Zadanie 7, Paper 2, specimen 2025, Biologia IB SL

(a) Maximum 4:

Mutations are changes in DNA sequence that arise without being directed towards a needed outcome. ✔

They may involve substitutions, insertions, deletions or rearrangements. ✔

Control of cell division can be lost in affected somatic cells. ✔

Abnormal cells may fail to carry out their usual functions. ✔

Repeated division of affected cells can form a tumour. ✔

(b) Maximum 4:

An appropriate example is phenylketonuria (PKU). ✔

Mutations affect the PAH gene OR the gene encoding phenylalanine hydroxylase. ✔

Both parents carry a disease-associated allele OR an affected person inherits such an allele from each parent (heterozygous carriers are usually unaffected). ✔

Without treatment, phenylalanine can accumulate to harmful concentrations OR tyrosine production is reduced. ✔

The enzyme defect reduces conversion of phenylalanine into tyrosine. ✔

Dietary management limits phenylalanine intake. ✔

(c) Maximum 7:

Mutations can produce new phenotypes. ✔

They increase genetic variation in a population. ✔

Some variants improve survival. ✔

The effect depends on environmental conditions or selection pressures. ✔

Competition or abiotic conditions can act as selection pressures. ✔

Natural selection changes the success of different variants. ✔

An advantageous variant can allow longer survival. ✔

Its carriers may then have more opportunities to reproduce. ✔

A heritable allele can pass to descendants. ✔

Allele frequencies can change over generations. ✔

Natural selection can therefore cause evolutionary change. ✔

Construction: 1 mark for clear, concise communication across the response.

Award any 4 listed points in (a), any 4 in (b) and any 7 in (c). Maximum 15 content marks plus 1 independent construction mark, total 16. In (a), general consequences of somatic mutation are covered by SL D1.3.6; detailed proto-oncogene and tumour-suppressor mechanisms are not required. In (b), accept another accurate autosomal recessive example with matching gene, inheritance, effects and treatment.