Glossary

Abstract

acanthocytosis The presence in the blood of distorted red cells with a “thorny” appearance. age of onset Age at which the effects of a genetic disorder become evident. allele Alternative forms of the gene occupying a specific site on a chromosome. allelic heterogeneity Multiple gene mutation in the same locus that are each capable of producing an aberrant phenotype. amino acids The major building blocks of polypeptides: Each of the 20 amino acids is encoded by one or more mRNA codons. amniocentesis Removal, through a needle inserted through the abdomen and uterus, of a small amount of the amniotic fluid, which is analyzed for fetal testing. amniotic bands Strands of tissue, thought to be from the rupture of the amnion (the sac around the fetus), leading to disruption of fetal development resulting in asymmetric congenital abnormalities (such as amputated digits, facial clefting). anal atresia (imperforate anus) Congenital absence of an anal opening. anencephaly Failure of the anterior neural tube to close, leading to complete or partial absence of the cranial vault (forebrain, overlying meninges, skull, and skin); a neural tube defect. aneuploidy Cells that contain a variant number of the normal diploid number of 46 chromosomes (either extra or missing chromosomes). Tripoloidy with 69 chromosomes is polyploidy. aniridia Absence of the iris of the eye.