Genes with various repeats or deletions of base pairs.

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.

The chemical composition of the molecules that contain the genes, which are the chemical instructions for cells to manufacture various proteins.

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

A person whose genotype includes a gene that is not expressed in the phenotype. The carried gene occurs in half of the carrier’s gametes and thus is passed on to half of the carrier’s children. If such a gene is inherited from both parents, the characteristic appears in the phenotype.

A condition in which a person has 47 chromosomes instead of the usual 46, with 3 rather than 2 chromosomes at the 21st site. People with Down syndrome typically have distinctive characteristics, including unusual facial features, heart abnormalities, and language difficulties. (Also called trisomy-21.)

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.

An in vitro fertilization technique in which a single sperm cell is injected directly into an ovum.

Referring to two genes of one pair that are exactly the same in every letter of their code. Most gene pairs are homozygous.

A variation that makes a gene different in some way from other genes for the same characteristics. Many genes never vary; others have several possible alleles.

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

Fertilization that takes place outside a woman’s body (as in a glass laboratory dish). The procedure involves mixing sperm with ova that have been surgically removed from the woman’s ovary. If a zygote is produced, it is inserted into a woman’s uterus, where it may implant and develop into a baby.

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

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

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.

Twins who originate from one zygote that splits apart very early in development. (Also called identical twins.) Other monozygotic multiple births (such as triplets and quadruplets) can occur as well.

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

Referring to two genes of one pair that differ in some way. Typically one allele has only a few base pairs that differ from the other member of the pair.

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

A general term for the techniques designed to help infertile couples conceive and then sustain a pregnancy.

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.

Referring to a trait that is affected by many factors, both genetic and environmental, that enhance, halt, shape, or alter the expression of genes, resulting in a phenotype that may differ markedly from the genotype.

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.

The chromosome pair that, in humans, determines sex. The other 22 pairs are autosomes, inherited equally by males and females.

Referring to a trait that is influenced by many genes.

The full set of genes that are the instructions to make an individual member of a certain species.

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

An international effort to map the complete human genetic code. This effort was essentially completed in 2001, though analysis is ongoing.

Consultation 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 may conceive.

The interaction of a heterozygous pair of alleles in such a way that the phenotype reflects one allele (the dominant gene) more than the other (the recessive gene).

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