Chromosome inheritance in the XX/XY model
| English | Español |
|---|---|
| chromosome pair/ˈkrəʊməsəʊm peə/ | chromosome pair |
| sex chromosome/seks ˈkrəʊməsəʊm/ | sex chromosome |
What would explain this observation?
- The specified XX/XY cross predicts equal chances at each fertilisation. It does not require every family to have equal numbers of the two outcomes.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Ordinary human body cells contain 23 pairs of chromosomes. The specification’s basic model uses 22 other pairs and one sex-chromosome pair 染色体对. It represents female cells as XX and male cells as XY. An egg carries X; sperm carry X or Y. The model assumes equal probabilities for the two sperm types.
- sex chromosome 性染色体: A chromosome in the sex-chromosome pair of the specified model; chromosome pair: Two corresponding chromosomes in an ordinary body cell.
How many chromosomes are in an ordinary body cell in the specified human model?
Combining X with X gives XX; combining X with Y gives XY. A cross with egg labels X and X and sperm labels X and Y has two XX and two XY boxes. Expected ratio is 1:1 and probability is 1/2 for each outcome. This is the GCSE chromosome-inheritance model, not a complete account of human sex development or gender.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Combining X with X gives XX; combining X with Y gives XY. A cross with egg labels X and X and sperm labels X and Y has two XX and two XY boxes. Expected ratio is 1:1 and probability is 1/2 for each outcome. This is the GCSE chromosome-inheritance model, not a complete account of human sex development or gender.
- Draw the labelled cross, distinguish ordinary cells from gametes and show where each chromosome comes from. Use fictional families or large model data, without asking students about their own chromosomes or identities. Explain why a previous outcome does not change the next independent probability.
Which two habits make the investigation or model in this case more defensible?
Draw the labelled cross, distinguish ordinary cells from gametes and show where each chromosome comes from. Use fictional families or large model data, without asking students about their own chromosomes or identities. Explain why a previous outcome does not change the next independent probability.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: a supplied model has 80 independent fertilisations with equal XX/XY probability. Expected XX count is 80×1/2 = 40 and expected XY count is 40. An observed 43:37 split is possible; expectations are not guarantees. A body cell has 46 chromosomes, whereas a gamete has 23, including one sex chromosome.
A model gives XX probability 1/2 in 150 independent fertilisations. Find expected XX outcomes. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A model gives XX probability 1/2 in 150 independent fertilisations. Find expected XX outcomes.
The result is 75 outcomes. Known: a supplied model has 80 independent fertilisations with equal XX/XY probability. Expected XX count is 80×1/2 = 40 and expected XY count is 40. An observed 43:37 split is possible; expectations are not guarantees. A body cell has 46 chromosomes, whereas a gamete has 23, including one sex chromosome.
Check the conclusion and its limits
- An egg in the specified model does not contain both X and Y. The sex-chromosome pair is included in the 23 pairs, not added as a twenty-fourth pair. Equal probability does not alternate outcomes or require the next child to balance previous outcomes.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
The sex-chromosome pair is additional to the 23 chromosome pairs. This claim is false: An egg in the specified model does not contain both X and Y. The sex-chromosome pair is included in the 23 pairs, not added as a twenty-fourth pair. Equal probability does not alternate outcomes or require the next child to balance previous outcomes.
Chromosome inheritance in the XX/XY model: Combining X with X gives XX; combining X with Y gives XY. A cross with egg labels X and X and sperm labels X and Y has two XX and two XY boxes. Expected ratio is 1:1 and probability is 1/2 for each outcome. This is the GCSE chromosome-inheritance model, not a complete account of human sex development or gender.
The sex-chromosome pair is additional to the 23 chromosome pairs.
An egg in the specified model does not contain both X and Y. The sex-chromosome pair is included in the 23 pairs, not added as a twenty-fourth pair. Equal probability does not alternate outcomes or require the next child to balance previous outcomes.
A chromosome in the sex-chromosome pair of the specified model: write the technical term.
sex chromosome means A chromosome in the sex-chromosome pair of the specified model.