Draw a labelled diagram of a mature sperm.

Outline the formation of chiasmata during crossing over.

Explain how an error in meiosis can lead to Down syndrome.

Award [1] for each of the following clearly drawn and correctly labelled.
head and midpiece/mid-section/body;
tail/flagellum; (at least four times length of the head and containing fibres)
acrosome; (shown as distinct structure near front of head)
nucleus; (occupying more than half the width or length of head)
mitochondria; (as repetitive structures inside membrane of mid piece)
centriole; (between head and midpiece)
(plasma) membrane; (shown as single line covering whole cell)
microtubules; (in 9 plus 2 array)

crossing over/chiasmata formed during prophase I of meiosis;
pairing of homologous chromosomes/synapsis;
chromatids break (at same point); (do not accept chromatids overlap)
non-sister chromatids join up/swap/exchange alleles/parts;
X-shaped structure formed / chiasmata are X-shaped structures;
chiasma formed at position where crossing over occurred;
chiasmata become visible when homologous chromosomes unpair;
chiasma holds homologous chromosomes together (until anaphase);
Accept the above points in an appropriately annotated diagram.

non-disjunction;
chromosomes/chromatids do not separate / go to same pole;
non-separation of (homologous) chromosomes during anaphase I;
due to incorrect spindle attachment;
non-separation of chromatids during anaphase II;
due to centromeres not dividing;
occurs during gamete/sperm/egg formation;
less common in sperm than egg formation / function of parents' age;
Down syndrome due to extra chromosome 21;
sperm/egg/gamete receives two chromosomes of same type;
zygote/offspring with three chromosomes of same type / trisomy / total 47 chromosomes;
Accept the above points in an appropriately annotated diagram.

In part (a) the sperm drawings were mostly neat but few candidates scored full marks. Five structures shown realistically and correctly labelled were needed. The nucleus was often shown insufficiently large. Fibres and microtubules were missing from the tail. Centrioles were missing from many drawings and the plasma membrane, head and mid-piece of the sperm were often not labelled

Many candidates also lost marks in part (b) by giving insufficient detail or by including errors in their answers. Candidates were expected to use the terms meiosis, homologous chromosomes and non-sister chromatids. A frequent error was to suggest that the tight linkage between sister chromatids that exists when crossing over takes place is broken prior to crossing over, and that regions of non-sister chromatids become linked instead. This would of course not result in the chiasmata that remain clearly visible throughout metaphase I of meiosis.  

Candidates should be taught that crossing over occurs by breakage of non-sister chromatids and their connection to each other, forming a knot-like chiasma. Chiasmata serve the essential function of preventing non-disjunction by holding homologous chromosomes together when the tight pairing or synapsis has ended.

Answers to part (c) were good in most cases. Diagrams were often included, candidates need to label them fully if they are to help answer the question. Some included more details of the normal process of meiosis than was expected and also symptoms of Down syndrome that were not really relevant.  

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