| Date | November 2013 | Marks available | 3 | Reference code | 13N.2.HL.TZ0.2 |
| Level | Higher level | Paper | Paper 2 | Time zone | TZ0 |
| Command term | Describe | Question number | 2 | Adapted from | N/A |
Question
The diagram shows a human karyotype.
Analyse this karyotype.
Outline the inheritance of hemophilia in humans.
Using an example, describe polygenic inheritance.
Markscheme
Male has (one X and) one Y chromosome / X chromosome is bigger than Y chromosome;
non-disjunction leads to three copies of chromosome 13/trisomy 13.
sex-linked/on X chromosome;
recessive allele / Xh;
more common in males than females;
heterozygous females are carriers / only females can be carriers;
more than one gene contribute to/control same characteristic;
as number of genes increase so does possible number of phenotypes;
leads to continuous variation;
specific example; (eg human skin color (due to differing amounts of melanin))
Award [2 max] for general points with no example.
Examiners report
The fact that there was a trisomy 13 as a result of non-disjunction eluded the majority, who seemed to register that pair 21 was OK, therefore nothing else could be wrong. Many lost a mark for not explaining why it was a male. Better prepared candidates were able to explain haemophilia and polygenic inheritance. For some candidates it seemed to be the first time that they had encountered them.
The fact that there was a trisomy 13 as a result of non-disjunction eluded the majority, who seemed to register that pair 21 was OK, therefore nothing else could be wrong. Many lost a mark for not explaining why it was a male. Better prepared candidates were able to explain haemophilia and polygenic inheritance. For some candidates it seemed to be the first time that they had encountered them.
The fact that there was a trisomy 13 as a result of non-disjunction eluded the majority, who seemed to register that pair 21 was OK, therefore nothing else could be wrong. Many lost a mark for not explaining why it was a male. Better prepared candidates were able to explain haemophilia and polygenic inheritance. For some candidates it seemed to be the first time that they had encountered them.
