to manifest the disease. Autosomal recessive diseases, however, require two copies of the deleterious allele for the disease to manifest. Because it is possible to possess one copy of a deleterious allele without presenting a disease phenotype, two phenotypically normal parents can have a child with the disease if both parents are carriers (also known as heterozygotes) for the condition. Autosomal aneuploidy can also result in disease conditions. Aneuploidy of autosomes is not well tolerated and usually results in miscarriage of the developing fetus. Fetuses with aneuploidy of gene-rich chromosomes—such as chromosome 1—never survive to term, and fetuses with aneuploidy of gene-poor chromosomes—such as chromosome 21— are still miscarried over 23% of the time. Possessing a single copy of an autosome (known as a monosomy) is nearly always incompatible with life, though very rarely some monosomies can survive past birth. Having three copies of an autosome (known as a trisomy) is far more compatible with life, however. A common example is Down syndrome, which is caused by possessing three copies of chromosome 21 instead of the usual two. Partial aneuploidy can also occur as a result of unbalanced translocations during meiosis. Deletions of part of a chromosome cause partial monosomies, while duplications can cause partial trisomies. If the duplication or deletion is large enough, it can be discovered by analyzing a karyogram of the individual. Autosomal translocations can be responsible for a number of diseases, ranging from cancer to schizophrenia. Unlike single gene disorders, diseases caused by aneuploidy are the result of improper gene dosage, not nonfunctional gene product. See also Aneuploidy (abnormal number of chromosomes) Autosomal dominant Autosomal recessive Homologous chromosome Pseudoautosomal region XY sex-determination system Genetic disorder References Chromosomes Cytogenetics