Final answer:
A locus influencing a trait with quantitative variation is known as a quantitative trait locus (QTL). It's a specific chromosome location associated with variation in a measurable trait, typically influenced by multiple genes and environmental factors.
Step-by-step explanation:
A locus that influences a trait showing quantitative variation is known as a quantitative trait locus (QTL). A QTL is a specific location on a chromosome that correlates with variation in a quantitative trait, which is a trait that can be measured and is usually influenced by multiple genetic and environmental factors. Traits that are qualitative, or discrete, like Mendel's peas, are usually controlled by a single gene with two alleles: a dominant allele and a recessive allele, represented by variables p for the dominant allele and q for the recessive allele in population genetics models.
In regard to the genetic context provided, if traits are dominant at each locus, the alleles can be represented as A? B? C? and if traits are dominant at two genes and recessive at the third, this reflects the combination of alleles at those loci which determine the phenotype. Additionally, a genetic marker is a gene or sequence on a chromosome identified to be linked with a specific trait, helping in the study of heredity. As explained, geneticists use the term phenotype to describe the observable traits expressed by an organism, influenced by both the genotype, the genetic constitution, and environment.