Revision: sex-linked traits, pedigrees and evaluating claims

AC9S10U01 AC9S10I08

By the end of this lesson you should be able to

• Use X and Y chromosomes with alleles (such as XA and Xa) in a Punnett square.
• Explain why X-linked recessive traits are more common in males and are not passed from father to son.
• Read a pedigree to work out genotypes and decide whether a trait is dominant or recessive, autosomal or X-linked.
• Evaluate a claim by testing each part against scientific evidence.

Hook

New York, 1910. Thomas Hunt Morgan had been breeding fruit flies, which normally have red eyes. One day he found a single male with white eyes.

He bred it with a red-eyed female. All the offspring had red eyes. He bred those offspring together. White eyes came back, but every single white-eyed fly was male.

Think–pair–share: what could make a trait turn up only in males? What is different about males' chromosomes? By the end of today you will be able to explain Morgan's result with a Punnett square.

Sex chromosomes and X-linked traits

Humans have 23 pairs of chromosomes. One pair is the sex chromosomes: females are usually XX and males are usually XY. The other 22 pairs are autosomes.

The X chromosome carries many genes. The Y carries very few. A trait whose gene is on the X is X-linked (sex-linked). We write the allele on the X: X^A (dominant) or X^a (recessive). The Y has no allele for these genes.

	Genotypes	Phenotype
Female	XAXA	unaffected
	XAXa	unaffected carrier (has the allele, does not show the trait)
	XaXa	affected
Male	XAY	unaffected
	XaY	affected

• Males are affected more often: they have only one X, so one recessive allele is enough. There is no second X to hide it.
• Fathers give their X to every daughter and their Y to every son. So an X-linked trait is never passed from father to son.
• Sons get their X from their mother.

Two X-linked recessive examples: white eyes in fruit flies (Morgan's flies), and Duchenne muscular dystrophy, a condition in which muscles gradually weaken. It affects mostly boys and is caused by a mutation in a gene on the X chromosome.

Key idea: X-linked recessive traits usually pass from a man, through his carrier daughters, to some of his grandsons.

Worked example: Morgan's fruit flies

Red eyes (X^R) are dominant to white eyes (X^r). Morgan crossed a carrier red-eyed female with a red-eyed male. Predict the offspring.

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Parents: carrier female X^RX^r × red-eyed male X^RY.

	XR	Y
XR	XRXR	XRY
Xr	XRXr	XrY

Father's gametes across the top, mother's down the side.

Daughters (left column): X^RX^R and X^RX^r. All red-eyed; half are carriers. They all got Dad's X^R.

Sons (right column): X^RY red, X^rY white. Each son has a 50% chance of white eyes.

All offspring: 1 box in 4 is white-eyed = 25%, and every white-eyed fly is male. That is exactly what Morgan saw.

Thinking to copy: check whether the question asks for the % of sons, of daughters or of all offspring. They give different answers.

Reading a pedigree

A pedigree is a family tree that shows who has a trait. Squares are males, circles are females, shaded means affected, half-shaded (if used) means carrier. A line between a male and a female joins parents, and the line down leads to their children. Generations are numbered with Roman numerals (I, II, III) and people with numbers, so II-4 is the 4th person in generation II.

Clues for deciding the pattern:

Clue in the pedigree	What it suggests
Two unaffected parents have an affected child	recessive (both parents carry it)
Every affected person has at least one affected parent; two affected parents have an unaffected child	dominant
Mostly males affected; passed from a man through his daughters to grandsons; never father → son	X-linked recessive
An affected daughter has an unaffected father, or an affected mother has an unaffected son	not X-linked recessive, so autosomal
Males and females affected about equally; father → son passing happens	autosomal

Worked example: genotypes in Pedigree A

Pedigree A shows an X-linked recessive trait. Write the genotypes of I-1, II-2, II-3 and III-1.

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Start with the affected people, then work outwards.

• I-1 is an affected male: X^aY.
• II-2 is his daughter. Carriers are not marked, so work it out: every daughter gets her father's X, so she must have his X^a. She is unaffected, so her other X is X^A: X^AX^a (carrier). Her affected son III-1 backs this up.
• II-3 is his son. Sons get Dad's Y, not his X, so II-3 got his X from his mother: X^AY, unaffected. This is the "never father → son" clue.
• III-1 is an affected male. His Y came from his father (II-1), so his X^a came from his carrier mother II-2: X^aY.

Evaluating a claim

To evaluate a claim, you judge how accurate it is using evidence. Don't just agree or disagree: break the claim into parts and test each one.

What a full answer includes:

1. State the claim (and split it into its parts).
2. Test each part against scientific evidence: genotypes, a Punnett square or a pedigree.
3. Say which parts are accurate and which are inaccurate, with a reason for each.
4. Give an overall judgement.

Worked example: "Red hair skips a generation, so it must be on the Y chromosome."

Evaluate this claim. (In a simple model, red hair is recessive and its main gene is on an autosome.)

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1. Claim: (a) red hair skips a generation; (b) so the gene must be on the Y chromosome.

