How X and Y chromosomes determine sex, why the chance of a boy is always 50%, and the inheritance of haemophilia and colour blindness.
5 narrated scenes. Press play, listen and read along — the points appear as they are explained. Use ‹ Prev / Next › or the scene bar to jump to any scene.
Why are boys far more likely than girls to be colour-blind?
Narration uses your device’s built-in voice. No sound? Switch to “Captions only” — the captions advance on their own.
Copy these into your notebook in your own words — that is how they stick.
Female XX, male XY. Eggs carry X; sperm carry X or Y.
Each child has a 50% chance of being male or female, every time.
Gene carried on X chromosome only (no allele on Y).
Males have one X; a single recessive allele is expressed.
Heterozygous female (XᴴXʰ): normal phenotype, can pass allele to children.
Haemophilia, red-green colour blindness.
Write alleles as superscripts on X: Xᴴ, Xʰ; Y carries no allele.
The kind of support a teacher gives after the lesson: worked examples to attempt first, a board illustration, key terms, common misconceptions and memory aids.
Try it in your notebook first, then reveal the solution one step at a time.
Try it in your notebook first, then reveal the solution one step at a time.
| Genotype | Sex | Phenotype |
|---|---|---|
| XᴮXᴮ | Female | Normal vision |
| XᴮXᵇ | Female | Normal vision — carrier |
| XᵇXᵇ | Female | Colour blind (rare) |
| XᴮY | Male | Normal vision |
| XᵇY | Male | Colour blind |
Females have two X chromosomes and can be carriers; males have one X and cannot be carriers.
“Sons get their X from Mum.” So X-linked conditions pass from carrier mothers to sons.
Haemophilia ran through several European royal families in the 1800s and 1900s. Queen Victoria was a carrier, and the allele passed through her daughters to sons in the Spanish, German and Russian royal families.
Choose an easily seen family trait and draw a family tree of three generations using squares for males and circles for females. Shade those who show the trait.
3 quick games on this lesson. Earn XP and badges as you play — your best score for each game is saved. Games are for practice and do not change your exam readiness.
8 items modelled on real CSEC Paper 1 questions. Choose one answer for each, then check. Aim for 80% or better.
Write full answers, then reveal the mark scheme and tick the points you earned (15 marks in total). Your answers are saved on this device.
Question 1 [8 marks]
“The probability of a child being male or female is the same each time a child is conceived.”
(a) Determine the accuracy of this statement using a complete genetic diagram and conclusion. [6]
(b) List the TWO possible genotypes of a female whose father is a haemophiliac and whose mother carries the allele. [2]
Question 2 [7 marks]
Red-green colour blindness is a recessive sex-linked condition (allele Xᵇ).
(a) Explain why colour blindness is more common in males than females. [3]
(b) A colour-blind man (XᵇY) and a woman with normal vision who is not a carrier (XᴮXᴮ) have children. Predict the outcome with a genetic diagram. [4]
Find these in your own past-paper booklets for extra timed practice. The practice above is adapted from them.