Zadanie 6, Paper 2, listopad 2024, TZ2, Biologia IB SL
Wzorowane na: Biology SL Paper 2, November 2024, TZ2, pytanie 6. Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
Polecenie
A health study considers sickle-cell anaemia in a malaria-endemic region of Uganda. Individuals with two sickle-cell alleles may develop severe anaemia; heterozygotes generally lack that severe phenotype and have some protection against malaria.
(a) Outline how a single-base substitution can cause sickle-cell anaemia.
(b) Construct a Punnett grid to show how two parents without sickle-cell anaemia can have an affected child. Define your allele symbols.
(c) Explain why the sickle-cell trait can persist in this malaria-endemic population despite the harmful effects of anaemia.
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Odpowiedź z klucza
(a) One DNA base changes in the haemoglobin gene. ✔
Transcription produces an altered mRNA codon. ✔
For the sickle variant, coding DNA GAG changes to GTG, so mRNA GAG changes to GUG. ✔
A different amino acid is incorporated OR valine replaces glutamic acid. ✔
The beta-globin amino-acid sequence is altered. ✔
Haemoglobin behaviour changes and red cells can become sickle-shaped or carry oxygen less effectively. ✔
(b) Define A as the usual allele and S as the sickle allele; severe anaemia occurs with SS. ✔
Both parents are AS, producing A and S gametes. ✔
| A | S | |
|---|---|---|
| A | AA | AS |
| S | AS | SS |
Correct offspring grid. ✔
SS is the affected genotype, with probability 1/4. ✔
(c) Individuals with the protective trait are heterozygous OR carry one copy of the sickle allele. ✔
This trait provides an advantage where malaria is common. ✔
Heterozygotes are less likely to die from malaria. ✔
They therefore have greater opportunity to reproduce. ✔
They pass the sickle allele to offspring. ✔
The allele can remain in the population over generations. ✔
SS individuals have reduced survival due to anaemia. ✔
AA individuals are more vulnerable to malaria than heterozygotes. ✔
This is natural selection. ✔
Where malaria is absent, the heterozygote advantage is absent. ✔
Quality of communication: clear on first reading, concise and free from unnecessary repetition or irrelevant material. ✔
Schemat punktowania
Part (a): any 4, maximum 4. Exact codons or amino-acid names are acceptable but not all are required. Part (b): 1 symbols, 1 parental gametes, 1 completed grid, 1 affected genotype/probability; maximum 4. Part (c): any 7 distinct points, maximum 7. Award 1 independent communication mark. Maximum 16.