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Date	November 2021	Marks available	2	Reference code	21N.2.HL.TZ0.2
Level	Higher level	Paper	Paper 2	Time zone	TZ0 / no time zone
Command term	Explain	Question number	2	Adapted from	N/A

Question

Hemophilia is a sex-linked disease that affected some members of royal families in Europe during the 19th and 20th centuries.

The diagram shows the structure of two chromosomes after the first division of meiosis, assuming that there was no crossing over or chiasmata formation.

Draw a Punnett square to show all the possible genotypes of Queen Victoria’s children.

[2]

a.i.

Deduce the genotype of Queen Victoria’s daughter Alice.

[1]

a.ii.

Draw the same chromosomes to show their structure at the same stage of meiosis if there had been one chiasma between two gene loci.

[1]

b.i.

State the stage of meiosis where chiasmata formation may occur.

[1]

b.ii.

Explain gene linkage and its effects on inheritance.

[2]

Explain the mechanism that prevents polyspermy during fertilization.

[2]

Markscheme

a. parental alleles shown as X^H and X^h (female) and X^H and Y (male);
b. Punnett square with genotypes of offspring shown as X^HX^H and X^HY and X^HX^h and X^hY;

a.i.

X^H X^h;

a.ii.

all four upper arms with one A and both chromosomes with one B and one b on the lower arms;

The chromatids can be shown as single lines rather than the wider versions in the question.

b.i.

prophase I;

b.ii.

a. located on the same chromosome;
b. genes/gene loci close together (on the same chromosome);
c. do not follow (the law of) independent assortment;
d. more chance of recombination if genes are further apart;
e. inherited together unless crossing over/recombination occurs;
f. ratios of offspring in dihybrid crosses are different from expected/non-Mendelian
OR
more offspring with parental phenotype combinations than expected;

a. cortical reaction (after first sperm nucleus enters the egg);
b. vesicles/cortical granules release their contents/enzymes (from the egg/zygote);
c. zona pellucida/glycoprotein coat/outer coat hardened / fertilization membrane formed;
d. enzymes of sperm/acrosome cannot digest (hardened coat)
OR
glycoproteins/ZP3 (in zona pellucida) altered so sperm cannot bind;

Examiners report

Generally this was well answered. Common errors were to omit X and or Y chromosomes, show alleles on both X and Y chromosomes, or show the male X chromosome with a recessive h allele.

a.i.

Any symbols were accepted for the alleles, as long as they were shown superscript to X and the genotype was heterozygous. 70 % of candidates answered correctly.

a.ii.

This was mostly well answered but some candidates showed one chromatid flopped over a non-sister chromatid, like crossed legs of a seated person, without any recombination having occurred. This showed a misunderstanding of the process of crossing over and chiasma formation. There were also a wide range of other answers showing confusion about events in meiosis.

b.i.

Mostly Prophase I was given as the answer but all other phases of Meiosis I were given by some candidates and others failed to specify 1st or 2nd division of meiosis.

b.ii.

The was some confusion between autosomal and sex linkage. Candidates tended either to have a secure understanding of the mechanisms at work in autosomal gene linkage, or none. This question and (d) correlated closely with candidates’ overall score on the paper, so distinguished particularly effectively between stronger and weaker candidates.

Answers were mixed. Most knew that there is a mechanism that prevents more sperm fusing with the egg after the first one and some gave accurate and detailed accounts of it.

Syllabus sections

Additional higher level » Topic 11: Animal physiology » 11.4 Sexual reproduction

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Additional higher level » Topic 11: Animal physiology

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