2–3. Part (a): partly accurate. Red hair is recessive, so two parents without red hair can both be carriers and have a red-haired child. It can look like it skips a generation. But it does not always skip: two red-haired parents usually have red-haired children.

Part (b): inaccurate. Only males have a Y. A Y-linked trait would appear only in males and pass from a father to every son, so it could not skip a generation of males. Red-haired females are common, so the gene cannot be on the Y. Skipping generations is evidence for a recessive allele, not for the Y chromosome.

4. Judgement: the observation is partly accurate, but the conclusion is inaccurate. The pattern fits an autosomal recessive trait.

Accurate or inaccurate? (pairs, 5 minutes)

For each statement, decide accurate or inaccurate and give a one-line reason. Use a Punnett square if it helps.

1. A boy always inherits his X chromosome from his mother.
2. A carrier female shows the X-linked recessive trait.
3. X-linked recessive traits are more common in males.
4. Every daughter of a man with an X-linked recessive trait carries the recessive allele.
5. A woman can have an X-linked recessive trait.

Answers: 1 accurate (his father gave him a Y). 2 inaccurate (her X^A hides the X^a). 3 accurate (one X, nothing to hide the allele). 4 accurate (every daughter gets her father's only X). 5 accurate, if she inherits X^a from both parents.

Guided example

Look at Pedigree B. Is the trait dominant or recessive? Is it X-linked recessive or autosomal?
Prompts: First look at I-1, I-2 and their daughter II-3. What does an affected child of two unaffected parents tell you? Then look at II-3's father (I-1) and her son III-3. If the trait were X-linked recessive, what would II-3's genotype be, and what would that mean for her father and her sons?

Show answer

Recessive: I-1 and I-2 are unaffected but their daughter II-3 is affected. Both parents must carry a hidden recessive allele (Aa × Aa).

Not X-linked: if it were X-linked recessive, II-3 would be X^aX^a. She gets one X from her father, so I-1 would be X^aY and affected, but he is not. Also, every son of an X^aX^a mother gets an X^a and would be affected, but III-3 is unaffected.

Conclusion: autosomal recessive. II-3 is aa. Her partner II-4 is unaffected, but their son III-2 is affected (aa), so II-4 must be Aa.

Self-check

How this works

Pick an answer to get feedback. Nothing is saved or sent anywhere; this is just for you.

1.

Why are X-linked recessive traits more common in males than in females?

A — Males have only one X chromosome, so a single recessive allele is expressed B — The Y chromosome carries the recessive allele C — Males inherit their X chromosome from their father D — Recessive alleles are stronger in males

Explanation

Male = XY: one X, so whatever allele is on it shows. Female = XX: she needs the recessive allele on both Xs to show the trait.

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2.

In fruit flies, red eyes (X^R) are dominant to white eyes (X^r). A homozygous red-eyed female (X^RX^R) is crossed with a white-eyed male (X^rY). What % of the daughters are expected to be carriers? (Type a number only.)

Worked solution

Every daughter gets Dad's only X, which is X^r, and an X^R from Mum. So every daughter is X^RX^r: red-eyed carriers, 100%. (The sons get Mum's X^R and Dad's Y, so they are all red-eyed.)

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3.

Use Pedigree A (an X-linked recessive trait).

What is the genotype of II-4?

A — X^AX^A B — X^AX^a C — X^aX^a D — X^aY

Explanation

Daughters of an affected father always receive his X^a. If unaffected, they are carriers (X^AX^a).

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4.

Use Pedigree B. The trait is recessive.

Which evidence in Pedigree B shows that the trait is not X-linked recessive?

A — Affected female II-3 has an unaffected father (I-1) B — Both I-1 and I-2 are unaffected C — III-2 is an affected male D — There are three generations

Explanation

For an X-linked recessive trait, an affected female's father and all her sons must be affected. II-3's father (I-1) and her son III-3 are unaffected, so the trait cannot be X-linked recessive. It is autosomal recessive.

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5.

A female fruit fly has the genotype X^RX^r (red eyes X^R are dominant to white eyes X^r). Which describes her?

A — Red-eyed, and a carrier of the white-eye allele B — White-eyed, because she has the white-eye allele C — Red-eyed, and she cannot pass on the white-eye allele D — This genotype is impossible, because only males can carry X-linked alleles

Explanation

A carrier has one copy of a recessive allele hidden by a dominant one. For X-linked traits, only females (XX) can be carriers: males have one X, so they either show the trait or do not have the allele.

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Exam skill: Sex-linked crosses, pedigrees and evaluating claims

What a full answer includes

• Sex-linked cross: both parents' genotypes written with X and Y (e.g. X^AX^a, X^AY); a Punnett square; the % for the group asked (sons, daughters or all offspring)
• Pedigree: start from affected people; use the clues (two unaffected parents with an affected child → recessive; affected female with an unaffected father or son → not X-linked); name the person (e.g. II-3) when you use them as evidence
• Evaluate a claim: state the claim and split it into parts; test each part with evidence; say which parts are accurate or inaccurate and why; give an overall judgement

Worked example

Worked example: sex-linked cross. Duchenne muscular dystrophy (DMD) is X-linked recessive. A woman who is a carrier and a man who does not have DMD are planning a family. What are the chances for their sons and daughters?

