Edexcel IGCSE Biology 3.27 Sex Determination At Fertilisation Questions

Practise using X and Y chromosome diagrams to show sex determination at fertilisation and calculate repeated offspring probabilities.

Exam points

complete a genetic diagram with XX female and XY male parents, their X and Y gametes, and possible XX or XY offspring
calculate the probability that several children are all one sex by multiplying a one-half chance at each fertilisation
explain why actual male and female offspring numbers may differ from the expected 1:1 ratio because fertilisation is random

[Maximum number: 6]

Human body cells contain a full set of chromosomes.
Human gametes contain a half set of chromosomes.

(a)

Which of these could be the chromosomes in a fertilised human egg cell?

one X chromosome and 22 other chromosomes

one Y chromosome and 22 other chromosomes

two X chromosomes and 22 pairs of other chromosomes

two Y chromosomes and 22 pairs of other chromosomes

[ 1 ]

(b)

Use a genetic diagram to show how the sex of an embryo is determined at fertilisation.

[ 3 ]

(c)

Two parents have four children.

Determine the probability that all the children are male.

probability =

[ 2 ]

calculation method not marking points

$\frac{1}{2} \times \frac{1}{2} \times \frac{1}{2} \times \frac{1}{2}$

0.0625 or 1 / 16 or 6.25%

full marks for correct answer

allow 1 mark for 0.5 or $\frac{1}{2}$ or 50% in working

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Edexcel IGCSE Biology 3.27 Sex Determination At Fertilisation Questions question 1

Question (a)

Question (b)

Question (c)