BIOL 205 Lecture Notes - Lecture 8: Nondisjunction, Down Syndrome, Turner Syndrome
Document Summary
Polyploidy: extra complete sets of chromosomes, e. g. , 3n (triploidy). This is found in some spontaneously aborted human fetuses. 2n + 1 = trisomy, the presence of an extra copy of one specific chromosome. 2n - 1 = monosomy, the absence of one copy of a specific chromosome. Origins of aneuploidy most frequently lie in meiotic nondisjunction. There are no viable human monosomies except that of the x chromosome: xo, turner syndrome. Other sex-chromosome aneuploidies associated with viability are xxy (klinefelter syndrome) and xyy. Most human autosomal trisomies are embryonic lethals, but ts 13 (patau), 18 (edwards) and. The most viable human trisomy is ts21, down syndrome. Incidence of nondisjunction-related ts21 increases with increasing maternal age. Abnormalities and lethality associated with monosomy and trisomy suggest tight dosage control, with no dosage compensation i. e only 1 transcriptionally active x so xx females make same amount of product as xy males. The most frequent structural aberrations include: deletions, duplications, inversions, and translocations.
