cell, 22 pairs look the same in everyone. The 23rd pair are the sex chromosomes, and they determine whether the person is female or male.
A female has two X chromosomes: XX. A male has one X and one Y: XY.
Gametes are haploid, so each gamete receives one chromosome from each pair — including one of the sex chromosomes.
Follow that through for the mother. She is XX, so every egg cell she makes carries an X. She has nothing else to give.
Now the father. He is XY, so half of his sperm carry an X and half carry a Y — in roughly equal numbers.
So the sperm decides the sex of the child. An X sperm fertilising the egg gives XX, a female; a Y sperm gives XY, a male.
Set the genetic diagram out in the order examiners mark it. First line: parents' sex chromosomes, XX (mother) and XY (father). Second line: gametes — X and X from the mother, X and Y from the father. Third line: the offspring from every combination — XX, XY, XX, XY.
If you prefer a Punnett square, put the mother's gametes X and X down the side and the father's gametes X and Y across the top. The four boxes then read XX, XY, XX and XY.
Read the result off: two XX and two XY, so a ratio of 1 female : 1 male, and a 50% (1 in 2) chance of a boy at each fertilisation. Say which way round the ratio goes — a bare '1:1' with nothing labelled is not an answer.
The trap, and it is a favourite: each fertilisation is independent. A couple who already have three daughters still have a 50% chance of a boy next time. The eggs do not remember, and the previous fertilisations do not change which sperm arrives first.
Two things that lose marks in genetic diagrams: not labelling which line is parents and which is gametes, and writing the chromosomes carelessly — a lower-case x is not the same symbol as X. Label the lines and write the letters as capitals.

Part 5 of 8

Bases and amino acids: what a gene actually specifies

DNA contains bases, arranged in a particular order along the molecule. That order is the information — nothing else about the molecule carries the message.
Proteins are built from amino acids joined end to end in a long chain. Which protein you get depends on which amino acids are used and in what order.
The rule joining the two is the objective, and it is worth having as one sentence: the sequence of bases in a gene determines the sequence of amino acids used to make a specific protein.
Note what you are explicitly not required to know: the objective ends 'knowledge of the details of nucleotide structure is not required'. You do not need the names of the bases, the pairing rules, or how many bases specify one amino acid. Do not spend revision time there.
Now the vocabulary discipline, because this is where answers fall apart. DNA is a sequence of bases. A protein is a sequence of amino acids. Writing 'DNA is made of amino acids', or 'the gene contains the amino acid sequence', mixes the two languages and is marked wrong.
'Specific' matters too. A gene codes for one particular protein — not 'for proteins' in general. It is the reason a change to one gene affects one protein and leaves the rest of the cell alone.

Part 6 of 8

Sequence, then shape, then function

The chain of amino acids does not stay stretched out. It folds up into a particular three-dimensional shape, and it folds the same way every time it is made.
How it folds depends on which amino acids are where along the chain. So different sequences of amino acids give different shapes to protein molecules. Change the sequence and you change the shape.
Shape is what makes a protein work, and you already know the clearest case. An enzyme's active site has a shape complementary to its substrate. Change the shape of the active site and the substrate no longer fits, so the reaction is not catalysed.
The same logic runs through the other proteins the syllabus names. Membrane carrier proteins have shapes that fit the particular molecule or ion they transport across a membrane. Receptors for neurotransmitters have shapes into which only that neurotransmitter fits.
So put the whole thing together: DNA controls cell function by controlling the production of proteins. Enzymes control the chemical reactions in the cell; carriers control what enters and leaves; receptors control what the cell responds to. Control the proteins and you have controlled everything the cell does.
Learn this as a chain you can write in one line, because most Explain questions in this subtopic are asking you to walk along it: sequence of bases → sequence of amino acids → shape of the protein → its function. Each arrow is usually a separate marking point.

Part 7 of 8

How a protein is actually made

Start with the problem, because the solution only makes sense as an answer to it. The gene that codes for the protein remains in the nucleus. But proteins are assembled in the cytoplasm, at ribosomes. The information has to get out; the gene does not go.
The solution is to send a copy. Messenger RNA (mRNA) is a copy of a gene. The gene stays; the copy travels.
mRNA molecules are made in the nucleus and move to the cytoplasm.
In the cytoplasm, the mRNA passes through ribosomes.
The ribosome assembles amino acids into protein molecules — it joins them in a chain, one after another.
The gene never leaves the nucleus. The copy does the travelling.
And the order it joins them in is not random: the specific sequence of amino acids is determined by the sequence of bases in the mRNA. Since the mRNA is a copy of the gene, the gene has specified the protein without ever leaving the nucleus.
The syllabus then says, in brackets, that knowledge of the details of transcription or translation is not required. Take it at its word. You do not need codons, anticodons or transfer RNA here. You do need those six statements, in that order.
Which is convenient, because on the paper this is usually a five- or six-mark Explain, and those six statements are near enough six marking points. Write them as six short sentences in sequence — one idea per sentence — rather than one long paragraph in which two of them blur together.
If you are asked why mRNA is needed at all, the answer is the problem you started with: the gene stays in the nucleus, and the ribosomes that build proteins are in the cytoplasm, so a copy of the gene must carry the information between them.