Parents: X^AX^a × X^AY.

	XA	Y
XA	XAXA	XAY
Xa	XAXa	XaY

Father's gametes across, mother's down.

Daughters: X^AX^A or X^AX^a: 0% affected, 50% chance of being a carrier. Sons: X^AY or X^aY: 50% chance each son has DMD. All children: 1 in 4 = 25%.

Families in this situation can talk to a genetic counsellor, who explains these chances and the options available.

Guided (use the prompts)

2.

In fruit flies, red eyes (X^R) are dominant to white eyes (X^r). Morgan crossed a white-eyed female with a red-eyed male.

Prompts: First write each parent's genotype using X^R, X^r and Y. Then put each parent's gametes along the top (male) and side (female), and fill in the offspring.

In the boxes, X^R is shown as Xᴿ and X^r as Xʳ.

blue = gametes from the red-eyed male · pink = gametes from the white-eyed female
Order rule: put the capital letter before the lower-case letter, and X before Y.

	…XᴿXʳY	…XᴿXʳY
…XᴿXʳY	…XᴿXᴿXᴿXʳXʳXʳXᴿYXʳY	…XᴿXᴿXᴿXʳXʳXʳXᴿYXʳY
…XᴿXʳY	…XᴿXᴿXᴿXʳXʳXʳXᴿYXʳY	…XᴿXᴿXᴿXʳXʳXʳXᴿYXʳY

Worked solution

Female white-eyed = X^rX^r (gametes X^r, X^r). Male red-eyed = X^RY (gametes X^R, Y). Every daughter gets Dad's X^R and Mum's X^r: X^RX^r. Every son gets Mum's X^r and Dad's Y: X^rY.

Guided (use the prompts)

3.

Use your square for the white-eyed female (X^rX^r) × red-eyed male (X^RY).

Daughters: all have …redwhite eyes, and they are …carriers of the white allelenot carriers.

Sons: all have …redwhite eyes, because a son's X chromosome comes from his …motherfather.

Worked solution

All daughters are X^RX^r: red-eyed carriers (they get Dad's X^R). All sons are X^rY: white-eyed, because their only X comes from Mum, who has only X^r.

Guided (use the prompts)

4.

For the same cross, white-eyed female (X^rX^r) × red-eyed male (X^RY), what % of the sons have white eyes? (Type a number only.)
Prompt: where does a son's X come from?

Worked solution

A son's X comes from his mother, and she only has X^r. So 100% of sons are X^rY (white).

Guided (use the prompts)

5.

Evaluate the claim: "White eyes in fruit flies only ever appear in males."
Prompts: First state the claim. Then ask: what genotype would a white-eyed female need, and which allele must each parent give her? Then test it: what do you get if you cross a white-eyed female (X^rX^r) with a white-eyed male (X^rY)? Finally, say what is accurate, what is inaccurate, and give a judgement.

Model answer

Claim: only male fruit flies can have white eyes.

Evidence: a white-eyed female would be X^rX^r, with one X^r from each parent. Crossing a white-eyed female (X^rX^r) with a white-eyed male (X^rY) gives only X^rX^r daughters and X^rY sons, so every offspring has white eyes, daughters included.

Accurate part: white eyes are more common in males, because a male needs only one X^r.

Inaccurate part: "only ever". A female has white eyes if she inherits X^r from both parents.

Judgement: inaccurate. It is based on a real pattern, but over-generalises it.

Your turn

6.

A carrier woman (X^AX^a) and an unaffected man (X^AY) have a family. What % of their daughters are expected to be carriers? (Type a number only.)

Worked solution

Every daughter gets X^A from her father, and X^A or X^a from her mother. So daughters are X^AX^A or X^AX^a: 50% are expected to be carriers.

Your turn

7.

Evaluate the claim: "Because a girl's father has an X-linked recessive condition, she will definitely have it too."

Model answer

Claim: a daughter of an affected father will definitely be affected.

Evidence: the father is X^aY. Every daughter gets her father's X, so she always inherits X^a. Whether she is affected depends on the X from her mother: if the mother is X^AX^A, every daughter is an unaffected carrier; if the mother is X^AX^a, each daughter has a 50% chance of being affected; only if the mother is X^aX^a is it 100%.

Accurate part: she always inherits her father's recessive allele.

Inaccurate part: she will not "definitely" have the condition. One X^A from her mother hides the X^a.

Judgement: inaccurate. She always inherits her father's recessive allele, but she is affected only if she also inherits a recessive allele from her mother.

Exit questions

1. Write the genotype of a carrier female and an affected male for an X-linked recessive trait.
2. In a pedigree, what is the clue that a trait is recessive?
3. Why can a female be a carrier of an X-linked recessive trait, but a male cannot?

Show answers

1. Carrier female X^AX^a; affected male X^aY.
2. Two unaffected parents have an affected child, so both parents must carry the hidden recessive allele.
3. A female has two X chromosomes, so a dominant allele on one can hide the recessive allele on the other. A male has only one X, so if it carries the recessive allele he shows the trait.

Want more?

• Crash Course Biology #9: Heredity (includes sex-linked traits)

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