NES Profile: Biology (305)

Which of the following is the most likely cause of a base pair mutation?

1. Sister chromatids fail to separate during meiosis.
2. A small portion of one chromosome is added to another chromosome.
3. A segment of a chromosome breaks, flips, and reinserts itself.
4. The DNA is not accurately duplicated during replication.

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D. This question requires the examinee to analyze the types and causes of chromosomal and gene mutations, the consequences of these genetic changes, and the genetic basis of common disorders and diseases. Mutations that involve nucleotide base pairs typically result from the substitution, insertion, or deletion of one or more incorrect bases. These most commonly occur during the replication of DNA when the new complementary strands are being produced.

The key provides symbols for Affected Male, Affected Female, Unaffected Male, and Unaffected Female. In the first generation, an unaffected female crosses with an affected male. This pair produces 4 offspring, which from left to right are an unaffected male who crosses with an unaffected female, an affected female who crosses with an unaffected male, an unaffected male who crosses with an unaffected female, and an affected female who crosses with an unaffected male. The first pair from the second generation produces 2 unaffected females. The second pair produces an unaffected male, an affected female, an affected male, and an unaffected female. The third pair produces an unaffected female, an unaffected male, and an unaffected female. The fourth pair produces an affected male and an unaffected female.

The pedigree above shows the sex-linked inheritance pattern of a gene. This pattern of inheritance suggests the allele is:

1. X-linked dominant.
2. X-linked recessive.
3. Y-linked dominant.
4. Y-linked recessive.

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A. This question requires the examinee to analyze patterns of inheritance and the relationship between genotypic and phenotypic frequencies. The allele cannot be inherited as Y-linked, since females in the first generation of offspring express the trait. If the trait is X-linked recessive, the affected females of the first generation must be homozygous recessive and their offspring should show sex linkage, with the males affected and the females unaffected. This is not the case. The pattern in the pedigree is consistent with an X-linked dominant, however. The affected females of the first generation must be heterozygous for the trait so the expression of the trait in their offspring would not show sex linkage. This is the pattern in the pedigree.

The occurrence of similar features in organisms from different phylogenetic lineages can be attributed to:

1. gradualism.
2. stabilizing selection.
3. convergent evolution.
4. uniformitarianism.

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C. This question requires the examinee to analyze evolutionary patterns and the mechanisms of speciation. Organisms that are only distantly related may adapt by developing similar characteristics in response to similar selective pressures. This is known as convergent evolution.

Which of the following would provide the strongest evidence of a relatively close evolutionary relationship between two species?

1. Both species occupy similar niches in their respective ecological communities.
2. Fossils of ancestors of both species are found in the same geographic area.
3. The embryos of both species look almost identical until late stages of development.
4. Both species use the same 20 amino acids to synthesize peptides and proteins.

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C. This question requires the examinee to analyze different kinds of scientific evidence for evolution. Homologous patterns of development that appeared early are shared by all members of an evolutionary lineage. Homologies shared by some but not all members of a lineage suggest that the species sharing these homologies have a more recent common ancestor than other members of the lineage. Thus species whose embryos differ from each other only in the very late stages of development are likely to be more closely related to each other than species that diverged at earlier stages of development.