Methylmercury (MeHg) is an organic form of mercury that can damage the developing brains of human fetuses. Women who consume methylmercury during pregnancy can bear children who have neurological issues because methylmercury has toxic effects on the nervous system during embryonic development. During the third week of gestation, the human nervous system begins to form in the embryo. During this gestational period, the embryo's nervous system is particularly susceptible to the influence of neurotoxins like methylmercury that can result in abnormalities. Furthermore, the fetal brain can incur damage despite the lack of signs of poisoning in the pregnant woman. In children, defects due to methylmercury can result in deficits in attention, behavior, cognition, and motor skills.

Purkinje cells, also called Purkinje neurons, are neurons in vertebrate animals located in the cerebellar cortex of the brain. Purkinje cell bodies are shaped like a flask and have many threadlike extensions called dendrites, which receive impulses from other neurons called granule cells. Each cell also has a single projection called an axon, which transmits impulses to the part of the brain that controls movement, the cerebellum. Purkinje cells are inhibitory neurons: they secrete neurotransmitters that bind to receptors that inhibit or reduce the firing of other neurons. Purkinje cells were the first neuronal cells identified. Researchers study the embryonic development of Purkinje cells to elucidate how they function in various mechanisms in the body.

Jan Evangelista Purkyne, also called Johannes or Johann Evangelist Purkinje, studied cells in the cerebellum, fibers of the heart, subjective visual phenomenon, and germinal vesicle, in eastern Europe during the early nineteenth century. His investigations provided insights into various mechanisms and structures of the human body. Purkyne introduced techniques for decalcification of bones and teeth, embedding of tissue specimens, and eye examinations. He was one of the first to adopt the microtome in his experiments and to use the term protoplasm when describing the contents of young animal embryos. Purkyne identified structures in the eggs of chickens, such as the germinal vesicle, from which he hypothesized the female reproductive cell (ovum) developed, and around which he said an embryo developed. Purkyne's results helped others locate ova in mammals.

The knowledge of medical genetics is currently used with prenatal testing, and the advancements in the field of behavioral genetics may someday allow for its use with prenatal testing as well. The use of prenatal procedures for medical phenotypes has its own implications and should these techniques be used for behavioral phenotypes, such implications can also apply. The complexity of behavior in terms of the factors that may affect it, along with the way it is conceptualized and perceived, adds further implications for prenatal testing of it. In this thesis, I discuss the qualitative, quantitative, and historical facets of prenatal testing for medical and behavioral phenotypes and the undercurrent of eugenics. I do so by presenting an example of the medical phenotype (cystic fibrosis) as a case for envisioning the implications of medical phenotypes before delving into examples of behavioral phenotypes (aggression, impulsivity, extraversion, and neuroticism) in order to explore the implications shared with those for medical phenotypes as well as those unique to it. These implications then set the foundation for a discussion of eugenics, and the considerations for how behavioral genetics with prenatal testing may give way to a modern form of it.