| active genetic-environmental interaction | A kind of interaction in which people's genes encourage them to seek out experiences compatible with their inherited tendencies.
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| allele | An alternate form of a gene; typically, a gene has two alleles, one inherited from the individual's mother, and one from the father.
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| alphafetoprotein (AFP) assay | A maternal blood test performed prenatally to detect such problems as Down syndrome, the presence of multiple embryos, and defects of the central nervous system.
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| amniocentesis | A technique for sampling and assessing fetal cells for indications of abnormalities in the developing fetus; performed by inserting a needle through the abdominal wall and into the amniotic sac and withdrawing a small amount of the amniotic fluid.
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| autosomes | The 22 paired non-sex chromosomes.
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| canalization | The genetic restriction of a phenotype to a small number of developmental outcomes, permitting environmental influences to play only a small role in these outcomes.
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| chorionic villi sampling | A technique for sampling and assessing cells withdrawn from the chorionic villi, projections from the chorion that surrounds the amniotic sac; cells are withdrawn either through a tube inserted into the uterus through the vagina or through a needle inserted through the abdominal wall.
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| chromosomes | Threadlike structures, located in the nucleus of a cell, that carry genetic information to help direct development.
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| codominance | A genetic pattern in which heterozygous alleles express the variants of the trait for which they code simultaneously and with equal force.
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| crossing over | The process by which equivalent sections of homologous chromosomes switch places randomly, shuffling the genetic information each carries.
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| deoxyribonucleic acid (DNA) | A ladder like molecule that stores genetic information in cells and transmits it during reproduction.
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| dizygotic | Characterizing fraternal twins, who have developed from two separate fertilized eggs.
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| dominant | The more powerful of two alleles in a heterozygous combination.
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| Down syndrome | A form of chromosome abnormality in which the person suffers disabling physical and mental development and is highly susceptible to such illnesses as leukemia, heart disorders, and respiratory infections.
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| evocative genetic-environmental inter action | The expression of the genes' influence on the environment through an individual's inherited tendencies to evoke certain environmental responses; for example, a child's smiling may elicit smiles from others.
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| fragile X syndrome | A form of chromosomal abnormality, more common in males than in females, in which an area near the tip of the X chromosome is narrowed and made fragile due to a failure to condense during cell division. Symptoms include physical, cognitive, and social problems.
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| gene | A portion of DNA located at a particular site on a chromosome and that codes for the production of certain kinds of proteins.
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| genotype | The particular set of genes a person inherits from his or her parents.
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| goodness of fit | A measure of the degree to which a child's temperament is matched by her environment; the more effectively parents and other agents of socialization accept and adapt to the child's unique temperament, the better this "fit."
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| hemophilia | A disorder, caused by an X-linked recessive gene, in which the blood fails to clot; found more often in males than in females.
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| heritability factor | A statistical estimate of the contribution heredity makes to a particular trait or ability.
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| heterozygous | The state of an individual whose alleles for a particular trait from each parent are different.
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| homozygous | The state of an individual whose alleles for a particular trait from each parent are the same.
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| human behavior genetics | The study of the relative influences of heredity and environment on the evolution of individual differences in traits and abilities.
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| Huntington disease | A genetically caused, fatal disorder of the nervous system that begins in mid-adulthood and is manifested chiefly in uncontrollable spasmodic movements of the body and limbs and eventual mental deterioration.
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| Klinefelter's syndrome | A form of chromosome abnormality in which a male inherits an extra X sex chromosome, resulting in the XXY pattern; many feminine physical characteristics, language deficits, and, sometimes, mental retardation are present.
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| meiosis | The process by which a germ cell divides to produce new germ cells with only half the normal complement of chromosomes; thus male and female germ cells (sperm and ovum) each contain only 23 chromosomes so that when they unite, the new organism they form will have 46 chromosomes, half from each parent.
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| mitosis | The process in which a body cell divides in two, first duplicating its chromosomes so that the new daughter cells produced each contain the usual 46 chromosomes.
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| modifier genes | Genes that exert their influence indirectly, by affecting the expression of still other genes.
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| monozygotic | Characterizing identical twins, who have developed from a single fertilized egg.
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| niche picking | Seeking out or creating environments compatible with one's genetically based predispositions.
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| nonshared environment | A set of conditions or activities experienced by one child in a family but not shared with another child in the same family.
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| nucleotide | A compound containing a nitrogen base, a simple sugar, and a phosphate group.
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| ovum | The female germ cell, or egg.
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| passive genetic-environmental interaction | The interactive environment created by parents with particular genetic predispositions who encourage the expression of these tendencies in their children.
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| phenotype | The visible expression of the person's particular physical and behavioral characteristics; created by the interaction of a person's genotype, or genetic makeup, with the environment.
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| phenylketonuria (PKU) | A disease caused by a recessive allele that fails to produce an enzyme necessary to metabolize the protein phenylalanine; if untreated immediately at birth, damages the nervous system and causes mental retardation.
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| range of reaction | The notion that the human being's genetic makeup establishes a range of possible developmental outcomes, within which environmental forces largely determine how the person actually develops.
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| recessive | The weaker of two alleles in a heterozygous combination.
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| sex chromosomes | In both males and females, the 23rd pair of chromosomes, which determine the individual's sex and are responsible for sex-related characteristics; in females, this pair normally comprises two X chromosomes, in males an X and a Y chromosome.
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| shared environment | A set of conditions or experiences shared by children raised in the same family; a parameter commonly examined in studies of individual differences.
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| sickle-cell anemia | A disorder, caused by a recessive gene, in which the red blood cells become distorted when low in oxygen, causing fatigue, shortness of breath, and severe pain and posing a threat to life from blockage of crucial blood vessels.
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| sperm | The male germ cell.
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| temperament | The individual's typical mode of response to the environment, including such things as activity level, emotional intensity, and attention span; used particularly to describe infants' and children's behavior.
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| Turner syndrome | A form of chromosome abnormality found in females in which secondary sex characteristics develop only if female hormones are administered, and abnormal formation of internal reproductive organs causes permanent sterility.
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| ultrasound | A technique that uses sound waves to visualize deep body structures; commonly used to reveal the size and structure of a developing fetus. Also called ultrasonography.
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| X-linked genes | Genes that are carried on the X chromosome and that, in males, may have no analogous genes on the Y chromosome.
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