Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.

Non disjunction & disorders

801 views

Published on

TYBSc Zoology Paper III Unit III: Genetics

Published in: Education
  • Be the first to comment

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

×