Sex-Linked Traits
Genes on the X chromosome affect males and females differently.
About this topic
Sex-linked genes lie on the sex chromosomes, usually the X. Because males (XY) have only one X, a recessive allele on it is expressed, whereas females (XX) need two copies and can be carriers. This is why red-green colour blindness and haemophilia are far more common in men.
Fathers pass X-linked traits to daughters but not sons; carrier mothers pass them to half their sons. Punnett squares use X and Y symbols with superscript alleles. Y-linked genes pass from father to son only.
Red-green colour blindness affects about 1 in 12 men but only 1 in 200 women. A carrier mother (XCXc) and normal father (XCY) have a 50 percent chance of each son being colour blind, and each daughter a 50 percent chance of being a carrier, but no daughter is affected. An affected father passes his X to all daughters and his Y to all sons, so he cannot pass the trait to a son.
Haemophilia, in which blood fails to clot properly, is another X-linked recessive condition, famously carried through European royal families from Queen Victoria. Males are not more susceptible because of a stronger gene effect; they simply have only one X. Students sometimes forget that the gene sits on the X chromosome and write the allele on the Y; draw the chromosomes in the Punnett square.
Key ideas
- Males are XY
- Colour blindness is X-linked recessive
- Carriers are heterozygous females
Quick check
Use cases: where Sex-Linked Traits helps
- Homework: review the key idea behind Sex-Linked Traits before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Sex-Linked Traits without notes.
- Visual learning: play with the interactive lab to see how Sex-Linked Traits changes when you adjust its inputs.
- Teaching a friend: explain Sex-Linked Traits in your own words, then compare your version with the key ideas.
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Frequently asked questions
What is Sex-Linked Traits?
Genes on the X chromosome affect males and females differently.
What will I learn in Sex-Linked Traits?
Males are XY. Colour blindness is X-linked recessive. Carriers are heterozygous females.
Is Sex-Linked Traits part of Biology?
Yes. It is one of 150 Biology topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Sex-Linked Traits lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Sex-Linked Traits?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.