The zygote refers to a fertilized egg. At eight weeks, the baby becomes known as an embryo, and fetus refers to the stage of development between 8 weeks and birth. 

Epistasis is when one gene is dependent on the prior function of another. Epistasis can influence the development of many mental disorders.

Our genes have an almost frightening level of influence on how we develop and grow. Genetic influence has been illustrated by many studies focused on twins. Many researchers have found that even when twins were reared in different environments, twins still had significant similarities in personality, intelligence, interests, and behavior. This is not to say that genes are the only factor, they are simply a significant factor in a sea of, nearly infinite, factors influencing development.

"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.