Advanced Placement (AP) Biology Practice Exam

In what way can the locus of a gene influence genetic traits?

1. By determining gene size

2. By affecting gene expression and location

3. By controlling the rate of mutation

4. By establishing the type of nucleotides

The correct answer is: By affecting gene expression and location

The locus of a gene plays a crucial role in influencing genetic traits primarily through its effect on gene expression and the specific location of the gene within the genome. The locus refers to the specific position or site of a gene on a chromosome, and this position can significantly influence how and when a gene is expressed. Gene expression is regulated by various factors such as the proximity to regulatory elements, the presence of enhancers and silencers, and the overall chromatin structure surrounding the gene. Different loci can have varying degrees of accessibility and interaction with transcription factors, leading to differences in how much of a gene is expressed, which in turn can affect the traits that are produced. Additionally, the location of the gene can influence its interaction with other genes and elements within the genome, which can further impact the phenotype. For example, genes located in heterochromatic regions may be less expressed than those in euchromatic regions due to differences in the compactness and accessibility of the chromatin. Thus, the correct answer highlights the pivotal role of a gene's locus in determining the level and timing of gene expression, which are fundamental factors in the manifestation of genetic traits.