special subjects
DNA Variants and Mutations
Extra or missing chromosomes
Abnormal numbers of autosomes (chromosomes that are not sex chromosomes) usually result in severe abnormalities. For example, extra autosomes typically cause abnormalities such as Down Syndrome (Trisomy 21) and other severe syndromes or can be fatal to the fetus. Absence of an autosome is generally fatal to the fetus. Because of X chromosome inactivation, having an abnormal number of X chromosomes is usually much less detrimental than having an abnormal number of autosomes. For example, the abnormalities resulting from the absence of one X chromosome are usually relatively minor (eg, in Turner Syndrome ). Also, females with 3 X chromosomes ( 