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day8_07-2 - Case 5: A Chromosomal Disease A four month old...

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Case 5: A Chromosomal Disease A four month old child is brought to a pediatric clinic for evaluation The male child is the product of an uneventful first pregnancy in a 35 year old The mother is concerned that the child looks “strange” (she says “mongoloid” ) and does not “act right” – The child has hypotonia (“floppy muscles”) – A large protruding tongue Is developmentally retarded Has epicanthal folds and vertical palpebral fissures (A fold of skin that comes down across the inner angle of the eye and opening between the eyelids in “doctor speak”) – Is suspected of having congenital (present at birth) heart and GI defects
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Case 5: The Child The pediatrician suspects Down Syndrome – (Note: Down Syndrome used to be referred to as Mongoloidism. Please do not use this term as it is insulting and inaccurate) To confirm the diagnosis the physician orders a karyotype study of the child. (Investigation of the structure and number of chromosomes)
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Case 5: The Child’s Chromosomes The child is aneuploid (has 47 or 2n + 1 chromosomes) and has three copies of chromosome 21 ( trisomy -21 ) This is diagnostic for Down Syndrome
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Case 5 Etiology and Pathogenesis Down syndrome is congenital and 95%+ of cases are caused by trisomy related to non- disjunction of chromosomes during meiosis This is almost always in the female gamete during first division, this starts in embryonic life Hence, this mother’s eggs were ~35 years old Down syndrome is clearly related to increased maternal age (and less so to increased paternal age Down phenotype is related to the extra copies of gene(s) on the distal part of 21q (away from center) Meiosis in Female
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Case 5: Genetic Counseling (1) The foetus of mothers>~35 of age should be tested for abnormal karyotype but Many cases of Down occur in younger mothers (where most births occur) Down syndrome may be inherited because of a defect in chromosome structure (no age effect) (see next page) Women with a history (or family history) of congenital abnormalities, spontaneous abortions should be tested regardless of age!
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Case 5: Genetic Counseling (2) One form of inherited Down is caused by a translocation of the long arm of chromosome 21 to the long arm of 14. (Individuals who are 45 t(14q21q) (that is have one translocated 14+21 and one each of a normal 14 and 21) are normal but they can transmit two doses of chromosome 21 to their offspring (resulting in trisomy 21 after fertilization) Such individuals may have a family history of Down Syndrome (or have multiple affected offspring
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Case 5: Genetic Counseling (3) Down Syndrome in young(er) women is a problem – It may occur because of translocations but also non-disjunction – Because most births are in young(er) women the number of such Down cases is concerning & costly (although the frequency/birth is low) – Fetal sampling to determine karyotype is associated with some risk to the foetus and is expensive – Several simple blood tests may help in identifying chromosome defects in young(er) women
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This note was uploaded on 01/09/2011 for the course PATH 128 taught by Professor Reisner during the Fall '10 term at UNC.

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day8_07-2 - Case 5: A Chromosomal Disease A four month old...

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