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DOWN SYNDROME. non-disjunction - the failure of homologs or sister chromatids to separate properly to opposite poles during meiosis or mitosis aneuploidy.

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Presentation on theme: "DOWN SYNDROME. non-disjunction - the failure of homologs or sister chromatids to separate properly to opposite poles during meiosis or mitosis aneuploidy."— Presentation transcript:

1 DOWN SYNDROME

2 non-disjunction - the failure of homologs or sister chromatids to separate properly to opposite poles during meiosis or mitosis aneuploidy - a chromosome number that differs from normal (does not include situations where entire sets of chromosomes are lost or duplicated) trisomy - an aneuploidy with an extra chromosome of one type, producing a chromosome number of 2N+1 monosomy - an aneuploidy in which one member of a chromosome pair is missing, producing a chromosome number of 2N-1

3 DOWN SYNDROME PHENOTYPE Epicanthic fold in eye corner IQ seldom above 70 Life expectancy short, few survive to 50 Have simian crease May have heart defects May have small heads May have furrowed tongues 20 fold increased risk of leukemia

4 GENERAL INFORMATION Down caused by non-disjunction of chromosome 21, correlated with age of mother and onset of menopause. Down also caused by attaching chromosome 21 to 14 (translocation), referred to as familial Downs. Amniocentesis and chorionic villi sampling developed because of defects such as Down. Newer, not yet perfected technology, samples fetal cells from maternal circulation. Once fully developed this method will be safer because it is not invasive. Extra copies of human chromosomes generally not well tolerated. Chromosome 21 has only 225 functional genes. Trisomy for chromosomes with more genes results in more pronounced defects; e.g. trisomy for chromosome 16 is common but lethal.

5 Primary non- disjunction 21/21 AI AII 75% of Down occur at MI 90% of all chromosome abnormalities are MI / maternal

6 2nd non- disjunction 21/21 21 Note: non-disjunction very unlikely to occur twice in same meiosis

7 Mothers gametes 21/ Fathers gametes 21 Offspring: 21/21/21 21/- 21/21 Down Lethal Normal 2N: 46+1 = 47 2N: 46-1 = 45 2N: 46 (2N+1 = 47) (2N-1 = 45)

8 Karyotype of Down Syndrome Individual extra chromosome 21 comes from mother 95% of time other viable individuals with extra chromosome are Patau (chromosome 13) and Edwards Syndrome (chromosome 18) …both die 3-4 months chromosomes 13, 18, and 21 have the fewest genes individuals missing one chromosome almost never seen and never viable; may unmask recessive lethal or haploinsufficient genes

9 Correlation Down Syndrome and Maternal Age probability of Down related to onset of menopause, aberrant hormone levels may affect oocyte maturation

10 Familial Down Syndrome long arms of chromosomes 14 and 21 fused with one common centromere (Robertsonian translocation) acentric fragments lost…. apparently without serious consequence for individual genes lost are primarily rRNA (encode RNA found in ribosomes)…..additional copies located elsewhere in genome fused centromere has chromosome 14 identity at meiosis 14 21

11 Offspring from Mating Between Translocation Carrier and Normal Individual 1/3 of surviving offspring expected to be Down another 1/3 surviving progeny expected to be carriers


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