Part 8 of 8

Same genes, different cells

Here is the fact that surprises people, and it is examined precisely because it surprises them: most body cells in an organism contain the same genes.
The reason is one you already know. Every body cell came, by mitosis, from the same fertilised egg — and mitosis produces genetically identical cells. So a nerve cell and a liver cell carry the same set of genes.
Then why is a nerve cell not a liver cell? Because in any one cell many genes are not expressed. The cell only makes the specific proteins it needs.
'Expressed' means the gene is being used — copied to mRNA and made into its protein. A gene that is not expressed has not been deleted; it is still there in the nucleus, just not being read.
Examples make it concrete. A cell in a salivary gland expresses the gene for amylase, so it makes that enzyme. A nerve cell expresses genes for receptors for neurotransmitters. A root hair cell expresses genes for the membrane carrier proteins it needs for active transport. All three cells still contain all three genes.
So when a question asks how two cells with identical genes can be so different, the answer is not 'they have different genes'. It is: they contain the same genes, but different genes are expressed, so different proteins are made.
And that closes the topic. DNA controls what a cell is by controlling which proteins are made in it — and which proteins are made depends on which genes that cell expresses.

Worked examples

Example 1. A couple have three daughters. Using a genetic diagram, explain the chance that their fourth child will be a boy.

Show the working

Write the parents' sex chromosomes first. The mother is XX and the father is XY. Label which is which — the labelling is itself a marking point.
Now the gametes, remembering that a gamete is haploid and gets one chromosome from each pair. The mother can only produce eggs carrying X. The father produces sperm carrying X and sperm carrying Y, in equal numbers.
Combine every egg with every sperm. Egg X with sperm X gives XX; egg X with sperm Y gives XY. Doing it for both eggs gives four outcomes: XX, XY, XX, XY.
Read the outcomes as a ratio and say what it is a ratio of: 2 XX to 2 XY, which is 1 female : 1 male.
Convert that to the chance asked for. One outcome in two is XY, so the probability of a boy is 1 in 2, or 50%.
Now deal with the three daughters, because the question put them there on purpose. Each fertilisation is independent: which sperm reaches the egg this time is unaffected by what happened before. The three daughters do not change the probability.

Answer: Parents XX and XY; gametes all X from the mother, X or Y from the father; offspring XX, XY, XX, XY — a ratio of 1 female : 1 male. The chance of a boy is 1 in 2 (50%), and it is still 1 in 2 for the fourth child because each fertilisation is independent of the ones before.

Example 2. In one person, a single base in the gene coding for an enzyme is different from the usual sequence. The enzyme this person makes cannot catalyse its reaction. Explain why.

Show the working

Begin at the level the change happened at. The sequence of bases in the gene determines the sequence of amino acids in the protein, so a different base can mean a different amino acid is used at that point in the chain.
Take the next step along the chain rather than jumping to the answer. Different sequences of amino acids give different shapes to protein molecules, so the chain now folds into a slightly different shape.
Ask which part of the shape matters for an enzyme. The active site — the region whose shape is complementary to the substrate.
State the consequence at the active site precisely. If the shape of the active site has changed, the substrate no longer fits into it.
Finish with the functional result, because that is what the question asked. With no enzyme–substrate complex formed, the reaction is not catalysed.

Answer: The changed base changes the sequence of amino acids in the protein. A different amino acid sequence gives a different shape, including a different shape of active site. The substrate is then no longer complementary to the active site and does not fit, so the reaction cannot be catalysed.

Example 3. A cell in the pancreas makes and secretes large amounts of the enzyme amylase. A nerve cell in the same person makes none. Explain how the pancreas cell makes amylase, and why the nerve cell does not, even though both cells contain the gene for amylase.

