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Date	November 2016	Marks available	1	Reference code	16N.2.HL.TZ0.5
Level	Higher level	Paper	Paper 2	Time zone	TZ0 / no time zone
Command term	Identify	Question number	5	Adapted from	N/A

Question

This is a pedigree chart of a family with hypophosphatemia, an X-linked condition, in which bone deformities occur because of poor absorption of phosphates into the blood.

Using the pedigree chart, deduce the type of allele that causes hypophosphatemia.

[2]

Identify the genotype of the individual marked with a star in the pedigree chart, using appropriate symbols for your answer.

[1]

Markscheme

a. dominant (allele)

Reject dominant disease/homozygous dominant.

b. all the offspring of the first generation would be affected if the allele was recessive (and one son is unaffected) / affected mothers could only have affected sons if the allele was recessive (and the pedigree shows that they can have both affected and unaffected sons) / affected mothers who have an unaffected son must be carriers of allele for being unaffected so the allele for being affected must be dominant / unaffected fathers could not have affected sons/daughters/children if the unaffected allele was dominant (and the pedigree shows that they can)

There must be a coherent argument here and not just observations about individuals on the pedigree chart, but the argument can be expressed in various ways and can be shown using a Punnett square or other genetic cross diagram. Do not accept arguments that involve ratios between the phenotypes.

X^HX^h «where H = hypophosphatemia and h = normal «absorption of phosphate»»

For the mark, allow any upper and lower case versions of the same letter, as long as they are shown superscript to an X to indicate
sex-linkage.

Examiners report

[N/A]

Syllabus sections

Core » Topic 3: Genetics » 3.4 Inheritance

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