Sex determination and sex-linked inheritance
Whether a baby is a boy or a girl is decided at fertilisation by one pair of chromosomes, the sex chromosomes. The same chromosomes explain why some conditions, such as haemophilia, are far more common in boys.
By the end of this page you can
- Explain how sex is decided, and why haemophilia and colour blindness are commoner in males
- Work out the children of a carrier mother
- Follow the Y chromosome from father to son
- Define sex chromosomes, sex-linked, carrier and recessive
- Show a sex-linked cross with a genetic diagram
Goals tick themselves off as you try the activities below.
Photo: Janko Ferlič, CC0, via Wikimedia Commons. Cropped, resized, JPEG q70.
Boy or girl was settled the moment a sperm met an egg, months before birth.
How often it shows up in past papers
Paper 1 counts come from our past-paper bank of CSEC Biology Paper 1s. Paper 2 is not included yet.
One pair decides, and carries more than sex
How sex is decided, and two conditions carried on the X chromosome. Tap each card.
A carrier mother, an unaffected father
H is the normal allele, h the haemophilia allele.
Who gets what?
Pick a person on the list.
Why mostly males? A male has one X, so one recessive allele is enough; a female needs two. Colour blindness is inherited the same way.
Follow the Y from father to son
Drag or tap each card into its slot.
What are the father’s sex chromosomes?
Seven terms to know cold
Say what each term means before you flip the card. Sex-linked answers need the letter X.
Six quick questions, then a real one
Pick an answer to see why it’s right or wrong.
Which chromosomes does a human egg cell contain?
Which parent decides the sex of a child?
What is the chance that a baby is a girl?
Haemophilia is more common in males because
A carrier mother (XH Xh) and an unaffected father (XH Y) have a son. What is the chance he has haemophilia?
A colour-blind man (Xc Y) and a woman with two normal alleles (XC XC) have daughters. The daughters are
exam style · 4 marks
A woman who is a carrier of haemophilia has children with a man who does not have haemophilia. Use a genetic diagram to show the chance that a son of theirs has haemophilia.
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Practice questions are written by My V.I.P. Academy. They are not official CXC papers.