📘 CodingMarble Learn

Sex-Linked Inheritance

Some genes sit on the X chromosome and have no partner on the short Y chromosome. Such X-linked genes pass from parents to children in a special pattern. A man (XY) has only one X, so one faulty recessive allele shows up in him. A woman (XX) needs two faulty copies; with one she is a healthy carrier. Red-green colour blindness and haemophilia follow this X-linked recessive pattern.

🎬 Step-by-step story

  1. Meet two chromosomes. X is long and carries many genes, including the colour-vision gene. Y is short. It has no copy of this gene.
  2. This mother has two X chromosomes. One X has a faulty colour-vision gene (grey). Her other X is good, so she sees normally. She is a carrier.
  3. This father has one X and one Y. His X has a good gene, so he sees colours normally.
  4. Each child gets one X from the mother. The father gives his X to a daughter and his Y to a son. That makes four possible children.
  5. Look at the son with the grey X. He has only one X, and Y cannot cover it. So he is colour-blind. This is why more boys have X-linked conditions.
  6. Your turn. Pick the mother's and father's genes and see which children are normal, carriers or colour-blind.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Why does the Y chromosome not cover the faulty gene?

Y is short and has no colour-vision gene at all, so there is no good copy to hide the faulty one.

How can the mother see normally if she has a faulty gene?

Her second X carries a good, dominant copy, which is enough to make the right pigment.

Can a son get colour blindness from his father?

No. The father gives a son his Y, never his X. Watch the sons in step 4: their X always comes from the mother.

Why do more boys have it than girls?

A boy needs one faulty X; a girl needs two. One bad copy is much more common than two.

Can a girl be colour-blind?

Yes, if her father is colour-blind and her mother is at least a carrier. Try it in free play.

Is a carrier woman ill?

Usually no. She is healthy but can pass the allele on.

What makes a gene sex-linked?

Humans have 23 pairs of chromosomes. One pair decides sex: females are XX and males are XY. The X chromosome is large and carries about 800 genes that have nothing to do with being male or female, such as genes for colour vision and blood clotting. The Y chromosome is small and carries only a few genes.

A gene found on the X chromosome (and not on Y) is called X-linked or sex-linked. Its pattern of inheritance depends on the sex of the child.

X-linked recessive traits

We write the gene as a letter on the X: XB = normal allele (dominant), Xb = faulty allele (recessive).

A male has only one X, so he cannot be a carrier: he is either normal or affected. This is called being hemizygous.

Key features of the recessive pattern

Examples: red-green colour blindness, haemophilia A (blood does not clot well), Duchenne muscular dystrophy.

X-linked dominant traits

Here one faulty allele is enough in anyone, male or female.

Examples: a rare form of rickets that does not respond to vitamin D (hypophosphataemic rickets) and Rett syndrome.

There are also a few Y-linked genes. They pass only from father to son, every time.

Solving sex-linked problems with a Punnett square

Always keep the gene on the X letter. Steps:

  1. Write each parent's genotype, e.g. mother XBXb, father XBY.
  2. List the gametes: mother XB, Xb; father XB, Y.
  3. Fill the 2×2 square: XBXB, XBXb, XBY, XbY.
  4. Read the answer separately for daughters and sons: all daughters normal (half carriers); half the sons affected.

In mixed exam problems, sex-linked genes are often combined with other patterns you already know: codominance (both alleles show, as in AB blood group), incomplete dominance (a blend, as in pink snapdragons) and multiple alleles (more than two forms, as in the ABO blood groups IA, IB, i). Solve each gene on its own, then combine.

Try it at home

Use two red cards (normal X), one grey card (faulty X) and one blue card (Y). Mother = red + grey, father = red + blue. Shuffle each pair, draw one card from each 20 times and count how many sons come out "grey + blue". You should get close to half of the sons.

Key formulas and definitions

Worked examples

1. A carrier woman marries a normal man. What fraction of their sons will be colour-blind?

Mother XᴮXᵇ gives Xᴮ or Xᵇ. Father XᴮY gives Y to every son. Sons: XᴮY or XᵇY. So ½ of the sons are colour-blind.

2. A colour-blind man marries a woman with normal genes (XᴮXᴮ). Predict the children.

Father XᵇY, mother XᴮXᴮ. Daughters XᴮXᵇ: all normal but carriers. Sons XᴮY: all normal. The trait is hidden for one generation.

3. A colour-blind girl is born. What must be true about her parents?

She is XᵇXᵇ, so she got Xᵇ from each parent. Her father must be colour-blind (XᵇY) and her mother must be at least a carrier (XᴮXᵇ or XᵇXᵇ).

4. Haemophilia: a carrier woman marries a man with haemophilia. What is the chance that a child (sex unknown) has haemophilia?

Mother XᴴXʰ × father XʰY. Children: XᴴXʰ, XʰXʰ, XᴴY, XʰY. Two of four are affected, so the chance is ½ (one affected daughter type and one affected son type).

Common mistakes

Practice quiz

1. An X-linked gene is a gene found on:
2. A colour-blind son always gets the faulty allele from his:
3. Genotype of a carrier female:
4. An affected father with an X-linked dominant trait passes it to:
5. Which is an X-linked recessive disorder?

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What is sex-linked inheritance in simple words?

It is the inheritance of genes that sit on the X (or Y) chromosome, so the result depends on whether the child is a boy or a girl.

Why is colour blindness more common in males?

Males have only one X chromosome. One faulty recessive allele on it is enough, while females need faulty alleles on both X chromosomes.

What is the difference between X-linked recessive and X-linked dominant?

Recessive traits need two copies in females and mostly affect males. Dominant traits need just one copy in anyone and pass from an affected father to all his daughters.

Where this is taught

Spain4º ESOGenetics and evolution
China高一Comp.2 Ch.2 Genes and chromosomes

Learn first

Learn next

Related lessons

All Biology lessons