Chromosome inheritance in the XX/XY model
הוצג| English | משמעות |
|---|---|
| chromosome pair/ˈkrəʊməsəʊm peə/ | זוג כרומוזומים |
| sex chromosome/seks ˈkrəʊməsəʊm/ | כרומוזום מיני |
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.
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?
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
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.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
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.
In the lab: 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?
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
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
- 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.
Your turn: A model gives XX probability 1/2 in 150 independent fertilisations. Find expected XX outcomes.
A model gives XX probability 1/2 in 150 independent fertilisations. Find expected XX outcomes.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
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.
Not true: The sex-chromosome pair is additional to the 23 chromosome pairs. 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.
- Ordinary human body cells contain 23 pairs of chromosomes.
- Combining X with X gives XX; combining X with Y gives XY.
- Watch for: an egg in the specified model does not contain both X and Y.
The sex chromosomes are one of the 23 pairs - the twenty-third pair.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
True. 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.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
sex chromosome means A chromosome in the sex-chromosome pair of the specified model.