Show the working

Start where the information is. The gene coding for amylase remains in the nucleus of the pancreas cell — it does not travel.
So a copy is made: an mRNA molecule is made in the nucleus as a copy of that gene.
The mRNA then moves out of the nucleus into the cytoplasm, where the ribosomes are.
The mRNA passes through a ribosome, and the ribosome assembles amino acids into a protein molecule.
State what fixes the order, because that is the marking point most often missed: the specific sequence of amino acids is determined by the sequence of bases in the mRNA, which is a copy of the gene.
Now the second half of the question. The nerve cell contains the same genes, because both cells came from the same fertilised egg by mitosis.
Give the reason for the difference in the syllabus's own terms: in the nerve cell the amylase gene is not expressed, because the cell only makes the specific proteins it needs — such as receptors for neurotransmitters.

Answer: In the pancreas cell the amylase gene stays in the nucleus; an mRNA copy of the gene is made there and moves to the cytoplasm, passes through a ribosome, and the ribosome assembles amino acids into the protein in the order determined by the sequence of bases in the mRNA. The nerve cell contains the same gene but does not express it, because a cell only makes the specific proteins it needs.

Example 4. A body cell of a rabbit contains 44 chromosomes. Determine the number of pairs of chromosomes in this cell, and the number of chromosomes in a rabbit sperm cell nucleus. State which of the two nuclei is haploid.

Show the working

Identify what kind of cell you have been given. A body cell is diploid, so its nucleus contains two sets of chromosomes.
Two sets means the chromosomes are in pairs, one of each type from each set, so the number of pairs is half the total: $44 \div 2 = 22$ pairs.
A sperm cell is a gamete, and a gamete has a haploid nucleus — a single set.
A single set is one chromosome from each pair, so 22 chromosomes.
Check the answer against the human case you know, to be sure the reasoning is right rather than the arithmetic being lucky: humans have 46 in a body cell, 23 pairs, 23 in a gamete. The rabbit follows the same pattern.

Answer: 22 pairs in the body cell; 22 chromosomes in the sperm cell nucleus. The sperm cell nucleus is the haploid one, because it contains a single set of chromosomes.

Where people go wrong

Mistake: Writing 'gene' where the answer needs 'allele' — 'he inherited the gene for the disease', 'the dominant gene'.

Instead: A gene is the length of DNA that codes for a protein; an allele is an alternative form of that gene. Everyone has the gene; individuals differ in which alleles of it they have.

Mark schemes in monohybrid inheritance print 'allele' as the required word. This is the single most common avoidable loss of marks in the whole of Unit 17, and it starts here.

Mistake: Saying chromosomes are made of genes, or that DNA is made of genes.

Instead: Chromosomes are made of DNA. Genes are lengths of that DNA, so one chromosome carries many genes.

The objective is worded as a chain — chromosomes are made of DNA, which contains genetic information in the form of genes — and reversing any link in it makes the sentence untrue rather than merely clumsy.

Mistake: Mixing bases and amino acids: 'DNA is made of amino acids', 'the gene contains the amino acid sequence'.

Instead: DNA is a sequence of bases. A protein is a sequence of amino acids. The base sequence determines the amino acid sequence.

These are two different molecules with two different alphabets, and the whole of protein synthesis is the translation between them. An answer that swaps the words has said the opposite of the biology.

Mistake: Writing that the DNA or the gene leaves the nucleus and goes to the ribosome.

Instead: The gene remains in the nucleus. An mRNA copy of the gene is made in the nucleus and that is what moves to the cytoplasm.

'The gene coding for the protein remains in the nucleus' is the first bullet of the objective, and it is the reason mRNA exists at all. Losing it usually costs two marks, not one, because the reason for the copy disappears with it.

Mistake: Explaining cell specialisation by saying different cells contain different genes.

Instead: Most body cells contain the same genes; they differ in which genes are expressed, so they make different proteins.

All body cells came from one fertilised egg by mitosis, which produces genetically identical cells. A question that says 'both cells contain the same genes' is telling you what the answer must be built on.

Mistake: Thinking that after three daughters, a son is more likely.

Instead: Each fertilisation is independent, so the chance is 1 in 2 (50%) every time.

Half of sperm carry X and half carry Y regardless of what happened at previous fertilisations. Examiners set this deliberately, and the mark is for saying the events are independent, not just for stating 50%.

