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Date	November 2018	Marks available	3	Reference code	18N.2.SL.TZ0.6
Level	Standard level	Paper	Paper 2	Time zone	TZ0 / no time zone
Command term	Describe	Question number	6	Adapted from	N/A

Question

Isolated communities in rural Finland, Hungary and some of the Scottish islands have a high incidence of red-green colour blindness. Describe the inheritance of red-green colour blindness.

[3]

The human hand is an example of adaptive radiation. Outline adaptive radiation.

[5]

Explain how the human body defends itself against pathogens.

[7]

Markscheme

a. sex linked/gene is on the X chromosome ✔

b. allele «for red-green colour blindness» is recessive/colour blindness is recessive trait/disorder ✔

c. heterozygous females are unaffected/carriers ✔

d. X^B denotes normal allele and X^b denotes colour blindness allele ✔

e. more frequent in males because they only have one X chromosome ✔

f. 50 % chance of colour blindness in sons whose mother who is heterozygote/X^BX^b✔

Accept any other letter for the alleles.

Award mpb, mpc, mpd and mpf if these points are clearly made on a Punnett grid.

a. «happens in a group of species that» evolve from a common ancestor ✔

b. evolution «of a structure» in different ways ✔

c. for different functions ✔

d. common features remain «despite the differences» ✔

e. homologous structures are evidence «of adaptive radiation»

Must see “homologous” ✔

f. an example of adaptive radiation ✔

g. example of specific adaptation ✔

h. second description of a specific adaptation ✔

example 1:

f. pentadactyl limb

g. human hand is adapted for grasping/climbing/manipulation

h. front limb of mole is adapted for digging»

example 2:

f. «Darwin’s» finches’/birds’ beaks

g. nectar feeding has a long/thin beak

h. seed feeding has a short/stout beak

a. skin/mucous membranes primary/first defence against pathogens ✔

b. tears/mucus contain enzymes/lysozymes which destroy pathogens ✔

c. stomach/skin/some mucus produces acid which kills pathogens ✔

d. phagocytic white blood cells/phagocytes/macrophages ingest pathogens
OR
lysosomes in phagocytes/macrophages release enzymes that digest pathogens ✔

e. phagocytes/macrophages give non-specific immunity «to diseases» ✔

f. specific immunity provided by lymphocytes ✔

g. lymphocytes divide to produce clones «of plasma cells» ✔

h. plasma cells/lymphocytes produce antibodies ✔

i. antibodies are specific to a pathogen/antigen ✔

j. memory cells provide immunity against future attacks by same pathogen ✔

k. blood clotting/fibrin closes opening in the body so pathogens cannot enter✔

Accept leukocytes instead of white blood cells