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Inheritance, Variation and Evolution

Inherited disorders and sex determination

  • 2 things to know
  • Paper 2

Overview

Polydactyly and cystic fibrosis, their dominant and recessive alleles, the case for and against embryo screening, the 23 chromosome pairs, XX and XY, and the chance of a boy.

What you need to know

Answer each one before you reveal it.

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Key term

Is polydactyly caused by a dominant or a recessive allele?

Key term

Which part of the cell is affected in cystic fibrosis?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What are the symptoms of polydactyly?

    Model answer

    Having extra fingers or toes.

  2. 2

    Is polydactyly caused by a dominant or a recessive allele?

    Model answer

    A dominant allele.

  3. 3

    Which part of the cell is affected in cystic fibrosis?

    Model answer

    The cell membranes.

  4. 4

    Is cystic fibrosis caused by a dominant or a recessive allele?

    Model answer

    A recessive allele.

  5. 5

    Why can two parents who do not have cystic fibrosis have a child who does?

    Model answer

    The allele is recessive, so each parent can carry one copy without showing the disorder and pass it on.

  6. 6

    A parent with one polydactyly allele and one normal allele has children with a parent who carries no polydactyly allele. What proportion of the children is expected to have polydactyly?

    Model answer

    Half of them, because the allele is dominant and half the children inherit it.

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