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Date	May 2018	Marks available	1	Reference code	18M.2.HL.TZ1.3
Level	Higher level	Paper	Paper 2	Time zone	Time zone 1
Command term	Distinguish	Question number	3	Adapted from	N/A

Question

The land snail Cepaea nemoralis is very common in North America and in Europe.
The base colour of its shell varies between brown, pink and yellow, and also in its intensity. Some shells are unbanded, but most show one to five bands of different width on top of the shell base colour.

The genes coding for the shell base colour and the presence of banding have linked loci. Allele C causes a dark pink shell and is dominant to allele c, which codes for a dark yellow shell. Allele B codes for the absence of bands and is dominant to allele b, which codes for the presence of bands.

C. nemoralis is a mollusc. Identify two external features that distinguish this snail from an arthropod.

[2]

a.i.

State the major chemical component of the shell in molluscs.

[1]

a.ii.

Distinguish between sex-linked genes and genes with linked loci.

[1]

b.i.

Outline how it can be shown that the genes for shell base colour (Cc) and presence or absence of bands (Bb) are linked.

[3]

b.ii.

Markscheme

a. unsegmented body (whereas arthropods are segmented)

b. shell (versus exoskeleton in arthropods)

c. muscular foot (which arthropods do not have)

d. no jointed appendages/jointed legs (whereas arthropods have jointed legs/appendages)

e. slimy/mucus-covered / arthropod is not slimy

Do not award marks for any answers after the first two given.

a.i.

calcium carbonate/CaCO₃

Do not accept carbon or calcium – the mass of oxygen in calcium carbonate is greater than both these and the chemical component is the compound not its constituent elements.

a.ii.

sex-linked genes are on sex/X chromosome(s)/on chromosome 23 whereas genes with linked loci are on the same autosome/chromosome

b.i.

a. perform a cross/test cross

b. (if) double heterozygotes/CcBb are crossed with double homozygous recessives/ccbb
OR
Punnett square/genetic diagram showing CcBb crossed with ccbb
OR

c. (then) expected ratio (for unlinked genes) is 1:1:1:1

d. (if) double heterozygotes/CcBb are crossed together
OR
Punnett square showing CcBb crossed with CcBb
OR

e. (then) expected ratio (for unlinked genes) is 9:3:3:1

f. no/fewer than expected recombinants if genes are linked
OR
fewer pink banded/yellow unbanded if the genes are linked
OR
linked genes are expressed together more often than expected

g. use chi-square test (for significance of difference)

h. linked genes are on the same chromosome/diagram showing this

b.ii.