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Flip to reveal answersWhat are the sex chromosomes in a human female and a human male?
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All 9 Flashcards — Sex determination & sex-linkage
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Question
What are the sex chromosomes in a human female and a human male?
Answer
**Female = XX**, **male = XY**.
Question
What are autosomes?
Answer
The **22 pairs of chromosomes that are not the sex chromosomes** — the same in males and females.
Question
Which parent's gamete determines the sex of a baby, and why?
Answer
The **father's sperm** — the egg always carries X, but a sperm carries **X or Y** (X→girl, Y→boy).
Question
What does 'sex-linked' mean?
Answer
The gene is carried on a **sex chromosome (usually the X)**, so its inheritance is tied to the offspring's sex.
Question
How do we write the alleles of an X-linked gene?
Answer
**On the X** — e.g. **Xᴮ** (dominant) and **Xᵇ** (recessive); the **Y has no matching allele**, written just **Y**.
Question
Why is an X-linked recessive condition more common in males?
Answer
Males have **only one X** and the **Y has no matching allele**, so a **single recessive allele shows**; a female needs it on **both** X chromosomes.
Question
What is a carrier (for an X-linked recessive condition)?
Answer
An **unaffected female (XᴮXᵇ)** who has one recessive allele, masked by the dominant allele on her other X; she can still pass it on.
Question
Deduce the possible genotypes of an unaffected female for colour blindness.
Answer
**XᴮXᴮ (homozygous dominant)** or **XᴮXᵇ (carrier)** — you usually can't tell which from her phenotype.
Question
In a carrier mother (XᴮXᵇ) × unaffected father (XᴮY) cross, who can be colour-blind?
Answer
Only **sons** — about **half** are colour-blind (XᵇY); **no daughter is affected**, though half are carriers.
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Inheritance
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