Allele		The reproductive cell, or gamete, of the female; the egg cell.
Dominant Gene		The range of possible phenotypes that an individual with a particular genotype might exhibit in response to the particular sequence of environmental influences he or she experiences.
Canalization		Describes a genotype consisting of distinct forms of chromosomes for the same gene.
Heterozygous		A reproductive cell (sperm or ovum).
Tay-Sachs Disease		Also called trisomy 21 because it is generally caused by an extra twenty-first chromosome or the translocation, or transfer, of part of the twenty-first chromosome onto another chromosome.
Hemophilia		Male gametes, or reproductive cells; produced in the testicles.
Recessive Gene		A prenatal diagnostic method in which a sample of amniotic fluid is withdrawn and tested to detect chromosomal abnormalities.
Mitosis		A medical procedure for detecting genetic abnormalities before birth in which a small bit of tissue surrounding the embryo is removed and examined microscopically.
Amniocentesis		A threadlike, rodshaped structure containing genetic information that is transmitted from parents to children; each human sperm or egg cell contains twenty-three chromosomes, and these determine a person’s inherited characteristics.
Behavior Genetics		A dominant gene disorder that results in a gradual deterioration of the central nervous system, causing uncontrollable movements and mental deterioration.
Chromosome		A prenatal diagnostic method that allows medical personnel and others to view the fetus by projecting high-frequency sound waves through the mother’s womb.
Range of Reaction		The set of genetic traits inherited by an individual.
Meiosis		One of several alternative forms of a gene.
DNA		A research method for studying the relative contributions of heredity and environment in which the degree of similarity between genetically identical twins is compared with the similarity between fraternal twins.
Gene		A disorder of the nervous system that occurs most frequently among people of German or Eastern European Jewish origin.
Gamete		The scientific study of the relationship between genotype and phenotype, especially with regard to intelligence, personality, and mental health.
Phenotype		A complex process by which gametes form; involves duplication and division of reproductive cells and their chromosomes.
Sperm		The set of traits an individual actually displays during development; reflects the evolving product of genotype and experience.
Genotype		A molecule containing hereditary information; has a chemical structure of two chains of polynucleotides arranged in a double helix or spiral.
Down Syndrome		In any paired set of genes, the gene that influences or determines physical characteristics only when no dominant gene is present.
CVS		The single new cell formed when a sperm cell attaches itself to the surface of an ovum.
Zygote		The creation of new cells through duplication of chromosomes and division of cells.
Twin Study		A genetic condition in which an individual inherits two identical genes at a particular location on a particular chromosome.
Turner Syndrome		A disease in which the blood fails to clot properly and therefore is characterized by excessive bleeding; example of a sex-linked recessive trait.
Homozygous		The tendency of many developmental processes to unfold in highly predictable ways under a wide range of conditions.
Sickel Cell Disease		Results from having only one sex chromosome; thus, affected individuals are, by default, female.
Ovum		A molecular structure, carried on chromosomes, containing genetic information; the basic unit of heredity.
Huntington Disease		A genetically transmitted condition in which a person’s red blood cells intermittently acquire a curved, sickle shape. The condition sometimes can clog circulation in the small blood vessels.
Adoption Study		In any paired set of genes, the gene with greater influence in determining physical characteristics that are physically visible or manifest.
Ultrasound		A research method for studying the relative contributions of heredity and environment in which genetically related children reared apart are compared with genetically unrelated children reared together.