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Chromosome Variations

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Chromosome Variations Alterations in Chromosome Number • Euploid – Have the normal chromosome number • Polyploid – A cell with extra sets of chromosomes – Ex: triploid (3n) • Aneuploid – Cells missing a single chromosome or having an extra chromosome Alterations in Chromosome Number • Nondisjunction: occurs when either – Homologs fail to separate during anaphase I of meiosis – Sister chromatids fail to separate during anaphase II • Result: one gamete has 2 copies of one chromosome and the other has no copy of that chromosome Alterations in Chromosome Number • If either of these gametes unites with another during fertilization, the result is aneuploidy (abnormal chromosome number). – Trisomy: extra chromosome (more common) – Monosomy: missing a chromosome Alterations in Chromosome Number • Frequency of nondisjunction is quite high in humans, but the results are usually so devastating to the zygote that miscarriage occurs very early in the pregnancy. • If the individual survives, he or she usually has a set of symptoms (a syndrome) caused by the abnormal dose of each gene product from that chromosome. Down Syndrome • Nondisjunction of autosome • Trisomy 21 • Characteristic facial features, short stature, “floppy” appearance, developmental milestones come slowly, heart & kidney defects, hearing & visual loss, suppressed immune system, greater chance of developing leukemia & Alzheimer disease • Most common autosomal aneuploid among liveborns • Likelihood increases with age of the mother • Average life expectancy is now 60 (in 1910 it was 9) • Genome editing to shut off one X chromosome Edward’s Syndrome • Nondisjunction of autosome • Trisomy 18 • Great physical & intellectual disabilities, almost every organ system affected: heart defects, displaced liver, growth retardation, oddly clenched fists, overlapping fingers, narrow flat skull, low-set ears, small mouth, unusual fingerprints, “rocker-bottom” feet • Most do not survive to be born; children with full trisomy 18 generally do not live more than a few months Patau Syndrome • Nondisjunction of autosome • Trisomy 13 • Fusion of the developing eyes; heart, kidney, brain, face, & limb abnormalities; cleft lip/palate; small head; extra fingers/toes; extra spleen; atypical liver; rotated intestines; abnormal pancreas • A few have survived to adulthood, but most rarely live more than a few months • Do not progress developmentally beyond the 6- month level Klinefelter Syndrome • Nondisjunction of sex chromosomes • 47XXY (extra X chromosome) • Male sex organs; but underdeveloped sexually (small testes and prostate gland, sparse pubic & facial hair) • Long arms & legs, large hands & feet, may develop breast tissue • About 1 in 500 males • Most common genetic or chromosomal cause of male infertility Trisomy X • Nondisjunction of sex chromosomes • 47XXX (extra X chromosome) • Females; healthy and fertile; usually cannot be distinguished from normal female except by karyotype • Only symptoms are tall stature and menstrual irregularities; tend to be less intelligent than siblings • About 1 in every 1000 females • All but one of the X chromosomes in inactivated Turner’s Syndrome • Nondisjunction of sex chromosomes • Monosomy X (45XO): one X chromosome & no 2nd sex chromosome (some are missing only part of an X chromosome) • Looks normal at birth, except puffy hands & feet • Wide-set nipples, soft nails that turn up, slight webbing at the back of the neck, short, coarse facial features, low hairline at the back of the head, impaired hearing, ear infections • At sexual maturity, sparse body hair, don’t ovulate or menstruate, breasts don’t develop, small uterus • The only viable monosomy in humans • Affects 1 in 2500 female births; 99% of XO fetuses are not born; life span is shortened slightly • More likely to develop osteoporosis, diabetes, colon cancer XXYY Syndrome • Nondisjunction of sex chromosome • XXYY (extra X chromosome & extra Y chromosome) • More severe behavioral problems, tend to develop foot & leg ulcers from poor circulation • Attention deficit disorder, obsessive compulsive disorder, autism, & learning disabilities typically develop by adolescence • Low testosterone, delayed development of secondary sexual characteristics, undescended testes • infertile • About 1 in 17,000 newborn boys • Until recently was diagnosed as Klinefelter’s XYY Males • Nondisjunction of sex chromosomes • 47XYY (extra Y chromosome) • Very tall, acne • 1 in 1000 males • criminally aggressive hypothesis disproven over time; 96% not destined to become criminals.
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