monozygotic twins: Twins who originate from one zygote that splits apart very early in development. (Also called identical twins.)

XX: A 23rd chromosome pair that consists of two X-shaped chromosomes, one each from the mother and the father. XX zygotes become females.

phenotype: The observable characteristics of a person, including appearance, personality, intelligence, and all other apparent traits.

allele: Any of the possible forms of a gene. Genes with various alleles are called polymorphic.

X-linked: A gene carried on the X chromosome. If a male inherits an X-linked recessive trait from his mother, he expresses that trait because the Y from his father has no counteracting gene. Females are more likely to be carriers of X-linked traits but are less likely to express them.

dizygotic twins: Twins who are formed when two separate ova are fertilized by two separate sperm at roughly the same time. (Also called fraternal twins.)

gamete: A reproductive cell; that is, a sperm or an ovum that can produce a new individual if it combines with a gamete from the other sex to form a zygote.

genome: The full set of genes for a certain species.

dominant–recessive pattern: The interaction of a pair of genes in such a way that the phenotype reveals the influence of one (the dominant gene) more than that of the other (the recessive gene).

chromosome: One of the 46 molecules of DNA (in 23 pairs) that almost every cell of the human body contains and that, together, contain all the genes. Other species have more or fewer chromosomes.

gene: A small section of a chromosome; the basic unit for the transmission of heredity. A gene consists of a string of chemicals that provide instructions for the cell to manufacture certain proteins.

genotype: An organism’s entire genetic inheritance, or genetic potential.

zygote: The single cell that is formed from the fusing of two gametes, a sperm and an ovum.

additive gene: A gene that contributes to the phenotype, usually with other additive genes.

XY: A 23rd chromosome pair that consists of an X-shaped chromosome from the mother and a Y-shaped chromosome from the father. XY zygotes become males.

carrier: A person whose genotype includes a gene that is not expressed in the phenotype. Such an unexpressed gene occurs in half the carrier’s gametes and thus is passed on to half the carrier’s children.

DNA (deoxyribonucleic acid): The molecule that contains the chemical instructions for cells to manufacture various proteins.

germinal period: The first two weeks of development after conception, characterized by rapid cell division and the beginning of cell differentiation.

implantation: The process, beginning about 10 days after conception, in which the developing organism burrows into the tissue that lines the uterus, where it will be nourished.

stem cells: Cells from which any other specialized type of cell can form.

embryo: The name for a developing human organism from two to eight weeks after conception.

Apgar scale: A quick assessment of a newborn’s health. Heart rate, respiratory effort, muscle tone, color, and reflexes are given a score of 0, 1, or 2, with the total compared with the ideal score of 10 (which is rarely attained).

Brazelton Neonatal Behavioral Assessment Scale (NBAS): A test often administered to newborns that measures responsiveness and records 46 behaviors, including 20 reflexes.

doula: A woman who helps with the birth process, including massage during birth and help with breast-feeding.

Down syndrome: A condition in which a person has 47 chromosomes instead of the usual 46, with three rather than two chromosomes at the 21st position. People with Down syndrome often have a distinctive appearance. (Also called trisomy-21.)

embryonic period: The stage of prenatal development from the end of the second week through the eighth week after conception, during which the basic forms of body structures, including internal organs but not sex organs, develop.

postpartum depression: The deep sadness and inadequacy felt by some new mothers in the days and weeks after giving birth.

couvade: Symptoms of pregnancy and birth experienced by fathers.

fetus: The name for a developing human organism from the start of the ninth week after conception until birth.

cesarean section (c-section): A surgical birth, in which incisions through the mother’s abdomen and uterus allow the fetus to be removed quickly, instead of being delivered through the vagina.

fetal period: The stage of prenatal development from nine weeks after conception until birth, during which the fetus grows in size and matures in functioning.

sonogram: An image of a fetus (or an internal organ) produced by using high-frequency sound waves. (Also called ultrasound.)

reflex: An unlearned, involuntary action or movement in response to a stimulus. A reflex occurs without conscious thought.

age of viability: The age (about 22 weeks after conception) at which a fetus may survive outside the mother’s uterus if specialized medical care is available.

small for gestational age (SGA): Having a body weight at birth that is significantly lower than expected, given the time since conception. For example, a 5-pound (2,265-gram) newborn is considered SGA if born on time but not SGA if born two months early. (Also called small-for-dates.)

threshold effect: A situation in which a certain teratogen is relatively harmless in small doses but becomes harmful once exposure reaches a certain level (the threshold).

fragile X syndrome: A genetic condition that involves the X chromosome and that causes slow development.

heritability: A statistic that indicates what percentage of the variation in a trait within a population, in a particular context and era, can be traced to genes.

fetal alcohol syndrome (FAS): A cluster of birth defects, including abnormal facial characteristics, slow physical growth, and intellectual disabilities, that may occur in the child of a woman who drinks alcohol while pregnant.

teratogen: Any agent or condition, including viruses, drugs, and chemicals, that can impair prenatal development, resulting in birth defects or complications.

extremely low birthweight (ELBW): A body weight at birth of less than 2 pounds, 3 ounces (1,000 grams).

cerebral palsy: A disorder that results from damage to the brain’s motor centers. People with cerebral palsy have difficulty with muscle control, so their speech and/or body movements are impaired.

preterm birth: A birth that occurs three or more weeks before the full 38 weeks of the pregnancy—that is, at 35 or fewer weeks after conception.

very low birthweight (VLBW): A body weight at birth of less than 3 pounds, 5 ounces (1,500 grams).

behavioral teratogens: Agents and conditions that can harm the prenatal brain, impairing the future child’s intellectual and emotional functioning.

immigrant paradox: The surprising fact that immigrants tend to be healthier than U.S. born residents of the same ethnicity. This was first evident among Mexican Americans.

low birthweight (LBW): A body weight at birth of less than 5½ pounds (2,500 grams).

false positives: The result of a laboratory test (blood, urine or sonogram) that suggests an abnormality that is not present.

anoxia: A lack of oxygen that, if prolonged, can cause brain damage or death.