Fig. 1 shows a pedigree from an optometry practice. Some males in the family have red-green colour blindness. The condition is caused by a recessive allele ...
Question 1 Report
Fig. 1 shows a pedigree from an optometry practice. Some males in the family have red-green colour blindness. The condition is caused by a recessive allele carried on the X chromosome. The normal allele is written XN and the colour-blind allele is written Xn. Individual II-2 has normal colour vision but has a colour-blind son.
(a) What is the genotype of individual II-2? [2]
(b) Describe why the colour-blind child in Fig. 1 is male. [2]
(c) Give the allele inherited from the father by the colour-blind son. [1]
(d) Complete a genetic diagram for a carrier mother and a father with normal colour vision. [2]
(e) Give the probability that a son of this couple will be colour blind. [1]
Answer Details
(a) Individual II-2 has normal vision but has a colour-blind son. She must therefore be a carrier: \(X^N X^n\). [2]
(b) A son receives a \(Y\) chromosome from his father. He has only one \(X\) chromosome, inherited from his mother, so a single \(X^n\) allele is expressed because there is no normal allele on a second \(X\) chromosome. [2]
(c) The colour-blind son inherited the \(Y\) allele-bearing chromosome from his father. [1]
(d) A carrier mother is \(X^N X^n\) and a father with normal colour vision is \(X^N Y\).
| \(X^N\) | \(Y\) | |
|---|---|---|
| \(X^N\) | \(X^N X^N\) | \(X^N Y\) |
| \(X^n\) | \(X^N X^n\) | \(X^nY\) |
The possible offspring are \(X^N X^N\), \(X^N X^n\), \(X^N Y\), and \(X^nY\). [2]
(e) Of the possible sons, half inherit \(X^n\) from the mother. The probability that a son is colour blind is \(50\%\), or one half. [1]
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