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Date	November 2018	Marks available	3	Reference code	18N.2.HL.TZ0.7
Level	Higher level	Paper	Paper 2	Time zone	TZ0 / no time zone
Command term	Describe	Question number	7	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]

Outline the causes of variation in one example of continuous variation in humans.

[5]

Explain how evolution occurs and which factors can cause the process to be rapid.

[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 is heterozygous/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. height/skin colour/other valid example ✔

b. with continuous variation any level of the variable/phenotype is possible/OWTTE ✔

c. polygenic inheritance/combined effect of more than one gene on the trait ✔

d. additive effects on the trait of alleles of multiple genes/OWTTE ✔

e. histogram showing effects of alleles of multiple genes ✔

f. environment «may» also affect the trait/sunlight affects skin colour/other example ✔

g. normal distribution curve drawn or described to show typical pattern with continuous variation ✔

a. evolution is a change in the heritable characteristics «of a species» ✔

b. natural selection «causes evolution» ✔

c. overpopulation/over-reproduction/more offspring «than the environment can support» ✔

d. competition «for resources/mates» ✔

e. variation in population/species ✔

f. mutation/meiosis/sexual reproduction contributes to variation ✔

g. adaptation increases chance of survival ✔

h. reproduction/offspring produced «by the better adapted/by those that survive» ✔

i. characteristics passed to offspring by reproduction/variation is heritable ✔

j. allele frequencies/number of organisms carrying a gene changes/gene pool changes ✔

k. environmental change stimulates/triggers/speeds up natural selection/evolution ✔

l. increase in rainfall/introduction of antibiotic/pollution on tree trunks/other valid example of environmental change/new selection pressure ✔

m. artificial selection/selective breeding can speed up evolution ✔

Mark points can be awarded if explained using an example.

Examiners report

[N/A]

Syllabus sections

Core » Topic 3: Genetics » 3.4 Inheritance

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