Lecture04 - BIS101/Engebrecht Lecture04 4/6/10...

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BIS101/Engebrecht Lecture04 4/6/10 Announcements: Discussions sections are meeting this week. Homework for discussion can be found under “Assignments” on SmartSite. The review quiz is due Friday, April 9 th online. Dosage Compensation : How does an organism deal with having different copies of genes on the sex chromosomes between males and females? The answer is through Dosage compensation = equalizing of the amount of gene product in the two sexes. Different organisms have different strategies. In mammals, females inactivate one X, producing a Barr body, a highly condensed chromosome that is relatively inactive for transcription. This is random and occurs after several divisions so in different clonal lines, different X’s will be inactivated resulting in a mosaic individual, with two genetically distinct cell types. A very visual example is the calico cat, where coat color is on the X. We discussed the findings from the sequencing of the X chromosome: 1. There is a large percentage of repeat sequences (~50%). It is believed that this is important for X inactivation and helps lead to the spreading of the heterochromatic DNA that results in silencing. 2. X inactivation is not complete, some genes on the X chromosome are still expressed even from the Barr body. It was found that 75% are inactive, 15% are expressed and 10% are variably expressed. 3. Large number of genes involved in brain function. It is hypothesized that the X chromosome may help define the difference between other mammals and our selves and help explain the differences between males and females. Recent studies indicate that the spectrum of genes that escape inactivation are more widespread then originally thought. I showed you a recent publication indicating that hypertension is influenced by X-linked genes. This also explains why Turner syndrome individuals have cardiovascular and cognitive problems in addition to being sterile. CQ1: If the single X chromosome of the father carried the orange alleles, no black females would be observed. Conversely, if the single X chromosome of the father carried the black allele, no orange females would be observed. Since we see no orange females, the father must be black and the cross is X O X o x X o Y, the answer is C. CQ2: This is due to chromosome nondisjunction resulting in a XXY male, the answer is C. I posed the question as to why Morgan didn’t see flies with red and white patches?
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This note was uploaded on 07/02/2010 for the course BIS 101 taught by Professor Simonchan during the Spring '08 term at UC Davis.

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Lecture04 - BIS101/Engebrecht Lecture04 4/6/10...

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