Describe the causes of Down syndrome.

Describe how human skin colour is determined genetically.

Explain the causes of sickle-cell anemia.

Down syndrome is caused by non-disjunction;
occurs during meiosis;
chromosome pairs fail to separate in meiosis I / chromatids in meiosis II / anaphase II;
some gametes have an extra chromosome;
can lead to zygotes/individuals with an extra chromosome / individual has 47 chromosomes;
in Down syndrome this would be trisomy 21/extra chromosome 21;
increased probability with increased age of mother/ages of parents;

skin colour is an example of polygenic inheritance;
many/more than two genes contribute to a person’s skin colour;
due to the amount of melanin in the skin;
combination of alleles determines the phenotype;
allows for range of skin colours / continuous variation of skin colour;
phenotypes do not follow simple Mendelian ratios of dominance and recessiveness;
the environment also affects gene expression of skin colour / sunlight/UV light stimulate melanin production;
the more recessive alleles there are, the lighter the skin colour; (vice versa)

caused by gene mutation;
(sickle-cell anemia) due to a base substitution (mutation);
changes the code on the DNA;
which leads to a change in transcription / change in mRNA;

DNA changes from CTC to CAC/GAG to GTG / mRNA changes from GAG to GUG; (accept DNA changes from CTT to CAT/GAA to GTA / mRNA changes from GAA to GUA)

which (in turn) leads to a change in translation / change in polypeptide chain/ protein;
(the tRNA) adds the wrong amino acid to the polypeptide chain;
glutamic acid replaced by valine;
produces abnormal hemoglobin;
causing abnormal red blood cell/erythrocyte shape / sickle shape;
which lowers the ability to transport oxygen;
sickle-cell allele is codominant;
homozygote/Hb^S Hb^S have sickle cell anemia/is lethal / heterozygote/Hb^S Hb^A has the sickle trait/is carrier (and is more resistant to malaria);

In part (a) most candidates got several marks but few the full five for describing the causes of Down syndrome. There was confusion as to when this occurs and how,. The most common statements were mp e, f and g.

Many candidates lost marks in part (b) by not knowing skin colour is an example of polygenic inheritance and describing a dihybrid inheritance. Others simply gave an incomplete account for 5 marks and many were confused as to the difference between alleles and genes. 

Answers to part (c) were good in many cases. In these scripts, there was complete detail on the cause, the DNA and amino acid changes and the effect on haemoglobin. This is obviously a topic that is well taught in many centres although some students are confused about the effect of the mutation on haemoglobin and its subsequent effect on the shape of the red blood cell.