MCB164+S08+ICE+3+key

MCB164+S08+ICE+3+key - The two homologs are held together...

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MCB164 S08 Names of group members:____________________________ April 8, 2008 In class exercise #3 _____________________________ Score: ____KEY______ _____________________________ The average number of crossovers per chromosome pair in mammals is ~1.4. A mechanism exists to assure that every chromosome receives at least one crossover per bivalent. When the distribution of crossing over is assayed along the length of chromosome 21 in humans with Down’s syndrome (trisomy 21), the crossover tends predominantly to be telomere proximal (i.e. near the telomeres) when compared to the normal distribution of crossovers in normal meioses. 1. In the figure shown below circle the structure of the bivalent that would resemble a telomere proximal crossing over event. Metaphase I 2. The maternal age effect for the increased incidence of births with trisomy 21 may be due in part by the location of the crossover. Why might this be the case?
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Unformatted text preview: The two homologs are held together by a much shorter region of sister chromatid cohesion compared to centromere proximal crossover (i.e. bivalent #4) NB: Telomere shortening is generally associated with lagging strand DNA replication. Since there is no DNA replication during the arrest period, telomeres, in principle, should not be getting shorter. 3. Diagram the chromosome segregation pattern in female meiosis leading to an egg disomic for chromosome 21 where the effect is due to a telomere proximal crossover event. 4. How many gametes are formed per meiosis in female mammals? One: The other chromosomes are contained in one of two polar bodies, which are degraded once the zygote forms. mom dad 1 st polar body (no chromosome 21 2nd polar body chromosome 21 disome Ovum chromosome 21 disome...
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