Are all traits in an organism controlled by only one gene per trait?
No - polygenic inheritance controls several traits - eg.height, weight, eye colour.
Are all traits in an organism controlled by only one gene per trait?
Not at all! Many traits are polygenic. Height is controlled by something like 50 genes, for example (from a purely genotypic standpoint). Check out this study (well, its abstract): http://www.ncbi.nlm.nih.gov/pubmed/19818695 Conversely, some genes are pleiotropic, and control multiple (often totally unrelated) traits. Marfan Syndrome is one example of pleiotropy.
How do genes control the traits of an organism?
Genes are coded for by triplets of DNA bases called codons. A single gene could have hundreds or thousands of codons. An allele is just another form of the same gene. For example blue and brown eyes, although in reality it's not as simple as that. Organisms from the same species will likely have the same genes but not necessarily the same alleles of those genes. When a particular gene is expressed it will result in a phenotype which we can observe, although some are.not.easily seen on the surface. I'm not entirely sure what else to write without writing your essay for you which is not the purpose of this site. I suggest you read a bit into DNA transcription and expression.
How does an organism know what genes are linked to what traits?
Organisms - not counting humans - have no idea what genes are linked to what traits. This is handled at the cellular level, and organisms are unaware that they have cells.If you’re asking how genes lead to traits, I’ll give you a quick overview.All life is made up of cells. Cells are actually like miniature cities; they are populated by organelles (analogous to organs in your body), they have roads (microtubules, actin, etc.), they have factories (ribosomes) they have a boss (DNA), they have all kinds of things in them. What types of cells you have determines your traits - obviously there are dramatic difference between brain cells and skin cells, for example.The nature of the cell itself is largely determined by its protein composition. Proteins are the building blocks of cells; most of a cell is made of protein (if you don’t count the water). Proteins are made by protein-factories called ribosomes, and protein factories make what the DNA tells them to make.DNA stays in the nucleus and ribosomes generally don’t, so DNA unzips itself and makes a miniature copy called messenger RNA (mRNA) which it sends out of the nucleus to give the ribosome the message of what protein to make.DNA is made of nucleotides that we abbreviate A, T, C, and G. It has millions or even billions of these nucleotides strung together. Proteins are encoded by a single gene (usually), so when DNA wants a protein made it only copies that one gene and sends it to the ribosomes.Proteins are made of amino acids linked together in a specific order that is written in the mRNA. Every 3 nucleotides make up a codon; every codon tells a ribosome what specific amino acid comes next. If a mutation in the DNA changes a nucleotide, it changes the codon which (sometimes) changes the amino acid.Changing a single amino acid in a protein at the right or wrong spot can have a dramatic impact. Many changes are so severe that we have never seen what they do because no one has ever survived past the first trimester with that mutation. Most changes do nothing, or are harmful, but occasionally changes are useful.Anyway, proteins make up the cells which are ultimately responsible for traits. DNA codes for proteins.Organisms are completely oblivious to this. 100 years ago, we were generally oblivious to this.
Are an organism's traits determined only by its genes?
No, not all of organism's traits are determined by its genes. For example, the height, which is controlled by multiple genes (polygenic) depends on interrelationships between genes AND environmental conditions, such as nutrition. Also, sometimes the gene gets "shut off" by environmental factor, which is the reason why many times "identical" twins look different when they grow up.
Are sex linked traits only controlled by recessive alleles?
There are a few sex-linked dominants in humans, but they are very rare. There is a form of ricketts that is a sex-linked dominant. Recessive alleles do not necesarily control sex-linked traits; it only looks that way because they show up in males more frequently. Since men have only one X chromosome, any abnormal recessive allele will show up n the phenotype.