Pedigrees and Sex Determination

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 3.24 — How to interpret family pedigrees
• 3.26 — Sex determination by one pair of chromosomes, XX and XY
• 3.27 — The determination of the sex of offspring using a genetic diagram

Specification section 3(b) Inheritance — also taught in DNA, Genes and Chromosomes, Protein Synthesis, Alleles, Genotype and Phenotype, Monohybrid Inheritance and Probability.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

A Punnett square works forwards: you are given the parents and you predict the children. A PEDIGREE works backwards. You are given a family — who is affected and who is not, across several generations — and you have to deduce genotypes that nobody has measured. It is the same genetics, run in the harder direction, and there is a reliable way in.

Deciding whether a condition is dominant or recessive — the two clues that settle it

1. LOOK FOR AN AFFECTED CHILD WITH TWO UNAFFECTED PARENTS. If you find one, the condition MUST be recessive — because if it were dominant, at least one parent would have had to carry the allele and would therefore have shown it.
2. That single observation is usually enough, and it is the first thing to search a pedigree for.
3. If instead every affected individual has at least one affected parent, and the condition appears in every generation, that is consistent with a DOMINANT allele.
4. Be careful with the second clue: it is weaker. A recessive condition can also appear in consecutive generations by chance, so 'it never skips' is suggestive rather than conclusive. The affected-child-of-unaffected-parents rule is the one that proves something.

Then work out the genotypes, in this order

1. FILL IN EVERY AFFECTED INDIVIDUAL FIRST, if the condition is recessive. Each of them must be homozygous recessive — aa — because that is the only genotype that shows it, and they must have inherited one recessive allele from each parent. These are free information.
2. WORK OUTWARDS FROM THEM. Every parent of an affected child must have passed on a recessive allele, so an unaffected parent of an affected child is definitely a heterozygous carrier, Aa.
3. Every CHILD of an affected individual must have received a recessive allele from them, so an unaffected child of an affected parent is also definitely Aa.
4. MARK WHAT REMAINS AS UNCERTAIN. An unaffected individual with no affected children and no affected parent may be AA or Aa, and honest answers say so — writing 'AA or Aa' scores where guessing does not.

Think of it like a crossword with a few letters already filled in

You do not start at the top left and work through. You start with the entries you are certain about — the ones with unambiguous clues — and then use each certainty to constrain its neighbours. Affected individuals in a recessive pedigree are the filled-in letters: their genotype is forced by their phenotype. Everything else is deduced by asking what must have been passed to them or received from them. And, as with a crossword, some squares stay genuinely open, and the skilled answer is the one that knows which.

Now the second half of the page, and it is a case where the genetic diagram always comes out the same. Of the 23 pairs of chromosomes in a human cell, 22 pairs are matched. The 23rd pair is the SEX CHROMOSOMES, and it decides sex: a female has two X chromosomes, XX, and a male has one X and one Y, XY.

Why the ratio is always 1 : 1

1. The mother is XX, so every egg she produces carries an X. She has no other option.
2. The father is XY, so half his sperm carry an X and half carry a Y.
3. At fertilisation, an egg (always X) meets a sperm that is equally likely to be X or Y.
4. X from the mother with X from the father gives XX — female. X from the mother with Y from the father gives XY — male.
5. So the expected ratio is 1 female : 1 male, or a 50 % chance for each — and the sex of a child is determined entirely by which sperm fertilises the egg, since the mother's contribution is the same either way.

That last point has a long and unhappy history attached to it. For most of recorded time women were blamed — and in some places still are — for the sex of their children, and the biology says flatly that they cannot be responsible for it. Every egg carries an X. The only variable in the entire system is which of the father's two kinds of sperm arrives first, and that is a matter of chance. It is a rare case where a straightforward piece of genetics settles a question people once fought over.

One thing worth being precise about: the Y chromosome is much smaller than the X and carries far fewer genes, but it carries the gene that triggers male development. So it is the presence of a Y that makes an embryo male, not the absence of a second X. That distinction matters because it explains why the father's gamete decides the outcome — he is the only parent who can supply a Y.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Reading a pedigree — the conventions, which are the same in every paper

Circle

A female.

Square

A male.

Shaded symbol

An individual affected by the condition — showing the phenotype in question.

Unshaded symbol

An unaffected individual. This does NOT mean their genotype is known.

Horizontal line between two symbols

A couple who have had children together.

Vertical line leading down

Leads to their offspring, who are shown side by side on the generation below.

Interpreting a pedigree — the routine, in order

1. Look for an affected individual whose parents are BOTH unaffected. If one exists, the condition is recessive — this is the decisive clue.
2. If the condition is recessive, write aa for every affected individual: their genotype is forced.
3. Any unaffected parent of an affected child must be a carrier, Aa, because they passed on a recessive allele.
4. Any unaffected child of an affected parent must also be Aa, because they received a recessive allele.
5. For anyone else who is unaffected, write 'AA or Aa' — stating the uncertainty is what scores.
6. To answer a probability question, draw a Punnett square for the specific couple asked about, using the genotypes you have deduced.

The clue that proves a condition is recessive

A. DECISIVE: an affected child with two unaffected parents. A dominant allele always shows, so neither parent could have carried it unseen — the condition must be recessive.

B. SUGGESTIVE ONLY: the condition appearing in every generation. That is consistent with a dominant allele, but a common recessive allele can do the same by chance.

