These questions ask you to place X and Y chromosomes in body cells and gametes, trace which parent supplies each chromosome, and interpret a Punnett square. Follow the chromosome that can vary instead of trying to remember separate answers.
Trace one chromosome from each parent
Begin with the parents' body-cell pairs. A female has XX, so meiosis can place only X in her eggs. A male has XY, so meiosis produces sperm carrying X and sperm carrying Y. At fertilisation an egg contributes X in every case. An X-carrying sperm produces XX, while a Y-carrying sperm produces XY. The two sperm types are predicted in equal proportions, so each child begins with the same equal chance; previous children do not alter the sperm available later.
| Signal | Conclusion |
|---|---|
| A human body cell | Its full chromosome set includes one pair of sex chromosomes. |
| An egg | It carries one chromosome from the mother's sex-chromosome pair, and that chromosome is X. |
| A sperm | It carries one member of the father's pair, either X or Y. |
| An offspring with XY | The X must have come through the egg and the Y through the sperm. |
| Chromosomes outside the sex pair | The other 22 chromosome types occur in both males and females and can pass to a child of either sex. |
Keep the scope human unless the question says otherwise. The XX and XY method is one biological system, not a rule for every species. Also remember that X and Y carry genes beyond their role in determining sex, just as the other chromosomes do.
Common wrong turns
| Tempting idea | How to check it |
|---|---|
| Adding the sex chromosomes after the full body-cell set | They are already one pair within that set. Adding them again would count two chromosomes twice. |
| Giving eggs a choice between X and Y | Look back at the mother's XX pair. There is no Y there for an egg to receive. |
| Putting both X and Y into one sperm | A gamete receives one member of the pair, not the whole body-cell pair. |
| Saying the mother determines the child's sex | Her contribution is X in every case. The changing contribution comes from the sperm. |
| Expecting a different result after several children of one sex | Each fertilisation is a new event. A previous sequence cannot make one sperm type due next. |
Try it
A Punnett square is drawn for the sex chromosomes of two human parents. Which statements about the completed square are true? Select all that apply.
- Only one box in four holds an X with a Y, a chance of 25 per cent
- Two of the four boxes hold two X chromosomes and two hold an X with a Y
- It predicts a 50 per cent chance of each sex for every child
- Three of the boxes hold two X chromosomes, because the mother supplies two of them
Which statements comparing the chromosomes of a human male and a human female are true? Select all that apply.
- Males and females carry entirely different sets of chromosomes
- The chromosomes that are not sex chromosomes occur only in females
- The 22 pairs that are not sex chromosomes are of the same types in both
- A male body cell holds only half as many chromosomes as a female body cell
- A gene on the pairs that are not sex chromosomes can be inherited by a child of either sex
A student states that every living thing settles sex with X and Y chromosomes in the way humans do. Why is that statement wrong?
- Because the human arrangement is one system among several that living things use
- Because sex is never determined by chromosomes in living things other than humans
- Because species differ in chromosome number, so no other species can have a sex chromosome pair
- Because every species has its own system, so no two species share one