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Date	May 2019	Marks available	4	Reference code	19M.2.HL.TZ2.7
Level	Higher level	Paper	Paper 2	Time zone	Time zone 2
Command term	Outline	Question number	7	Adapted from	N/A

Question

Distinguish between the structures of DNA and RNA.

[3]

Mendel found the same pattern of inheritance in all the crosses that he performed. Outline, with examples, different types of inheritance that produce non-Mendelian ratios.

[4]

Explain the cause of sickle cell anemia and how this disease affects humans.

[8]

Markscheme

A table format is not required but clear distinctions must be apparent.

The full names of the bases must be given.

a. some traits may involve many genes/be polygenic eg: height, skin colour «correct example required» ✔ Accept any verifiable examples of these types of inheritance.

b. linked genes/alleles of different genes on same chromosome ✔

c. «small numbers of» recombinant phenotypes due to crossing over «between linked genes» ✔

d. co-dominance of specific alleles/intermediate forms eg: pink flowers «from red and white ones»/blood groups «correct example required» ✔

e. sex-linked effects eg: colour blindness «correct example required» ✔

f. environmental influence on inheritance/epigenetics/methylation ✔

g. any other example of non-Mendelien inheritance with a specific example ✔

a. caused by a single nucleotide/base substitution mutation/GAG to GTG ✔

b. «mutation of» a gene of β-globin/a subunit of hemoglobin ✔

c. mRNA copies the mutation of DNA and substitutes an amino acid in hemoglobin «subunit» ✔

d. glutamic acid is substituted by valine ✔

e. sickle cell anemia involves distorted hemoglobin protein/HbS ✔

f. «distorted HbS causes» distortion/sickling/shape change of red blood cells ✔

g. «distorted/sickled red blood cells» block capillaries/blood flow ✔

h. HbS/sickled red blood cells cannot carry enough oxygen «for the body»/leads to fatigue ✔

i. low oxygen concentration seriously affects structure of HbS ✔

j. homozygous «HbS/HbS» state causes severe anemia/death at low oxygen concentrations ✔

k. heterozygous state has less anemia/minor effects/less effect of structure of hemoglobin
OR
heterozygous state only affected at high altitude/extreme exercise/low levels of oxygen ✔

l. «heterozygous state» provides protection against malaria parasite/selective advantage in malaria areas ✔

OWTTE

Examiners report

This question was well answered by most. Students are cautioned against representing bases with letter symbols such as A,C,T,G and U. The expectation of the mark scheme is that students write out the full names.

This question was less well answered. Students commonly indicated linked genes and sex-lined genes as examples of non-Mendelian inheritance. Accurate discussion of co-dominance was less frequent.

This question was well answered. The mutation and its impact was well understood by most. The genetic causes were less frequently discussed.

Syllabus sections

Core » Topic 3: Genetics » 3.4 Inheritance

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