Sex-Linked Sleuth

Sex-Linked Sleuth 🧬

Whether a baby is a boy or a girl comes down to a single chromosome — and so do some conditions that mysteriously run through the men of a family. Time to play genetic detective and solve both puzzles.

The sex chromosomes 👶

Human body cells have **23 pairs** of chromosomes. The **23rd pair** are the **sex chromosomes**: • **Females are XX** • **Males are XY** All **eggs carry an X**. **Sperm carry either an X or a Y** — so it is the **father's sperm** that determines a baby's sex.

Cross: XX × XY

An interactive activity.

Who decides?

Whose gamete determines the sex of a baby, and why?

• The father's sperm — it carries either an X or a Y chromosome
• The mother's egg — it carries either an X or a Y
• Both parents equally, at random
• Neither — sex is decided after birth

X-linked disorders ♂️

**(Higher tier)** Some genes sit only on the **X chromosome** — they are **X-linked**. For an X-linked **recessive** disorder such as **red-green colour blindness** or **haemophilia**: • A **male (XY)** has only **one X**, so a **single** recessive allele is enough to affect him. • A **female (XX)** has **two X**s, so she needs **two** recessive alleles — with just one she is an unaffected **carrier**. That is why these conditions affect **males far more often**.

The colour-blind son

An interactive activity.

Why mostly males?

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

• Males have only one X chromosome, so a single recessive allele shows
• Males have two X chromosomes
• The disorders are carried on the Y chromosome
• Females never inherit the alleles

True about sex and inheritance

Select ALL THREE statements that are TRUE.

• Females are XX and males are XY
• The father's sperm determines a baby's sex
• A female with one recessive X-linked allele is an unaffected carrier
• The mother's egg determines the baby's sex
• X-linked recessive disorders are more common in females
• Every sperm carries a Y chromosome

Match each term to its meaning

Sleuth summary

Females are _____ and males are XY, so the father's sperm (X or Y) determines a baby's sex. A cross of XX × XY gives a _____ ratio of girls to boys. X-linked recessive disorders affect _____ more often because they have only one X chromosome. A female with one recessive allele is an unaffected _____.