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May 2025 Section II

0/34 marks

2025 · 6 questions · 32 parts · 34 marks

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1(A)1(B)(i)1(B)(ii)1(B)(iii)1(C)(i)1(C)(ii)1(C)(iii)1(D)(i)1(D)(ii)2(A)2(B)(i)2(B)(ii)2(C)(i)2(C)(ii)2(D)(i)2(D)(ii)3(A)3(B)3(C)3(D)4(A)4(B)4(C)4(D)5(A)5(B)5(C)5(D)6(A)6(B)6(C)6(D)

Context for question 6

The ald gene of fruit flies encodes the ALD protein, which is associated with both the centromeres of chromosomes and protein filaments produced during meiosis. In the absence of functional ALD proteins, gamete-producing cells enter anaphase I before homologous chromosomes are correctly aligned. As a result, the gametes produced do not contain the correct numbers of chromosomes. Scientists generated four mutations in the ald gene: ald1, ald3, ald23, and del, which was a deletion of the gene. To study the role of the ALD protein in meiosis, scientists used gameteforming metaphase cells from groups of flies with different ald genotypes. Some of the flies were homozygous for the wild-type allele of ald: WT/WT. Other flies were heterozygous for different ald alleles: WT/del; ald1/del; ald3/ald23; ald23/del. The scientists measured the percent of metaphase cells that contained ALD-associated filaments (Figure 1A) and the amount of ALD protein produced by each of the cell types (Figure 1B). Figure 1. (A) The average percent of gamete-forming metaphase cells that contained filaments associated with ALD and (B) the amount of ALD protein produced by each cell type. A thicker band indicates a greater amount of ALD protein.

6(A)Genome Structure, Mutations and MeiosisEasy1 mark

Based on Figure 1A, identify the fly genotype in which the average percent of metaphase cells with ALD-associated filaments is close to 12%.

6(B)Genome Structure, Mutations and MeiosisEasy1 mark

Based on Figure 1B, describe the difference in ALD protein production between gamete- forming metaphase cells of flies with the genotype ald3/ald23 and flies with the genotype ald23/del.

6(C)Mendelian and Sex-Linked InheritanceEasy1 mark

Scientists hypothesize that gamete-forming metaphase cells can produce a normal amount of ALD-associated filaments even when they produce about half as much ALD protein as the wild-type cells produce. Use the data in Figures 1A and 1B to support the scientists’ hypothesis.

6(D)Genome Structure, Mutations and MeiosisEasy1 mark

For gamete-forming metaphase cells of the WT/del and ald1/del flies, explain why the phenotypes observed in Figure 1A differ even though the amount of ALD protein produced (Figure 1B) does not.