"Nature vs. nurture" refers to a long-standing debate about the degree to which our inherited genetic and biological qualities ("nature") or our environment/surroundings ("nurture") influence and shape us. It is generally agreed upon that both influence our behavior in differing ways, but there is still theoretical controversy as to how much each one does. 

The 5-TT gene has been shown to play a role in depression due to its influence on serotonin levels. In a research study, it was found that those with a shorter 5-TT gene were more prone to become depressed after a stressful event compared to those with a longer 5-TT gene. It is difficult to say it is directly linked however, since a strict cause-effect relationship has not been definitively proven, but a strong correlation has been indicated.

A host of family, twin, and adoption studies have shown that all of the disorders provided as answer choices share a hereditary component. Other mental illnesses such as major depression and panic disorder have also shown indications of genetic influence. Genetics will play a role in virtually any disease, but due to the complex nature of the brain and environmental factors, the direct genetic factors influencing psychological disorders are difficult to pinpoint.

Because John's phenotype is brown eyes and brown eyes are dominant, his genotype must be either BB or Bb. However, because we know he carries the gene for blue eyes as well, one of his alleles must be a b (small b). The only genotype that fits these conditions is Bb.

DNA creates a code or blueprint for genes. Genes group together and condense in order to create chromosomes. Genome would be an incorrect answer because it refers to an organism’s entire DNA sequence. Centromeres would also be incorrect as an answer because it is a part of the chromosome. This is the point on a chromosome that will attach to spindle fibers during mitosis and meiosis.

Identical twins come from a monozygotic egg—one egg that's been fertilized and then splits into two. They come from the same egg and sperm; therefore, they will be genetically identical. Fraternal twins come from dizygotic fertilized eggs - two separate eggs, each fertilized by its own sperm. This renders two children that will not be genetically identical (i.e. normal siblings with the same parents).

Identical twins share 100% (or nearly 100%) of their genetic makeup, and fraternal twins only share about 50%. Because of this, twin studies can help clarify what traits have a genetic component rather than those associated with the environment. If a trait has a genetic component, then it will be clear because the identical twins will be more likely to share that trait than fraternal twins.

Identical twins are also called monozygotic (one fertilized egg that split) and fraternal twins are called dizygotic (two separate eggs that were fertilized). Siblings and parents are not the correct answers because the question asks about twin studies, not family studies.

Adoption studies are a great way to study heritability because it separates genetic influences (e.g. biological parents) from environmental influences (e.g. adoptive parents). By examining traits in adoptees, one can reasonably conclude the specific influences of biological heredity. In this case, children scored more similarly to their adoptive parents, so it's likely that the environment had more to do with the expression of the specific trait than their genetic makeup did. 

Recessive genes require that two genes be present in order for the person to display the phenotype or trait. This is true for blond and red hair. Dominant genes only require one gene to be present, which is true for brown hair. The definition of a phenotype is a person's observable characteristics. Sex-linked genes are located on sex chromosomes. Last, color blindness is sex-linked. 

Recessive genes are usually not expressed in a person's phenotype, that is, the set of observable traits a person possesses. Environmental factors can "bring them to light" in a manner of speaking but they are usually not expressed. Dominant genes, such as the gene for dark-colored eyes, are always expressed in the phenotype.