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Lecture 15

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Western University
Biology 1001A
Tom Haffie

Reasons why, if identical twin women have sons with identical twin men, the sons will not be identical - ● The gametes of the two couples will be different as sexual recombination occurs and will result in variation Definition or explanation of "heritability" with respect to human disease risk - ● Heritability of a trait is the observable differences between individuals due to genetic differences ● With respect to human disease risk, heritability causes some genes to be similar in related individuals and thus it means that if one of those individuals has a disease, the other will have a greater risk of developing that disease as well due to similar genes Characteristics that would, and would not, be different between monozygotic ("identical") twins - ● Differences: ○ expression of genes ○ mutations can differ in each person ○ random X inactivation in females; inactivated different X’s in different tissues ○ their gametes are different due to sexual recombination ● Similarities Process of random X inactivation leading to genetic mosaicism - ● Fertilized zygote divides and at about 100 cells stage, each cell will inactivate one of it’s X chromosomes, either the maternal or paternal X ● This process of either X being inactive will occur at random with different cells having different X chromosomes being inactive ● Now, as each of those cells divide, every subsequent cell that their divide into will have their respective X inactivated; if a cell has it’s maternal X inactivated, every cell that is produced through division from that cell will also have the maternal X inactivated ● This is a kind of stable gene regulation as it persists from division to division ● Mosaicism is that two different genetic expressions or cell lines as a result from the same zygote How a mosaic is different from a heterozygote - ● Role of Xist RNAin X inactivation - ● Reason why the X inactive chromosome gets so tightly condensed is because normally the chromosomes are accessible and open to a transcription factories (RNA polymerasse etc) ● However, expression of Xist RNA(only made by inactive X) coats the chromosome and keeps it away from transcription factories ● Xist RNAdirectly associated with the protein, it is the binding of non coding RNA, not base pairing, don’t really know what but it inactivates the X chromosome and it is stable, cell division to cell division ● This is an epigenetic change as there is no mutation and it persists through generations Role of Tsix RNAin regulation of Xist RNAexpression - ● If Xist is expressed, it shuts down a chromosome but it must be controlled so that only one chromosome is shut off ● To control and regulate this, Tsix, an antisense strand is transcribed and its expression shuts of Xist; it is antagonistic to Xist ● Xist and tsix genes are transcribed in opposite directions at the same
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