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NSWBiologyQuick questions
Module 5: Heredity
Quick questions on Sex-linked inheritance explained: HSC Biology Module 5
10short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is why sex-linked matters?Show answer
In mammals (including humans), biological sex is determined by the sex chromosomes. Females are XX; males are XY. Most genes on the X chromosome have no equivalent on the much shorter Y chromosome.
What is notation?Show answer
Use superscripts on the X chromosome to show the allele.
What is why no male carriers?Show answer
A male only has one X chromosome. He either has the recessive allele (and is affected) or he doesn't (and is unaffected). There is no carrier state for males in X-linked recessive inheritance.
What is standard carrier-mother cross?Show answer
Carrier mother ($X^H X^h$) × unaffected father ($X^H Y$).
What is affected father, unaffected mother?Show answer
Affected father ($X^h Y$) × unaffected non-carrier mother ($X^H X^H$).
What is worked example?Show answer
A carrier mother and an affected father have children. Predict offspring outcomes.
What is wrong notation?Show answer
Always write the allele as a superscript on the X. Writing just "H" or "h" without the X is a 1-mark deduction because it hides the sex-linkage.
What is forgetting the Y has no allele?Show answer
The Y chromosome does not carry the X-linked gene. Males are NEVER carriers of X-linked traits.
What is wrong denominators?Show answer
"50% of sons are affected" is different from "25% of all children are affected sons." Read the question.
What is confusing X-linked dominant and X-linked recessive?Show answer
Most exam questions focus on X-linked recessive. X-linked dominant is rare; if a question says dominant, the pattern flips.