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Human Chromosomes and Chromosome Behavior

Human Chromosomes and Chromosome Behavior. Karyotype = stained and photographed preparation of metaphase chromosomes arranged according to their size and position of centromeres. Human Chromosomes. Each chromosome in karyotype is divided into two regions (arms) separated by the centromere

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Human Chromosomes and Chromosome Behavior

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  1. Human Chromosomes and Chromosome Behavior

  2. Karyotype = stained and photographed preparation of metaphase chromosomes arranged according to their size and position of centromeres

  3. Human Chromosomes • Each chromosome in karyotype is divided into two regions (arms) separated by the centromere • p = short arm (petit); q = long arm • p and q arms are divided into numbered bands and interband regions based on pattern of staining • Within each arm the regions are numbered.

  4. Centromeres • Chromosomes are classified according to the relative position of their centromeres • Chromosomes with no centromere, or with two centromeres, are genetically unstable

  5. Chromosomal Abnormalities

  6. Mistakes during segregation • During anaphase, homologous chromosomes separate • Sometimes, they do not separate, leading to the formation of a gamete with two copies of one chromosome • Called Nondisjunction • After fertilization, the zygote will have 3 copies of one chromosome – called trisomy

  7. Can occur during meiosis: • During the S phase of cell cycle – mistakes in making sister chromatids. • During prophase 1 – when crossing over occurs • During anaphase 1 – separation of homologous pairs • During anaphase 2 – separation of sister chromatids

  8. Four types of errors in gene sequence • Deletion: Gene sequence 1 2 3 4 5 goes to 1 2 4 5; 3 deleted • Duplication: Gene sequence 1 2 3 4 5 goes to 1 2 3 2 3 4 5; section 2,3 copied twice • (during crossover, can get one chromosome with deletion & one with duplication) • Inversion: 1 2 3 4 5 goes to 1 2 5 4 3 • One segment is turned around so order reversed • Translocation: 1 2 3 4 5 goes to 1 2 3 8 9 4 5 • A segment from another place or another chromosome is inserted in a gene sequence

  9. Two types of prenatal testing • Amniocentesis – remove amniotic fluid from around fetus; culture cells; do karyotype • Chorionic villi sampling – extensions of placenta growing into uterus, sample tissue and do karyotype

  10. Chromosomal Disorders • Trisomy – an extra copy of one chromosome; results from non-disjunction • Trisomy 21 – Down Syndrome (mental retardation) • Trisomy 18 – Edwards Syndrome (1/10,000); thin, frail, feeding problems • Trisomy 13 – Patau Syndrome (1/5000) • Translocation Down (etc) – extra chromosome linked to another (usually 21, 13, 22, 15) – can be inherited • Mosaic – disorder appears in some body cells, but not all

  11. Down Syndrome • Incidence 1/700 • 2/3 of down fetus spontaneously abort • Trisomy 21 in 94% of cases with extra chromosome from mother mostly(95%) • Risk correlate with maternal age • <25 y/o 1/1600 • >40 y/0 1/80 • 2% are mosaic

  12. Other Clinical features • Hypotonia without weakness • Hypotonia is a disorder that causes low muscle tone (hypotonic condition) • Clinodactaly protruded tongue,small ears,brachycephaly,small up turned nose, depressed nasal bridge. • Mental retardation, socially do better with good environment (Happy children)

  13. Clinical issues • Cardiac and GI • Hypothyriodism • Transient leukemoid reaction • Alzheimer’s disease up to 25% over 40 y/o • Early death relate to cardiac dysfunction

  14. Trisomy 18 • Incidence 1/8000 • Overlaps with trisomy 13 • Sever Mental retardation • >90% dead in 1st year

  15. Trisomy 18 • Small face with prominant occiput • Small sternum and pelvis • Flexion deformity of the finger • VSD and horseshoe kidney

  16. triploidy • Complete extra set of chromosomes • Mostly miscarriages • Fetal wastage skeleton more than cephalic, 2% survive to be recognized • Large hydatidiform placenta • Genital and CNS abnormalities

  17. Trisomy 13 • Sever developmetal retardation • Incidence 1/20000 • 90% dead in the 1st year

  18. Trisomy 13 • Midline brain defect • Malformed ear • Microophalmos and coloboma • Scalp defect

  19. Deletions • Cri-du-chat– deletion in chromosome 5; mental retardation; delayed development; low birthweight; small head; poor muscle tone; high pitched cry • D-George syndrome – deletion on chromosome 22; congenital heart disease; cleft palate; learning difficulties • Prader-Willi (from Mom)/Angelman (from Dad) – deletion in chromosome 15; different symptoms – PW mental retardation; short; obese; small hands & feet; - Angelman jerky movements; inappropriate laughter

  20. Duplications • Pallister Killian – duplication in chromosome 12; mental retardation, coarse facial features, prominent forehead, thin upper lip, thick lower lip, hearing impaired; may live until 40

  21. INVERSIONS A B C C B A B B A A C C

  22. Female Abnormalities • Turner Syndrome – only one X chromosome (XO); short, webbed necks, small jaws; lack prominent female sex characteristics; do not ovulate (1/3000 to 1/5000 live female births) • Metafemales – 1 or more extra X chromosomes; taller than average w/ long legs, slender torsos 1/1000 live births

  23. Male Abnormalities • Klinefelter syndrome – males w/ one or more extra X chromosomes – high pitched voices, breast enlargement, sterile, testes & prostate are small; treat w extra testosterone • XYY syndrome – super-males; tall, seem normal; can live normal lives

  24. Klinefelter syndrome • 20% of aspermic adult male (blocked spermatogenesis • 47 XXY in 80% and mosaic in 20% • IQ is 98 (normal) with mild decrease in verbal IQ • Scoliosis, decrease libido may improve with testesterone, gynecomastia

  25. Fragile X Syndrome • Moderate to sever mental retardation • Speech delay, short attention, hyperactivity • Poor motor coordination and mouthing objects • Poor socialization, temper tantrum • Mood disorder (bipolar), schizophrenia

  26. Fragile X syndrome • Long protruding ears • Long face and prominent jaw • Flattened nasal bridge • High arch palate • Macroorchidism • Genetic is complex, 80% penetration in male and 30% penetration in female

  27. Angelman syndrome • Severe mental retardation • Inappropriate laughter • Decrease pigmentation of choroid or iris (pale blue eyes) • Ataxia and jerky eye movement • Sever speech proplem • Deletion of b15q11q13, maternal in origin • Paternal uniparental disomy

  28. Prader-willi syndrome • (A fat red faced boy in state of somnolency) Charles Diickens • Early hypotonia • Obesity • Short stature as adult • Almond shaped blue eyes • Mental retardation (mild to moderate) • Narrow hands

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