Result two of the gametes have normal numbers of

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Result: Two of the gametes have normal numbers of chromosome, one has an extra chromosome, and the other one is missing a chromosome. Nondisjunction in meiosis II
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The risk of trisomy 21 increases with the age of the mother Risk of a 48 year old woman to have a child with trisomy 21 is 1/14, and any chromosomal aberration is 1/10! Oocytes are arrested in the prophase of Meiosis I for 10-40 years!
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Trisomy 21, 13, and 18 are the only three human autosomal anneuplod genotypes where some babies survive to birth Trisomy 13, Patau syndrome, very few survive to birth, many are mosaic for the trisomy genotype, cleft palate, brain development defects, feeding difficulties, extra fingers and or toes (polydactyly) frequency- 1/10,000 live births Trisomy 18, Edwards syndrome, half die prior to birth, joint contractures, poor developmental delays, difficulty in feeding and breathing, mental retardation, small head (microcephaly), frequency 1/3,000 live births * Chromosomes 13, 18, and 21 have the highest densities of G-bands = most gene- poor chromosomes Bill McGinnis
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Why don t we observe trisomies of other chromosomes? Embryos that have them do not survive to birth. It is estimated that 1/2 to 2/3 of human conceptions do not develop successfully to term. Chromosomal abnormalities are thought to be the cause for most naturally terminated pregnancies.
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Mental retardation in Down syndrome 1. Brain morphology and cognitive abilities at birth essentially normal, but postnatal development slows down 2. Affected regions: hippocampus, cerebellum and prefrontal cortex. Changes include: reduced volume of these regions, reduced neuronal density 3. Cognitive impairments: spatial learning, language skills, 3. Degeneration of basal forebrain cholinergic neurons (BFCN), which synthesize and release into the brain Acetylcholine, a neurotransmitter essential for learning And much more
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Clicker question A non-disjunction of Chromosome 21 occurs in meiosis I of an egg cell. Assuming that all 4 gametes were fertilized by normal sperm cells, what would the karyotypes of the resulting zygotes (early embryos)? A. 2 zygotes with normal karyotype, 1 monosomic for Chr. 21, one trisomic for Chr. 21 B. 3 zygotes with normal karyotype, one trisomic for Chr. 21 C. 2 zygotes trisomic for Chr. 21, two monosomic for Chr.
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