Describe the process of blood clotting.

Factor IX is a blood clotting protein which some hemophiliacs lack. In the future hemophilia could be treated using clotting factors synthesized by genetically modified bacteria. Outline the basic technique used for this gene transfer.

Explain how males inherit hemophilia and how females can become carriers for  the condition.

Remember, up to TWO “quality of construction” marks per essay.

a. clotting factor released by platelets/damaged tissue/cells;
b. cascade/series of reactions;
c. prothrombin (activated) to thrombin;
d. soluble fibrinogen to insoluble fibrin / thrombin converts fibrinogen to fibrin;
e. mesh of fibrin/fibres seals wound/traps platelets/red blood cells;

Remember, up to TWO “quality of construction” marks per essay.

a. mRNA/gene coding for factor IX extracted from human cell/tissue;
b. mRNA copied to DNA/cDNA (using reverse transcriptase);
c. plasmids used (for gene transfer);
d. restriction enzyme/endonuclease used to open plasmid/cut DNA;
e. complementary bases/sticky ends on gene and plasmid/link gene to plasmid;
f. sealed using ligase;
g. recombinant plasmid/plasmid containing desired gene taken up by bacteria;
h. isolate/clone the recombinant/transformed bacteria;
i. bacteria cultured/grown in fermenter to produce factor IX;

Remember, up to TWO “quality of construction” marks per essay.

a. hemophilia is due to a recessive allele/is a recessive trait / X^H is normal allele and X^h is hemophilia allele;
b. hemophilia is sex linked;
c. allele/gene is on the X chromosome;
Reject disease/hemophilia carried on X chromosome.
d. (sex chromosomes in) females are XX while males are XY;

e. Y chromosomes do not have the allele/hemophiliac males are X^hY;
f. males inherit their X chromosome from their mother/do not pass the allele to sons;
g. males have only one copy so recessive trait/allele is not masked;
h. males have a 50% chance of hemophilia/receiving the allele if mother is a carrier;

i. carrier is heterozygous for the gene/is X^HX^h;
j. dominant/normal allele masks the recessive allele (so clotting is normal);
k. females inherit one X chromosome from father and one from mother;
l. affected/hemophiliac males have carrier daughters;
m. hemophilia allele could have been inherited from either parent;

Accept the points above explained either in text or clearly using a Punnett grid or genetic diagram, but not for simply reproducing an unlabeled Punnett grid or diagram without explanation.

There were many general accounts of the sealing up of cuts with clotted blood but what was needed here was the process that leads to clotting. The programme specifies which stages in the cascade of reactions are expected and better answers included these and scored full marks without difficulty.

Marks for this part of the question covered the whole range. Among weaker candidate there were various misunderstandings about gene transfer and many confused gene transfer with gene therapy, describing the transfer of the Factor IX gene to haemophiliacs rather than to bacteria. In almost every case the method of gene transfer described in successful answers was that using plasmids. There were some detailed and accurate accounts of this process.  

Almost all candidates knew something about the inheritance of hemophilia. The mark scheme rewarded a wide range of relevant points as long as they were clearly made. Punnett grids could be used to illustrate particular points but they did not score marks in themselves. One area of confusion among weaker candidates was the difference between genes and chromosomes, with answers referring to dominant or recessive X chromosomes or implying that X was the gene for hemophilia.