Chromosome inheritance in the XX/XY model
Đã giới thiệu| English | Ý nghĩa |
|---|---|
| chromosome pair/ˈkrəʊməsəʊm peə/ | cặp nhiễm sắc thể |
| sex chromosome/seks ˈkrəʊməsəʊm/ | nhiễm sắc thể giới tính |
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?
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An ordinary human body cell has 46 chromosomes - 23 pairs including the sex pair. 23 is the gamete count, and 48 would be an extra pair.
Match each technical term to its precise meaning.
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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?
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Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
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.
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The result is 75 outcomes. Known: XX probability 1/2; 150 independent fertilisations. Expected XX = 150 x 1/2 = 75 outcomes. Expectations do not force any observed split to balance.
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: The sex chromosomes are one of the 23 pairs - the twenty-third pair. They are included in the count of 46, not additional to it.
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.
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The sex chromosomes are one of the 23 pairs - the twenty-third pair. They are included in the count of 46, not additional to it.
A chromosome in the sex-chromosome pair of the specified model: write the technical term.
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sex chromosome means A chromosome in the sex-chromosome pair of the specified model.