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Unformatted text preview: • 1959: Most people with the disorder have three copies of
chromosome 21 (a condition called trisomy).
• In rare cases, people with the disorder are trisomic for smaller
parts of chromosome 21.
• Compared these smaller fragments to establish the Down
Syndrome Critical Region, or DSCR. Introduction:
Trisomy 21 results in Down Syndrome • The DSCR is 5 million base pairs long and contains 33 genes.
• Down Syndrome likely results from the overexpression of many
genes that interact in complex ways. Chromosome Morphology • A functional chromosome has:
– Centromere, where spindle ﬁbers attach
– Two telomeres at the ends, that stabilize the chromosome Chromosome Morphology • Four basic types of chromosomes:
– The centromere is in the middle
– Two arms of approximately equal length Chromosome Morphology 2. Submetacentric
– Centromere is closer to one end
– Long (in humans called q) and short arms (in humans
– Long arm and a knob, or satellite Chromosome Morphology 1. Acrocentric
– Centromere is very close to one end
– Long arm and a knob, or satellite
– Centromere is essentially at one end Karyotype
• Complete set of chromosomes
possessed by an organism is called
• Presented as a picture of
metaphase chromosomes lined up
in descending order of their size. Karyotype
• Staining techniques can
chromosomes of similar sizes • e.g. Giemsa dye, which stains
areas that are A-T rich (called
“G bands”) For Wednesday: We’ll discuss aneuploidy and begin to discuss the most
common cause of aneuploidy
Read Chapter 7: Recombination, linkage, and gene mapping...
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