Check yourself

Complete the chain: chromosomes are made of ____, which contains genetic information in the form of ____.
Answer
DNA; genes.
Define a gene.
Answer
A length of DNA that codes for a protein.
Define an allele, and say in one sentence how it differs from a gene.
Answer
An allele is an alternative form of a gene. The gene is the length of DNA coding for the protein; an allele is one particular version of that gene, and a diploid cell carries two alleles of each gene, one on each chromosome of a pair.
Why does the sperm, rather than the egg, determine the sex of a child?
Answer
The mother is XX so every egg carries an X. The father is XY, so his sperm carry either X or Y. Whether the fertilising sperm carries X or Y decides whether the child is XX (female) or XY (male).
State the difference between a haploid and a diploid nucleus, and give the number of pairs of chromosomes in a human diploid cell.
Answer
A haploid nucleus contains a single set of chromosomes; a diploid nucleus contains two sets. A human diploid cell has 23 pairs (46 chromosomes).
What does the sequence of bases in a gene determine?
Answer
The sequence of amino acids used to make a specific protein.
Why do two proteins with different amino acid sequences behave differently?
Answer
Different sequences of amino acids give the molecules different shapes, and a protein's function depends on its shape — for example the shape of an enzyme's active site.
Give three kinds of protein through which DNA controls what a cell does.
Answer
Enzymes, which control the cell's chemical reactions; membrane carrier proteins, which control what is transported across the membrane; and receptors for neurotransmitters, which determine what the cell responds to.
Why is mRNA needed at all?
Answer
The gene coding for the protein remains in the nucleus, but proteins are assembled at ribosomes in the cytoplasm. An mRNA copy of the gene is made in the nucleus and carries the information to the cytoplasm.
What determines the order in which a ribosome joins amino acids together?
Answer
The sequence of bases in the mRNA, which is a copy of the sequence of bases in the gene.
A muscle cell and a skin cell contain the same genes. Explain how they can be different cells.
Answer
Many genes in a particular cell are not expressed. Each cell only makes the specific proteins it needs, so different genes are expressed in the two cells and different proteins are made.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1.

(a) Define a gene. 1 mark No calculator

Mark scheme

B1 a length / section of DNA that codes for a protein

(b) State what a chromosome is made of. 1 mark No calculator

Mark scheme

B1 DNA. Ignore additional reference to protein

(c) Describe the difference between a gene and an allele. 2 marks No calculator

Mark scheme

B1 a gene is a length of DNA coding for a (specific) protein
B1 an allele is an alternative form / version of a gene, so a gene may exist as two or more alleles

(d) Describe what is meant by a haploid nucleus and by a diploid nucleus, and state the number of pairs of chromosomes in a human diploid cell. 3 marks No calculator

Mark scheme

B1 haploid — a nucleus containing a single set of chromosomes
B1 diploid — a nucleus containing two sets of chromosomes / a pair of each type of chromosome
B1 23 pairs

Question 2. A couple have two children, both girls. They are expecting a third child.

(a) Explain, using a genetic diagram, why approximately half of all children born are male. 4 marks No calculator

Mark scheme

B1 parents shown and labelled: mother XX, father XY
B1 gametes shown and labelled: all eggs carry X; sperm carry X or Y, in equal numbers
B1 offspring shown: XX and XY
B1 ratio stated as 1 female : 1 male, or 50% / 1 in 2 chance of a male

(b) Predict the probability that the third child will be a boy, and justify your answer. 2 marks No calculator

Mark scheme

B1 1 in 2 / 50% / 0.5
B1 each fertilisation is independent — the sex of the earlier children does not affect whether an X or a Y sperm fertilises the egg

Question 3. An enzyme in a plant is made using the instructions in one gene. In one variety of the plant, a single base in that gene is different, and the enzyme produced by that variety does not work.

(a) Explain how the sequence of bases in a gene determines the shape of the protein that is produced. 3 marks No calculator

Mark scheme

B1 the sequence of bases determines the sequence of amino acids
B1 the amino acids are joined in that order to form the protein
B1 different sequences of amino acids give different shapes to protein molecules

(b) Suggest why the enzyme produced by this variety does not work. 2 marks No calculator

Mark scheme

B1 a different amino acid is used, so the shape of the enzyme / active site is changed
B1 the substrate is no longer complementary to the active site / does not fit, so the reaction is not catalysed

(c) State two other types of protein, apart from enzymes, through which DNA controls the function of a cell. 2 marks No calculator

Mark scheme

B1 membrane carriers / carrier proteins in the cell membrane
B1 receptors for neurotransmitters

Question 4. A cell in the wall of the small intestine makes large amounts of a membrane carrier protein. A cell in the same person's skin contains the gene for this protein but does not make it.

