Distinguish between the structure of the chromosomes of prokaryotes and eukaryotes.

Outline the causes of sickle cell anemia.

Identify, with a reason, the sex of this individual.

State the haploid number for this nucleus.

A fly that is homozygous dominant for both body colour and wing size mates with a fly that is recessive for both characteristics. In the table, draw the arrangement of alleles for the offspring of this mating and for the homozygous recessive parent.

The offspring, which were all heterozygous for grey body and normal wings, were crossed with flies that were homozygous recessive for both genes. The table shows the percentages of offspring produced.

Explain these results, based on the knowledge that the genes for body colour and wing size are autosomally linked.

a. prokaryotes have circular DNA/chromosome but eukaryote chromosomes linear/OWTTE ✔
OR
eukaryotes have telomeres/centromeres whereas prokaryotes do not ✔

b. some prokaryotes have plasmids whereas eukaryotes do not ✔

c. eukaryotes have multiple chromosomes whereas prokaryotes «typically» have only one ✔

d. histones/nucleosomes/proteins associated with DNA in eukaryotes but not in prokaryotes/naked DNA in prokaryotes
OR
eukaryote DNA can coil/supercoil/condense «due to histones» but not prokaryote DNA ✔

a. genetic disease/caused by a gene
OR
inherited «from parents»
OR
caused by mutation «of a gene» ✔

b. base substitution
OR
GAG → GTG ✔

c. hemoglobin gene mutated / different allele/form/version of hemoglobin gene
OR
Hb^A → Hb^S ✔

d. leads to change in amino acid sequence «in hemoglobin»
OR
glutamic acid → valine ✔

e. only homozygotes have full disease/sickled cells / heterozygote has milder form
OR
hemoglobin crystallizes at low oxygen concentration ✔

f. «selected for/spreads in population» as it gives resistance to malaria ✔

male because «X and» Y chromosome present
OR
male because sex chromosomes/last two chromosomes/pair 21 are unpaired/different «from each other»/not homologous ✔

The answer must include “male” and the reason.

21

a. not a 1:1:1:1 ratio «because of linkage»
OR
not independent assortment
OR
grey normal and black vestigial types/parental combinations/double dominant and double recessive were commoner than 25 %/commoner than expected ✔

b. «linked genes» so were on the same chromosome ✔

c. grey body vestigial wing and black body normal wing are recombinants
OR
2 % plus 3 % of the offspring are recombinants ✔

d. recombinants due to crossing over/exchange of genes between «non-sister» chromatids
OR
2 % and 3 % of offspring were due to crossing over
OR
genes inherited together unless separated by crossing over ✔

e. crossing over between the two loci/between the two genes on the chromosomes ✔

f. few recombinants/not much crossing over because genes/gene loci close together ✔

Accept any of these points from an annotated diagram.

Answers to this question were variable. Some candidates wrote about cell structure rather than chromosome and many wrote about the location of chromosomes in the cell rather than their structure. Weaker candidates did not know the difference between prokaryotes and eukaryotes and some think that prokaryotes are plants and eukaryotes are animals. The term ‘naked’ was often used for DNA not enclosed in a nuclear membrane, when in this context it should be reserved for DNA that is not associated with histone proteins.

This question also elicited a wide range of answers. Some were accurate and detailed but some showed no knowledge of this genetic disease. There was lack of clarity in many answers, for example the disease was described as the mutation and sickling of cells was known but not linked to haemoglobin. The weakest answers tended to describe anemia in general, with iron deficiency sometimes given as the cause.

90 % of candidates correctly identified the individual as male, because the two sex chromosomes were different in size so must be an X and a Y. The other 10% mostly thought that there were two X chromosomes present, perhaps because pair 20 were X shaped.

Slightly more than 50 % of candidates got this right. A common wrong answer was 42 – diploid number. There were a variety of other answers, some of which must have been guesses from candidates who did not understand the term haploid.

This was generally well answered, with more than two thirds of candidates getting at least one of the genotypes correct. The best answers gave the alleles on lines that symbolised linked genes on a chromosome, but there was a potential pitfall here. If the alleles were arranged in a way that was impossible, given the nature of the cross, one mark was deducted. For example, each chromosome must have one of each gene, not two copies of one gene.

Most candidates found it very hard to explain the non-Mendelian ratio. A good start would have been to state the expected Mendelian ratio for unlinked genes in this type of cross and then compare this with the actual percentages. Answers tended to state which traits tended to be inherited together rather than explain the mechanism. An obvious answer was that that the two genes are located on the same chromosome but only a minority of candidates stated this and even fewer made the point that crossing over between the two gene loci results in the small percentage of recombinants. Many students referred to the 9:3:3:1 ratio, even though the cross performed in this case would not have given this ratio, even if there had not been gene linkage. Also many candidates claimed that more individuals showed dominant phenotypes than recessive, which was not supported by the percentages.