abortion Premature, spontaneous or induced expulsion of the embryo or fetus from uterus. aneuploidy Having one extra or one less chromosome relative to the parental chromosome number. autosome Any chromosome of a type that is the same in males and females of the species. crossing over At prophase I of meiosis, an interaction in which non-sister chromatids of a pair of homologous chromosomes break at corresponding sites and exchange segments; genetic recombination is the result. deletion At cytological level, loss of a segment from a chromosome. At molecular level, loss of one to a few base pairs from a DNA molecule. disease Outcome of infection when defenses aren't mobilized fast enough and a pathogen's activities interfere with normal body functions. double-blind study Different investigators independently collect, then compare data. duplication Gene sequence repeated several to many hundreds or thousands of times. Even normal chromosomes have such sequences.

genetic abnormality A rare or less common version of a heritable trait.

genetic disorder Any inherited condition that causes mild to severe medical problems.

genetic recombination Result of any process that puts new genetic information into a DNA molecule (e.g., by crossing over). homologous chromosome Of cells with a diploid chromosome number, one of a pair of chromosomes identical in size, shape, and gene sequence, and that interact at meiosis.

Nonidentical sex chromosomes (e.g., X and Y) also interact as homologues during meiosis. in-vitro fertilization Conception outside the body (''in glass'' petri dishes or test tubes).

independent assortment Mendelian theory that by the end of meiosis, each pair of homologous chromosomes (and linked genes on each one) are sorted before shipment to gametes independently of how the other pairs were sorted. Later modified to account for the disruptive effect of crossing over on linkages. inversion Part of a chromosome that became oriented in reverse, with no molecular loss. karyotype Preparation of metaphase chromosomes sorted by length, centromere location, other defining features. linkage group All genes on a chromosome. mosaicism Cells of same type express genes differently, so phenotypic differences emerge in same type of tissue. E.g., occurs by X chromosome inactivation in female mammals; also by nondisjunction in any cell after fertilization (only descendants of altered cell inherit the abnormal chromosome number). Non-disjunction Failure of sister chromatids or a pair of homologous chromosomes to separate during meiosis or mitosis. Daughter cells end up with too many or too few chromosomes. polyploidy Having three or more of each type of chromosome in the nucleus of a eukaryotic cell at interphase. reciprocal cross A paired cross. In the first cross, one parent displays the trait of interest. In the second, the other parent displays it. sex chromosome A chromosome with genes that affect sexual traits. Depending on the species, somatic cells have one or two sex chromosomes of the same or different type (e.g., in mammals, XX females, XY males). syndrome A set of symptoms that may not individually be a telling clue but collectively characterize a genetic disorder or disease. translocation Of cells, movement of a stretch of DNA to a new chromosomal location with no molecular loss. Of vascular plants, distribution of organic compounds by way of phloem. X chromosome A type of sex chromosome. An XX mammalian embryo becomes female; an XY pairing causes it to develop into a male. Y chromosome A type of sex chromosome. An XX mammalian embryo becomes female; an XY pairing causes it to develop into a male.