Final answer:
In genetics, 'modifier' genes change how another gene expresses a trait, whereas 'regulatory' genes initiate or stop a gene's expression. 'Epistasis' is when the expression of one gene pair affects another gene pair's expression. Gene expression is often regulated by transcription factors that can turn other genes on or off.
Step-by-step explanation:
The question you're asking relates to how genes interact with each other in the process of gene expression. In terms of genetic interactions, ___ genes change how another gene expresses a trait while the ___ genes start or stop another gene's expression. The first blank should be filled with 'modifier' genes, as they can alter how another gene expresses a trait, often by enhancing or diluting its effect. The second blank refers to 'regulatory' genes, which are responsible for initiating or halting the expression of other genes. An important concept in genetic interaction is epistasis, which occurs when the expression of one gene pair masks or modifies the expression of another gene pair.
Another key point in understanding gene expression is the role of regulatory proteins, such as transcription factors, which can turn genes on or off through positive or negative regulation. Inducible genes are switched on by stimulatory proteins when needed, while constitutive genes are expressed at a constant level without regulation. A classical example of gene regulation is seen in the fruit fly Drosophila, where the slx gene determines sex based on the presence of X chromosomes and directly affects the splicing of the tra gene, which in turn influences the expression of other genes related to sexual characteristics.