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AP Biology 6.7.B: How Genotype Changes Affect Phenotype

Practise AP Biology 6.7.B questions by connecting DNA damage, chromosome changes and mutation effects to altered protein function, genetic variation and phenotype.

Syllabus
Effective Fall 2025
Course
AP Biology

Exam points

  • explain how replication or repair errors, radiation and chemical mutagens create DNA mutations
  • predict how genotype changes alter protein structure or function and produce context-dependent phenotypes
  • explain how nondisjunction and chromosome-structure changes can alter gene dosage and phenotype

6.7.B—Explain how changes in genotype may result in changes in phenotype question 1

[Maximum number: 4]

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.

Figure for Question 6.7.B—Explain how changes in genotype may result in changes in phenotype question 1 — AP Biology

Question (a)

(a)

Based on Figure 1A, identify the fly genotype in which the average percent of metaphase

cells with ALD-associated filaments is close to 12\%.

[ 1 ]

Question (b)

(b)

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.

[ 1 ]

Question (c)

(c)

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.

[ 1 ]

Question (d)

(d)

For gamete-forming metaphase cells of the W T / 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.

[ 1 ]
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