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

DNA is stored on?

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.

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.

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

An organism's entire genetic inheritance, or genetic potential.

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

The full set of genes for a certain species.

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).

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.

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.

The first two weeks of development after conception, characterized by rapid cell division and the beginning of cell differentiatio

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.

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

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

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

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.)

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

A body weight at birth of less than 51⁄2 pounds (2,500 grams).

A body weight at birth of less than 3 pounds, 5 ounces (1,500 grams).

A body weight at birth of less than 2 pounds, 3 ounces (1,000 grams).

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.