Maladies caused by defects in the human genome. A single-gene disorder is caused by a single mutant gene. Chromosomal disorders arise from disruptions in development produced by an excess or deficiency of whole chromosomes or chromosome segments (e.g., Down syndrome). Chromosomal disorders usually do not run in families because their effects are so detrimental or lethal. They affect about seven individuals per thousand births and account for about half of all spontaneous first-trimester abortions. Polygenic disorders are determined by combined action of a number of genetic loci, each with a small effect. Single-gene disorders are cataloged in two online databases. See GeneCards, OMIM.
Subjects: Genetics and Genomics.