Edexcel IGCSE Biology 3.27 Sex determination at fertilisation
Practise using X and Y chromosome diagrams to show sex determination at fertilisation and calculate repeated offspring probabilities.
- Syllabus
- First assessment 2019
- Course
- Biology 4BI1
Exam points
- complete genetic diagrams showing XX female parents, XY male parents and XX or XY offspring
- calculate probabilities for several children all being one sex using repeated 1/2 chances
- explain why actual numbers of male and female offspring may differ from the expected ratio
3.27 Sex determination at fertilisation question 1
[Maximum number: 6]
Human body cells contain a full set of chromosomes.
Human gametes contain a half set of chromosomes.
Question (a)
(a)
Which of these could be the chromosomes in a fertilised human egg cell?
A
one X chromosome and 22 other chromosomes
B
one Y chromosome and 22 other chromosomes
C
two X chromosomes and 22 pairs of other chromosomes
D
two Y chromosomes and 22 pairs of other chromosomes
[ 1 ]
C
Question (b)
(b)
Use a genetic diagram to show how the sex of an embryo is determined at fertilisation.
[ 3 ]
parent genotypes XX and XY / eq
- gametes X and X or Y / eq
- offspring XX and XY / eq
Allow mp 123 from Punnet square
No ECF
For gametes either circled or clear separation of X Y
No credit for other letters
Question (c)
(c)
Two parents have four children.
Determine the probability that all the children are male.
probability =
[ 2 ]
calculation method not marking points
21×21×21×21
0.0625 or 1 / 16 or 6.25%
full marks for correct answer
allow 1 mark for 0.5 or 21 or 50\% in working
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