The sex-determination diagram — statement 3.27, laid out step by step

1. Parents: mother XX, father XY.
2. Gametes: the mother produces eggs carrying (X) only; the father produces sperm carrying (X) or (Y).
3. Combine in a Punnett square: X with X gives XX, and X with Y gives XY.
4. Offspring: XX, XX, XY, XY — a ratio of 1 female : 1 male.
5. The probability of a child being male is therefore 1/2, or 50 %, at every conception.

The sex chromosomes

Facts about the sex chromosomes that questions rely on

• Humans have 23 pairs of chromosomes; 22 pairs are matched and the 23rd pair determines sex
• The Y chromosome is much smaller than the X and carries far fewer genes
• It is the PRESENCE of a Y that causes male development, not the absence of a second X
• The mother cannot influence the sex of her children — every egg carries an X
• The expected ratio is 1 : 1 at every conception, independently of any previous children

Model answer [4 marks]

A pedigree shows two unaffected parents who have an affected daughter. Explain what this shows about the allele causing the condition, and give the genotypes of both parents. [4]

The allele must be recessive. If it were dominant, an individual carrying it would show the condition, so at least one parent would have been affected. The affected daughter must therefore be homozygous recessive, having received a recessive allele from each parent. Both parents are unaffected but each passed on a recessive allele, so both must be heterozygous carriers.

Model answer [4 marks]

Draw a genetic diagram to show how the sex of a child is determined. [4]

The mother's genotype is XX and the father's is XY. The mother's gametes all carry X; the father's gametes carry either X or Y. Combining these gives offspring XX, XX, XY and XY. XX is female and XY is male, so the expected ratio is 1 female : 1 male, and the probability of a male child is 50 %.

Model answer [3 marks]

Explain why the father, and not the mother, determines the sex of a child. [3]

The mother is XX, so every egg she produces carries an X chromosome and she has no other kind to contribute. The father is XY, so half his sperm carry an X and half carry a Y. Whether the child is XX or XY therefore depends entirely on which of the father's two kinds of sperm fertilises the egg.

Mark-losing trap. An affected child of two unaffected parents proves the condition is RECESSIVE. Look for this first.

Mark-losing trap. Unshaded does not mean AA. Write 'AA or Aa' where the pedigree genuinely cannot decide — the uncertainty is the mark.

Mark-losing trap. In a sex diagram write XX and XY, never a single X or Y for a parent.

Mark-losing trap. Show the GAMETES in a sex-determination diagram — X only from the mother, X or Y from the father. It is usually a separate mark.

Mark-losing trap. The 1 : 1 sex ratio applies at every conception. Three sons do not make a daughter more likely next time.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — What are the sex chromosomes of a human male and of a human female?
2. Grade 7 · Explain [2 marks] — In a family pedigree, two unaffected parents have a son who is affected by a genetic condition. What does this show about the allele causing the condition?
3. Grade 8 · Explain [5 marks] — Explain how the sex of a child is determined, and why the mother cannot influence it. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [4 marks] — A woman whose brother is affected by a recessive condition has parents who are both unaffected. She is unaffected herself. Calculate the probability that she is a carrier of the recessive allele.
5. 9+ · Analyse [5 marks] — A pedigree shows a condition appearing in all four generations of a family. Every affected individual has at least one affected parent, and no unaffected couple has ever had an affected child. A student concludes the condition is caused by a dominant allele. Select every statement that belongs in a full-mark evaluation.

The people behind this science

Two ways into the same idea — the one who could design his crosses, and the one who deduced a structure she could not see. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Gregor Mendel — the one who could design his crosses

Mendel could choose which plants to cross, produce hundreds of offspring from each pairing, and repeat anything that looked ambiguous. A pedigree offers none of that: you take the family you are given, the offspring are few, and the crosses were made without regard to your question. Everything that makes this page difficult is the absence of what made his method work — which is why deduction from a pedigree needs a rule as sharp as the affected-child-of-unaffected-parents test to get started at all.

• “Why did being able to choose your crosses matter so much?”
• “What could you have concluded from a family of only four offspring?”
• “How would you study inheritance in an organism you could not breed at will?”
• “Which of your results would have been invisible in a small sample?”

Rosalind Franklin — the one who deduced a structure she could not see

A pedigree asks you to deduce something invisible — a genotype — from a pattern of visible effects, and to be honest about which parts remain undetermined. That is precisely the discipline of Franklin's work: an X-ray photograph does not show a molecule, it shows a pattern of spots from which the structure has to be inferred, with measurements constraining what is possible and the rest left open. Her caution about announcing conclusions the data did not yet force is the same caution a good pedigree answer shows when it writes 'AA or Aa'.

• “How do you reason from a pattern to the thing that caused it?”
• “How do you decide when the evidence forces a conclusion and when it does not?”
• “Why were you reluctant to announce a result others were happy to guess at?”
• “What is the cost of claiming more than the data supports?”

Carry on through the course

• Previous lesson: Monohybrid Inheritance and Probability
• Next lesson: Mitosis and Meiosis
• Sexual and Asexual Reproduction — Two ways to make a new organism — one produces copies, the other produces something that has never existed before. Why any species would want both.
• Flowers, Pollination and Fertilisation — Two completely different solutions to one problem — a plant that cannot move has to get its male gamete to another plant's female gamete.
• Seeds and Germination — What a seed needs to start growing, what it lives on before it has a single leaf — and why light is not on the list.
• The Human Reproductive Systems — Two systems built around the same problem — making gametes and bringing them together — and the hormones that switch them on.
• The Menstrual Cycle — Four hormones, two organs and one repeating question — should the uterus lining be built up, held, or let go?
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification