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. 

The brain's four lobes are the frontal lobe, temporal lobe, parietal lobe, and occipital lobe. Each lobe exists in a distinct part of the cerebral cortex and serves a different primary function. The frontal lobe is located toward the front of the brain, directlly behind the forehead, and is used to process higher level thinking such as conscious thought. The temporal lobe is located toward the outside of the head, behind the temple and ears, and is responsible for auditory processing. The parietal lobe is located superior to (above) the temporal lobe at the top of the head and is primarily responsible for processing motor and sensory functions. The occipital lobe is located at the back of the head and processes visual signals.

The corpus callosum is a thick band of nerve fibers that connects the left and right hemispheres of the cerebal cortex. Its main purpose is to facilitate communication between the two hemispheres.

The precentral gyrus and central sulcus are topical features of the brain (a ridge and a groove, respectively) that separate the frontal and parietal lobes.

The cerebellum, located in the rear of the brainstem, helps coordinate our voluntary movements and helps us maintain balance.

The medulla oblongata controls our breathing and heart rate. It is located at the base of the brainstem. Note that most structures in the brainstem are associated with essential, involuntary functions.