(a) Explain how this protein is made in the cell in the small intestine. 5 marks No calculator

Mark scheme

B1 the gene coding for the protein remains in the nucleus
B1 an mRNA molecule, a copy of the gene, is made in the nucleus
B1 the mRNA moves out of the nucleus to the cytoplasm
B1 the mRNA passes through a ribosome
B1 the ribosome assembles amino acids into the protein, in an order determined by the sequence of bases in the mRNA

(b) Explain why the skin cell does not make this protein, even though it contains the same gene. 2 marks No calculator

Mark scheme

B1 the gene is not expressed in the skin cell
B1 a cell only makes the specific proteins it needs. CON if the answer states that the skin cell does not contain the gene

How this is examined

The six steps by which a protein is made: a five- or six-mark Explain — 'Explain how a protein is made in a cell' — sometimes anchored to a named protein such as an enzyme or a membrane carrier Six short statements, in order, each its own marking point: 'the gene coding for the protein remains in the nucleus'; 'mRNA is a copy of the 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 molecule'; 'the sequence of amino acids is determined by the sequence of bases in the mRNA'. Details of transcription and translation are not required and earn nothing extra.
Base sequence to amino acid sequence to shape to function: a three- or four-mark Explain, usually applied — a changed base, a protein that no longer works, or 'explain how DNA controls the function of a cell' The links, written separately: 'the sequence of bases in the gene determines the sequence of amino acids'; 'different sequences of amino acids give different shapes'; 'the shape of the active site is no longer complementary to the substrate, so the substrate does not fit'; 'so the reaction is not catalysed'. For the general version, name the protein types the syllabus names — enzymes, membrane carriers, receptors for neurotransmitters.
Inheritance of sex from X and Y chromosomes: a three- or four-mark question asking for a genetic diagram or Punnett square, then the ratio or the probability, often with a Suggest about a family that already has children of one sex Parents labelled XX and XY; gametes labelled — all eggs X, sperm X or Y; offspring XX and XY shown; and the conclusion stated as '1 female : 1 male' or '50% / 1 in 2 chance'. For the family part, the mark is for 'each fertilisation is independent', not merely for repeating 50%.
The exact wording of gene, allele and chromosome: one-mark Define or State parts, usually opening a longer question, and then as the required vocabulary throughout every later question in the unit 'A gene is a length of DNA that codes for a protein'; 'an allele is an alternative form of a gene'; 'chromosomes are made of DNA, which contains genetic information in the form of genes'. In any later part, use 'allele' whenever you mean one version — 'the dominant gene' does not score where 'allele' is required.
Same genes in every body cell, different genes expressed: a two- or three-mark Explain contrasting two specialised cells, frequently with the stem already telling you both cells contain the same genes 'Most body cells contain the same genes (they came from the same fertilised egg by mitosis)'; 'many genes in a particular cell are not expressed'; 'the cell only makes the specific proteins it needs'. Saying the cells contain different genes contradicts the stem and scores zero for the whole part.
Haploid, diploid and the chromosome arithmetic: a two-mark State or Describe, or a Determine giving the chromosome number of an unfamiliar species and asking for the number in a gamete or the number of pairs 'A haploid nucleus contains a single set of chromosomes'; 'a diploid nucleus contains two sets'; 'in a diploid cell there is a pair of each type of chromosome, and in a human diploid cell there are 23 pairs'. For the arithmetic, halve the diploid number for a gamete and show it. Defining haploid only as 'half the number' is not enough — name the set.

Words to know

Chromosome: A structure in the nucleus made of DNA, which contains genetic information in the form of genes.
DNA: The molecule that chromosomes are made of. Its sequence of bases carries the genetic information.
Gene: A length of DNA that codes for a protein.
Allele: An alternative form of a gene. A diploid nucleus carries two alleles of each gene, one on each chromosome of a pair.
Base: One of the units whose order along a length of DNA carries the information. The sequence of bases in a gene determines the sequence of amino acids in a protein.
Amino acid: One of the units joined in a chain to build a protein. Their order determines the protein's shape.
Protein: A molecule made of a chain of amino acids folded into a specific shape — for example an enzyme, a membrane carrier or a receptor.
mRNA (messenger RNA): A copy of a gene, made in the nucleus, which moves to the cytoplasm and passes through a ribosome.
Ribosome: The structure in the cytoplasm that assembles amino acids into protein molecules, in the order determined by the mRNA.
Expressed: Describes a gene that is being used to make its protein. An unexpressed gene is still present in the nucleus, but is not being used.
Haploid nucleus: A nucleus containing a single set of chromosomes — 23 in humans.
Diploid nucleus: A nucleus containing two sets of chromosomes — in humans, 23 pairs, so 46 chromosomes.
Sex chromosomes: The 23rd pair of chromosomes in humans: XX in a female, XY in a male.

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