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genotype  the particular set of genes that a person inherits from his or her parents.
phenotype  created by the interaction of a person's genotype, or genetic makeup, with the environment; the visible expression of the person's particular physical and behavioral characteristics.
ovum  the female germ cell, or egg.
sperm  the male germ cell.
chromosomes  threadlike structures, located in the central portion, or nucleus, of a cell, that carry genetic information to help direct development.
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.
crossing over  the process by which equivalent sections of homologous chromosomes switch places randomly, shuffling the genetic information each carries.
mitosis  the process in which a body cell divides in two, first duplicating its chromosomes so that the new, daughter cells each contain the usual 46 chromosomes.
autosomes  the 22 paired non-sex chromosomes.
deoxyribonucleic acid (dna)  a ladderlike molecule that stores genetic information in cells and transmits it during reproduction.
nucleotide  a compound containing a nitrogen base, a simple sugar, and a phosphate group.
gene  a portion of dna that is located at a particular site on a chromosome and that codes for the production of certain kinds of proteins.
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.
homozygous  describing the state of an individual whose alleles for a particular trait from each parent are the same.
heterozygous  describing the state of an individual whose alleles for a particular trait from each parent are different.
codominance  a genetic pattern in which heterozygous alleles express the variants of the trait for which they code simultaneously and with equal force.
dominant  describing the more powerful of two alleles in a heterozygous combination.
recessive  describing the weaker of two alleles in a heterozygous combination.
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 y chromosomes.
x-linked genes  genes that are carried on the x chromosome and that, in males, may have no analogous genes on the y chromosome.
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.
modifier genes  genes that exert their influence indirectly, by affecting the expression of still other genes.
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.
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.
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.
Turner syndrome  a form of abnormality of the sex chromosomes found in females, in which secondary sex characteristics develop only if female hormones are administered and in which abnormal formation of internal reproductive organs leads to permanent sterility.
Klinefelter's syndrome  a form of chromosome abnormality in which a male inherits an extra x sex chromosome, resulting in the xxy pattern, and has many feminine physical characteristics as well as language deficits and, sometimes, mental retardation.
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 appears narrowed due to a failure to condense during cell division, causing it to be fragile. this failure to condense is caused by an abnormal number of cgg repeats from 200 to thousand of repeats. symptoms include physical, cognitive, and social problems.
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.
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.
huntington disease  a genetically caused, fatal disorder of the nervous system that begins in midadulthood and is manifested chiefly in uncontrollable, spasmodic, movements of the body and limbs and eventual mental deterioration.
alphafetoprotein assay (afp)  a blood test performed prenatally to detect such problems such as down syndrome, the presence of multiple embryos, and defects of the central nervous system.
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.
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.
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.
niche picking  seeking out or creating environments that are compatible with one's own (genetically based) predispositions.
human behavior genetics  the study of the relative influences of heredity and environmental forces on the evolution of individual differences in traits and abilities.
heritability factor  a statistical estimate of the contribution made by heredity to a particular trait or ability.
monozygotic  characterizing identical twins, who have developed from a single fertilized egg.
dizygotic  characterizing fraternal twins, who have developed from two separate fertilized eggs.
shared environment  a set of conditions or experience that is shared by children raised in the same family with each other; a parameter commonly examined in studies of individual differences.
nonshared environment  a set of conditions or activities that is experienced by one child in a family and not shared with another child in the same family.
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.
psychopathology  psychological and behavioral dysfunction as it occurs in various types of mental disorder.
concordance rate  the percentage of cases in which a characteristic or trait exhibited by one member of a twin pair is also exhibited by the other twin.

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