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Non disjunction & disorders


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TYBSc Zoology Paper III Unit III: Genetics

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Non disjunction & disorders

  1. 1. Non-disjunction & disorders TYBSc Zoology Paper III Unit III Biren Daftary
  2. 2. Non disjunction of autosomes
  3. 3. Downs’ syndrome • Initial hypothesis: chromosomal abnormality • Trisomy of chromosome no 21 • Probability: 1 in 800/1000 • 50% of babies with disorder born to women above 35 years (accounting for 15% of all births) • 4-5% sporadic cases are paternal in origin • Beyond the age 35 of mother, frequency rises dramatically (70% chance that 45 yr old will have an affected child compared to a 20 yr old) • Trisomy 21 is highly lethal in-utero • Delayed development & premature aging with brain abnormalities
  4. 4. • 2% of the affected individuals are mosaics • Important to detect whether the cause is germinal mosaicism of parent or embryo mosaicism • If the cause is non disjunction in parent gametes then it can lead to multiple Down syndrome births in a family
  5. 5. Edwards’ syndrome • Trisomy of chromosome no 18 • Probability: 1 in 6000 • Cause: Mostly maternal errors in meiosis II • Gross abnormalities of nearly all the systems leading to an early death • Most distinctive feature is the occurrence of simple arch patterns on 3 or more finger tips & a long skull which bulges at the rear • Fists are tightly clenched with the index finger bent sideways across the third finger • Limbs and joints are so stiff that they cannot be moved
  6. 6. • Many affected babies have rocker bottom feet with big toes that are short and bent upwards • Half of them have extra folds of skin on the neck • Facial features: round face, small and wide spaced eyes & small mouth • 80% births are female • Low birth weight and mean survival time is about 2-5 months • Most cases die during the first year
  7. 7. Rocker bottom foot
  8. 8. Pataus’ syndrome • Trisomy of chromosome no 13 • First reported in 1957 by Patau and his associates describing a case of multiple malformations in a new born child • Distinct traits: Small head, cleft palate or lip, wine coloured birth marks & extra fingers/toes • Most affected babies appear to be deaf and some are blind • Males have undescended testes & females may have a uterus divided into 2 parts • Fingers may be abnormally bent
  9. 9. • Nose is often large & triangular • Wide spaced & defective eyes • Mean survival time: 3 months • Most affected die within a year
  10. 10. Undescended testes/ parted uterus Undescended testes/parted uterus
  11. 11. Non disjunction of sex chromosomes
  12. 12. • In the late 1950s, scientists discovered that Y chromosome plays a major role in sex determination • Before that it was thought that two X’s lead to femaleness and one X leads to maleness and Y is required only for fertility in males • However after the human chromosome no. was established the role of Y in sex determination was clarified • The presence or absence of Y is much more important to male development than X
  13. 13. Klinefelters’ syndrome • XXY males with 47 chromosomes in total • Phenotypic changes detected usually after puberty • Very small testes, sperms absent & androgen deficiency • Poorly developed male secondary sexual characters & breast development in some • Sexual behaviour normal in young affected males but impotency common among older ones • Presence of long limbs and taller by 2 - 4.5” compared to a normal male • Many affected males show normal height and intelligence & function well in society
  14. 14. • Many exhibit no symptoms apart from infertility and they live without the slightest hint that they are in anyway abnormal • Some exhibit mild retardation with increased tendency to emotional and social problems • Probability: 1 in 600/1000 live births, 1 in 300 abortions • Some are known to be karyotypically normal and 10% have different karyotypes viz. 48 XXXY, 48XXYY, 49 XXXXY, 49XXXYY
  15. 15. Turners’ syndrome • XO females with 45 chromosomes • Females who lack secondary sexual characters & breast development • They are sterile • Very short females with short and webbed necks • Forearms show greater than normal angling away from the body when the palms are faced upward • Most of them have primitive gonads that lack germ cells and hormone producing tissue • The external genitalia remain small and infantile
  16. 16. • Affected females have a narrowed aorta • They also show kidney and some minor skeletal deformities • Hairline at the base of the neck is low • Females identified by characteristic skin folds on the back of the neck, swelling of hands and feet, abnormally large fingerprint patterns & low birth weight • IQ in most cases is normal although some may show a moderate decrease • Probability: 1 in 2500 females born, highly lethal in embryos
  17. 17. Skin folds on the back