Describe the characteristics of stem cells that make them potentially useful in medicine.

Outline the inheritance of a named sex-linked condition in humans.

Explain the use of karyotyping in human genetics.

(stem cells) have/retain the capacity to divide;
can be used to produce cell cultures/large number of identical cells;
can be used to repair/replace damaged/lost cells/tissue;
(stem cells) are undifferentiated / have not yet differentiated/specialized;
can differentiate/specialize in different ways / are pluripotent/totipotent;
can be used to form a variety of different tissues / form organs;
used in medical research;
used in treatment of (named) disease;

genes that are located on just one of the sex chromosomes/X or Y are sex-linked;
(sex-linked) genes present on the X chromosome are absent from the Y chromosome / vice versa;
named recessive X-linked condition (e.g. colour blindness / haemophilia / other valid example);
sex-linked conditions tend to be more commonly expressed in males;
female can be homozygous or heterozygous/carrier for a sex-linked/X-linked condition;
affected males have only one copy of the gene / have carrier daughters but cannot pass the condition on to sons;
carrier/heterozygous females can have affected sons/carrier daughters;
for a female to be affected (homozygous recessive) the father must be affected;

If the example used is of a recessive X-linked condition, use marking points c–h.
Make appropriate adjustments if the example is of a dominant X-linked trait or a Y-linked trait.
Accept any of the above points shown in a suitable diagram/chart/Punnett square/pedigree.

Definition and construction of karyotypes:
karyotype is the number and type / image of chromosomes in a cell;
cells collected from chorionic villus / by amniocentesis;
requires cells in metaphase / stimulate cells to divide and reach metaphase;
burst cells and spread chromosomes / photo taken of chromosomes;
chromosomes are arranged in pairs;
according to size/structure/position of centromere/banding pattern;

Uses for karyotypes:
karyotypes used to identify sex/gender;
male is XY and female XX;
used to identify chromosome mutations/abnormal numbers/non-disjunction;
Down syndrome due to extra chromosome 21 / other trisomy/aneuploidy example;
used for pre-natal diagnosis of chromosome abnormalities;
may lead to a decision to abort the fetus;
prepare for consequences of abnormality in offspring;

Many candidates knew that stem cells retain the capacity to divide and can differentiate into different tissues. It was frequently mentioned that stem cells can replace damaged cells and form a variety of tissues. Candidates knew about medical applications for stem cells such as in treatment for leukemia and for growing skin to help burn victims.  

Many candidates showed excellent knowledge about sex linked inheritance, using hemophilia as their example.  Although many candidates gained the max of 5 marks, the importance of the X chromosome in sex-linked conditions was not always understood.  There was a lack of understanding of “carrier” vs “affected.” A few candidates gained marks with annotated Punnet Squares. For those who failed this question, there was an inability to explain the genetics of inheritance and the inaccurate choice of sickle cell anemia as the example. Some regarded alleles as chromosomes. There appears to be weakness in the learning of Topic 4 among more than a few candidates.  

There were many strong answers to this question. The technique and the uses of karyotypes were well stated. Collectively, candidates answered all the marking points. Some candidates failed to point out the obvious e.g. XY is male and XX is female. Some candidates referred to Down syndrome as inherited on chromosome 23. Others did not use the term chromosome for the abnormality and referred to genes instead. As mentioned earlier in this report, karyotyping was confused with a variety of other laboratory procedures from DNA fingerprinting to gel electrophoresis.