What is meant by the term epistasis? Distinguish between epis tasis and dominance. Do not use examples in answering this question.

Genetic interactions are the cornerstone of understanding the complex ballet of gene expression, leading to an organism's observed characteristics, or phenotype. These interactions refer to the interplay between different genes and their respective alleles, how they influence one another, and ultimately shape the traits of an organism.

In a given organism, numerous genes are at play, often affecting similar or interconnected traits. The influence one gene might have over another can be direct or indirect, causing genes to be silenced, modified, or expressed more vigorously. It's this intricate network of 'on' and 'off' signals, and varying levels of expression that makes the genetic landscape rich and sometimes unpredictable.

Epistasis is a particular type of genetic interaction where one gene can override the effects of another, which is different from dominance, where one allele masks another within the same gene. Understanding genetic interactions helps elucidate why certain genes have the power to determine the presence or absence of a trait in ways that single-gene models cannot explain.

Allele expression refers to how the genes we inherit from our parents manifest through their alleles, or different versions of a gene. Each parent contributes one allele for every gene, resulting in pairs of alleles for each gene in their offspring. How these alleles are expressed governs the organism's phenotype.

There are dominant alleles, which can express the associated trait even when paired with a different (or recessive) allele. Recessive alleles, on the other hand, require pairs—two copies of the recessive allele—to exhibit the trait.

Factors including regulatory sequences, environmental influences, and the presence of other genes (like in epistasis) play critical roles in allele expression. Through these interactions, alleles might be expressed, suppressed, or modified, highlighting the importance of understanding not just which alleles are present but also the broader context of genetic expression.

Dominance in genetics describes the relationship between alleles of the same gene and their subsequent influence on the phenotype. In this hierarchical interaction, the dominant allele asserts itself whenever it is present, masking the expression of a recessive allele in a heterozygous pair.

While dominant alleles are not necessarily more common or 'stronger' in a physical sense, they have the ability to determine the phenotypic expression even in the presence of contrasting alleles. This makes dominance an essential concept in predicting the inheritance patterns of certain traits, particularly in simple single-gene inheritance scenarios.

Genes operate under a variety of dominance interactions—including complete dominance, incomplete dominance, and codominance—which further nuance our understanding of how traits are passed on and expressed.

The phenotypic outcomes of an organism—its physical appearance and biological functions—are the end result of genetic interactions, including the effects of both epistasis and dominance. The phenotype is what we can observe, from eye color and hair texture to enzyme function and disease resistance.

These outcomes can be simple, resulting from the straightforward expression of dominant-recessive pairs, or complex, involving interactions from multiple genes affecting a single trait in the case of epistasis.

By studying these phenotypic outcomes, scientists can trace back through the layers of genetic interactions to understand the underlying genetic architecture of traits. This approach helps clarify how heritable diseases manifest, or why certain characteristics are prevalent in populations, exemplifying the importance of genetics in deciphering the stories written in our DNA.