Alleles are variations in a genes nucleotide sequence at a specific locus. This is caused by ______.

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The question asks about the cause of allelic variation at a specific locus, which is tied to changes in the nucleotide sequence.
Option 1: 'RNA mutations'—In general, mutations in RNA are not the primary source of heritable genetic variation at the DNA level; RNA mutations can occur but are typically not passed on through reproduction unless they reflect underlying DNA changes or occur in retrotransposed elements. This makes it an unlikely primary cause of alleles.
Option 2: 'natural selection'—Natural selection acts on existing variation within a population and changes allele frequencies over time, but it does not itself create new alleles or variations in the nucleotide sequence at a locus. It explains the distribution of alleles, not their origin.
Option 3: 'DNA mutations'—Mutations in DNA change the nucleotide sequence of a gene at a specific locus, creating new variants (alleles). This is the direct mechanism that generates allelic variation.
Option 4: 'crossing over'—Crossing over during meiosis shuffles existing genetic material between homologous chromosomes, which increases genetic recombination and variation between chromosomes, but it does not create new allelic variants at a locus itself. It alters combinations rather than introducing new allele sequences per se.
Therefore, among the options, DNA mutations are the direct cause of allelic variations by introducing changes in the nucleotide sequence at the locus.

APBIO141-031 with labs 14.04 to 14.06 - Video Quiz

Alleles are variations in a genes nucleotide sequence at a specific locus. This is caused by ______.

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