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SCIPAD - BIG IDEAS - BOOK 2
THE BASIS OF HEREDITY
Sex Determination
After completing the karyotype activity, you should have noted that humans have 23 pairs of
chromosomes.
The final pair (pair 23), are called your sex chromosomes because they determine your biological
sex. If you have 2 ‘X’ chromosomes (XX) you are biologically female. Biological males have one
of each type (XY).
In this activity, we are going to ignore all the other chromosomes and just look at pair 23.
1. Complete the diagram by showing the possible combinations of sex chromosomes that
could form in the fertilised ova.
2. What is the probability that any baby produced would be a boy?
3. What is the chance that any baby produced would be a girl?
4. If a couple already had 2 children, both of whom were girls, what is the probability that the
next baby will be a boy?
5. Explain your answer to question 4.
During the production of ova, the
female sex chromosomes separate. This
means all ova receive an ‘X’.
During the production of sperm, male
sex chromosomes separate. This means
half of the sperm receive an ‘X’ and the
other half receive a ‘Y’.
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