In humans the sex of a child is determined by the sex chromosomes carried in the sperm and egg. Body cells of females contain two X chromosomes, while body ...
Question 1 Report
In humans the sex of a child is determined by the sex chromosomes carried in the sperm and egg. Body cells of females contain two X chromosomes, while body cells of males contain one X and one Y chromosome. When gametes form, each egg carries a single X chromosome, but the sperm may carry either an X or a Y chromosome. The sex of the child therefore depends on which type of sperm fertilises the egg at fertilisation. The Punnett square below has been started to show the cross between the sex chromosomes of the two parents.
(a) State the sex chromosomes present in a human male and in a human female. [2]
(b) Copy and complete the Punnett square to show the sex chromosomes of the offspring. [3]
(c) State the ratio of male to female children expected from many such crosses. [1]
(d) Calculate the probability that a couple's next two children will both be girls. [2]
Answer Details
This question tests the inheritance of sex using a Punnett square and simple probability.
(a) Sex chromosomes. [2] A human male is XY [1]; a human female is XX [1].
(b) Complete the Punnett square. [3] Each egg carries one X; a sperm carries either X or Y. Combining the gametes gives:
| sperm X | sperm Y | |
|---|---|---|
| egg X | XX | XY |
| egg X | XX | XY |
The four offspring boxes are XX, XX, XY, XY [3] (two females and two males).
(c) Ratio of male to female. [1] 1 male : 1 female [1], because half the sperm carry Y and half carry X.
(d) Probability both next two children are girls. [2] Each birth is independent with a \(\tfrac{1}{2}\) chance of a girl, so \[\frac{1}{2}\times\frac{1}{2}=\frac{1}{4}\;(25\%)\] [2]. Exam tip: multiply the separate probabilities for "both" events; do not add them.
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