DNA
(Deoxyribonuleic Acid)
The molecule that contains the chemical instructions for cells to manufacture various proteins.
Chromosome
One of the 46 molecules of DNA (in 23 pairs) that each cell of the human body contains and that, together, contain all the genes. Other species have more of fewer chromosomes.
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.
Zygote
The single cell that is formed from the fusing of two gametes, a sperm and an ovum.
Gene
A section of a chromosome and the basic unit for the transmission of heredity, consisting of a string of chemicals that are instructions for the cell to manufacture certain proteins.
Genotype
An organism's entire genetic inheritance, or genetic potential.
Phenotype
The observable characteristics of a person, including appearance, personality, intelligence, and all other traits.
Allele
Any of the possible forms in which a gene for a particular trait can occur.
Genome
The full set of genes that are instructions to make an individual member of a certain species.
Monozygotic Twins
Twins who originate from one zygote that splits apart very early in development. (Also called "identical twins.")
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.")
XX
A 23rd chromosome pair that consists of two X-shaped chromosomes, one each from the mother and the father. XX zygotes become females.
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.
Polygenic
Referring to a trait that is influenced by many genes.
Multifactorial
Referring to a trait that is affected by many factors, both genetic and environmental.
Regulator Gene
A gene that directs the interactions of other genes, controlling the expression, duplication, and transcription.
Additive Gene
A gene that adds something to some aspect of the phenotype. Its contribution depends on additions from the other genes, which may come from either the same or the other parent.
Dominant-Recessive Pattern
The interaction of a pair of alleles in such a way that the phenotype reveals the influence of one allele (the dominant gene) more than that of the other (the recessive gene.)
Carrier
A person whose genotype includes a gene that is not expressed in the phenotype. Such as unexpressed gene occurs in half the carrier's gametes and thus is passed on to half the carrier's children, who will most likely be carriers, too. Generally, the chara
Heritability
A statistic that indicates what percentage of the variation in a particular trait within a particular population, in a particular context and era, can be traced to genes.
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 typically have distinctive characteristics, including unusual facial features (thick tongue, r
Fragile X Syndrome
A genetic disorder in which part of the X chromosome seems to be attached to the rest of it by a very thin string of molecules. The cause is a single gene that has more than 200 repetitions of one triplet.
Genetic Counseling
Consulation and testing by trained experts that enable individuals to learn about their genetic heritage, including harmful conditions that they might pass along to any children they conceive.
Phenylketonuria (PKU)
A genetic disorder in which a child's body is unable to metabolize an amino acid called phenylalanine. The resulting buildup of phenylalanine in body fluids causes brain damage, progressive mental retardation, and other symptoms.
Germinal Period
The first two weeks of prenatal development after conception, characterized by rapid cell division and the beginning of cell differentiation.
Embryonic Period
The stage of prenatal development from approximately the third through the eighth week after conception, during which the basic forms of all body structures, including internal organs, develop.
Fetal Period
The stage of prenatal development from the ninth week after conception until birth, during which the fetus grows in size and matures in functioning.
Placenta
The organ that surrounds the developing embryo and fetus, sustaining life via the umbilical cord. The placenta is attached to the wall of the pregnant woman's uterus.
Implantation
The process, beginning about 10 days after conception, in which the developing organism burrows into the placenta that lines the uterus, where it can be nourished and protected as it continues to develop.
Embryo
The name for a developing human organism from about the third through the eighth week after conception.
Fetus
The name for a developing human organism from the start of the ninth week after conception until birth.
Sonogram
An image of an unborn fetus (or an internal organ) produced by scanning it with high-frequency sound waves. (Also called "ultrasound.")
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.
Apgar Scale
A quick assessment of a newborn's body functioning. The baby's heart rate, respiratory effort, muscle tone, color, and reflexes are given a score of 0, 1, or 2 twice-at one minute and five minutes after birth- and each time the total of all five scores is
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.
Low Birthweight (LBW)
A body weight at birth of less than 51/2 pounds (2,500 grams).
Very Low Birthweight (VLBW)
A body weight at birth of less than 3 pounds, 5 ounces (1,500 grams).
Extremely Low Birthweight (ELBW)
A body weight at birth of less then 2 pounds, 3 ounces (1,000 grams).
Preterm Birth
A birth that occurs 3 or more weeks before the full 38 weeks of the typical pregnancy have elapsed- that is, at 35 or fewer weeks after conception.
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 no SGA if born two months early. (Also called "small-for dates.")
Kangaroo Care
A child-care technique in which the mother of a low-birthweight infant holds the baby between her breasts, like a kangaroo that carries her immature newborn in a pouch on her abdomen.
Postpartum Depression
The sadness and inadequacy felt by some new mothers in the days and weeks after giving birth.
Parent-Infant Bond
The strong, loving connection that forms as parents hold, examine, and feed their children.
Fetal Alcohol Syndrome (FAS)
A cluster of birth defects, including abnormal facial characteristics, slow physical growth, and retarded mental development, that may occur in the child of a woman who drinks alcohol while pregnant.
Teratogens
Agents and conditions, including viruses, drugs and chemicals, that can impair prenatal development and result in birth defects or even death.
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).
Turner Syndrome
Only one X. Usually short. Female Organs are under developed and cannot conceive children.
Klinefelter Syndrome
Born with XXY. Penis does not grow during puberty. Fat accumulates around the breast area. Sometimes not recognized until adulthood.