Does it make sense to say that the DNA sequence determines the organism?

Geneticists are often said to find a gene responsible for a complex trait, as the quotes below show. What exactly is meant by a gene “for” a trait?

“‘Fat’ gene found by scientists” (The Times, April 13, 2007)

“Twins hold key to unravelling maths gene” (Guardian, August 7, 2005)

“Evidence for homosexuality gene: a genetic analysis of 40 pairs of homosexual brothers has uncovered a region on the X chromosome that appears to contain a gene or genes for homosexuality” (Science, July 16, 1993)

Often, we think of a simple quantitative trait such as the height of a human, the weight of a plant’s seeds, or the number of bristles on a fly. However, real phenotypes, taken as a whole, are far more complex: Think of the shape of an insect’s wing, the pattern of growth (i.e., body size through time), the chance of survival with increasing age, or crop yield as a function of amount of fertilizer. Explain how these kinds of trait can be described by quantitative genetics.

What is meant by a “variance component”?

Organisms function through an immensely complex network of interactions between multiple genes (e.g., Fig. 14.1). Why, then, can a simple additive model be used to describe variation in genetically complex traits?

Figure 14.15 shows estimates of the heritability of mental abilities, based on a study of twins. What does knowing that the heritability is about 50% tell us?

The first quantitative trait locus was mapped nearly a century ago (Payne 1918). Why, then, has QTL mapping only recently become widely used?

Why is it so difficult to find the precise variant that contributes to variation in a trait? (This problem is sometimes referred to as finding the QTN, or “quantitative trait nucleotide.”)