Ch
ươ Đ T BI N NHI M SĂC TH
ng 4 Ễ
Ộ
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N i dung chính
ộ
ộ ấ ế
ng NST ộ
ạ ủ
ệ
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1. Đ t bi n c u trúc NST – L p đo n ặ ạ – M t đo n ạ ấ – Đ o đo n ạ ả – Chuy n đo n ạ ể 1. Đ t bi n s l ố ượ ế – Đa b i hoá ộ – L ch b i hoáBB t ho t c a NST X ấ
ng l ch b i (aneuploidy)
ệ ượ
ệ
Hi n t • Nguyên nhân và c chơ
ộ ế
Hi n t
ng l ch b i
ng
ệ ượ
ộ ở
ệ
i ườ
Th d h p ể ị ợ chuy n đo n ạ ể
Giao tử
Cân b ng Không cân b ng
ằ
ằ
Gtbt
Bth
ng
D h p ị ợ CĐ (45)
Monosomic 21 (45)
Trisomic 21 (46)
ườ (46) >1/3
>1/3
<1/3
(ch t) 0ế
Th Barr và b t ho t X
ể
ấ
ạ
Structural
• Translocations, inversions, insertions, deletions,
rings
– normal, – balanced – abnormal
• What happens at meiosis? • Formation of gametes that are:
• Associated with increased miscarriages • Most chromosome abnormalities incompatible
with life
Robertsonian translocation
Pericentric inversion • If cross over occurs within the
inverted segment
– Normal gametes – Balanced gametes
• inverted
– Unbalanced gametes
• Duplication of proximal end
& Deletion of distal end
• Deletion of proximal end & Duplication of distal end
Paracentric inversion
• If cross over occurs within the
inverted segment
– Normal gametes – Balanced gametes
(inverted)
– Acentric – dicentric
Insertion
• If carrying balanced deletion/insertion OK
• But 50% gametes will
be abnormal
• Could carry the
A derA derB B
deletion, insertion or both
Deletions
• Deletions are rare, as are monosomies
– due to translocation or inversion in parent
• Can be de novo or inherited
• Would not reproduce
Deletions
• Interstitial • Williams, 7q11.2,
– microdeletion (FISH)
• Terminal • Cri du chat, 5p15 • Wolf-Hirschhorn,
4p36
• Retinoblastoma,
13q14
• Prader-Willi,
15q11.2
• Angelman, 15q11.2 • DiGeorge, 22q11.2
Cri du Chat
– 5p15
• Terminal deletion
• Cries like cat
• Mental retardation
Ring chromosomes
• Often unstable in mitosis
• Often only find ring in proportion of cells
• Other cells usually monosomic as lack ring
Diagnosis of chromosome abnormalities
– take blood and look at lymphocytes
• Child born
– Chorionic villus sampling (CVS) – Amniocentesis (AF) – Fetal Blood Sampling (FBS)
• Unborn child • Prenatal Diagnosis
Screening
• Both in first and second trimester • Serum
• Ultrasound
• Indicates which should go forward for
invasive procedure
CVS
Prenatal Diagnosis Amniocentesis
• Most common method used • Usually perform around 16 weeks • 10-20 ml amniotic fluid • Culture for 8-14 days • Do G banding • Can do a quick diagnosis on uncultured
amniocytes using FISH or QF-PCR – Only examine a few chromosomes
• 0.5-1% miscarriage rate
Amniocentesis
Ambiguous results Amniocentesis
• Usually establish 2-3 cultures • 1 abnormal cell in 1 culture = artefact – level 1 mosaicism or pseudo mosaicism
• 2 or more abnormal cells in 1 culture = could be
artefact or real – level 2 mosaicism, 20% chance real mosaicism • 2 or more abnormal cells in 2 or more cultures =
true mosaicism – level 3 mosaicism
– To resolve need to repeat amniocentesis or do FBS
Molecular Cytogenetics
• FISH
– Use DNA probes for specific chromosomes – Can paint metaphase – Useful for quick result and identifying small areas – Eg deletions, ESACs
• QF-PCR
– Quantitative fluorescent PCR – Use polymorphic sites to define number of copies
present
– Useful for quick result in prenatal diagnosis
Quantitative Fluorescent PCR
Trisomy detection in prenatal samples
Hypervariable region on chromosome 21 amplified by F-PCR
Ratio:
1 : 1 : 1
Ratio:
1 : 2
Quick result from Amniocentesis
• FISH
– Use probes for 13,21 and X, Y, 18 on two different
slides
– takes 24 hours
• QF-PCR
– Use polymorphic markers for chromosomes 13, 18,
21
– Results in 24 hours – Becoming more common
– Can only detect abnormalities for these chromosomes – Usually go on and do full karyotype - ???
The future
• Fetal cells in the maternal circulation • Free fetal DNA in maternal plasma
• If diagnostic – no need for CVS or
amniocentesis to detect chromosome abnormality
NG 4
BÀI T P CH Ậ
ƯƠ
ộ
ợ
ượ ạ ồ
Ở ờ
ồ
ệ ấ ấ ồ
ấ ồ ượ i ả ạ
• Bài 1: M t ru i gi m cái thân đen mun ồ ấ c cho lai ee (e = ebony) đ ng h p đ ồ v i m t con đ c hoang d i đ ng h p ợ ự ộ ớ (e+e+) đã đ đ i con có c chi u tia X. ế ượ xu t hi n m t ru i cái thân mun. Khi cho ộ ru i gi m này lai v i ru i gi m F1 thu ớ c ½ thân mun: ½ hoang d i. Hãy gi đ thích k t qu này. ế ả
• Bài 2: Gi ủ
ả ử ằ ệ ồ
ề ố ượ ố ủ ờ
ế
ng gây ườ ế ạ
tr ng thái đ ng h p). s r ng F1 c a phép lai bài ở ở t p 8 ta phát hi n đ c hai con ru i gi m ượ ậ ấ đ c và cái đ u có thân đen mun. Có th ể ề ự ng đ i c a đ i con nói gì v s l ng t ươ n u cho hai con ru i gi m F1 đó lai v i ớ ấ ồ t các m t đo n th nhau? (Cho bi ấ ch t ợ ế ở ạ ồ
ộ ị ợ
ộ ớ ờ
ạ ấ ệ
ng trong k t qu này và b n có ả ườ ạ
ề
ủ
ợ ề ể ể ị ợ ợ ọ
• Bài 3: Cho m t cây d h p ABCDE/abcde lai v i m t cây abcde/abcde, và đ i con xu t hi n sáu d ng sau: ABCDE; abcde; Abcde; aBCDE; ABCDe; abcdE. Có đi u ề gì b t th ế ấ i thích đi u đó ra sao? (G i ý: s th gi ự ể ả thi u v ng c a hai ho c nhi u ki u hình ặ ắ ế c kỳ v ng g i ra m t th d h p đ o đ ộ ượ đo n có liên quan đ n các gene liên k t). ế ạ ả ế
