| Date | November 2016 | Marks available | 1 | Reference code | 16N.2.HL.TZ0.5 |
| Level | Higher level | Paper | Paper 2 | Time zone | TZ0 |
| 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.
Identify the genotype of the individual marked with a star in the pedigree chart, using appropriate symbols for your answer.
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.
XHXh «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.
