TYBSc Zoology Paper III Unit III: Genetics

1. Non-disjunction & disorders
TYBSc Zoology Paper III Unit III
Biren Daftary

2. Non disjunction of autosomes

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. • 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

7. 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

8. • 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

10. Rocker bottom foot

11. 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

12. • Nose is often large & triangular
• Wide spaced & defective eyes
• Mean survival time: 3 months
• Most affected die within a year

14. Undescended
testes/ parted
uterus
Undescended
testes/parted
uterus

16. Non disjunction of sex chromosomes

17. • 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

18. 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

19. • 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

21. 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

22. • 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

24. Skin folds on the back