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6 - Biol 241 2010 Lecture 11&12 Chromosomal abnormalities I...

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Biol 241 - 2010 Lecture 11&12: Chromosomal abnormalities I) Variations in chromosome number 1) Aneuploidy and nondisjunction A) Aneuploidy – a condition where the organism gains or loses one or more chromosomes. The loss of a single chromosome (2n-1) is called monosomy . The gain of one chromosome (2n+1) is called trisomy . B) Euploidy – a condition where the complete haploid sets of chromosomes are present. C) Aneuploidy is caused by nondisjunction , the failure of chromosomes or chromatids to disjoin and move to opposite pole during cell division. Nondisjunction can occur in either first or second meiotic divisions. D) Monosomy for X (45, X) in human is known as Turner syndrome. Monosomy for any of the autosomes is not usually tolerated in animals. The lethality may be caused the unmasking of recessive lethal mutations or disruptions of a delicate equilibrium of gene expression for embryonic development. Plants, unlike animals, can tolerate aneuploidy. E) Partial monosomy (segmental deletions). For example, deletion of the short arm of chromosome 5 causes the cri-du-chat (cry of the cat) syndrome. The incidence of this disorder is1 in 50,000 births. F) Trisomy of autosomes in human can sometimes be tolerated. They usually involve smaller autosomes (which carry less genes): 13 (Patau syndrome), 18 (Edwards syndrome), and 21 (Down syndrome). Trisomy of other autosomes are embryonic lethal. G) 75% of Down syndrome are caused by nondisjunction in Meiosis I. The ovum (egg) is the source in about 95% of trisomy 21 cases. Incidence of trisomy 21 increases with maternal age. 2) Polypoidy - a condition where more than two sets of chromosomes are present. Those with three sets are called triploid , and those with 4 are called tetraploid . Polyploidy is not common in animal species (but does occur in some lizards, amphibians, and fish). It is more common in plants. A) Even number polyploids are more common. Odd numbers of chromosomes are not usually found in species depending solely upon sexual reproduction for propagation.
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