In a particular sex-linked disease, progressive weakening of the muscles and loss of coordination lead to death before the age of five - HSC - SSCE Biology - Question 15 - 2008 - Paper 1
Question 15
In a particular sex-linked disease, progressive weakening of the muscles and loss of coordination lead to death before the age of five.
Using this pedigree, what is ... show full transcript
Worked Solution & Example Answer:In a particular sex-linked disease, progressive weakening of the muscles and loss of coordination lead to death before the age of five - HSC - SSCE Biology - Question 15 - 2008 - Paper 1
Step 1
Individual 1
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Answer
Individual 1 is a normal male. He does not show any symptoms of the sex-linked disease, so we can conclude he has the genotype for being normal, represented as X^N Y (where X^N is the normal X chromosome).
Step 2
Individual 2
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Answer
Individual 2 is a normal female. Her phenotype suggests that she has at least one normal X chromosome. Therefore, her genotype can be either X^N X^N (homozygous normal) or X^N X^n (heterozygous carrier). Since her offspring include affected males, she must be a carrier, thus her genotype is X^N X^n.
Step 3
Individual 3
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Individual 3 is an affected male. Males inherit only one X chromosome from their mothers. As Individual 2 is a carrier (X^N X^n), Individual 3 inherits the X^n chromosome, making his genotype X^n Y.
