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GENERAL BIOLOGY Notes (Part 7) - 4.0ed the course!

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Department
Biological Sciences
Course Code
01:119:115
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A L L

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Chromosomal basis of inheritance 10/22/2013 Given that female mammals (including humans) inherit two X chromosomes, do they make twice the  amount of proteins coded by x­linked genes? No Genes on X chromosome are important to females and males In females, one X chromosome in each cell is inactivated during embryonic development (X inactivation in  females) Inactive X chromosome condenses into an object called a Barr body Therefore, at functional level, females are also hemizygous Barr body – very dense, stains darkly, and lies inside of nuclear envelope Active X looks indistinguishable from other autosomes Inactivation involves modification of DNA and histone proteins Inactivation of X chromosome is random and independent in each embryonic cell X inactivation in tortoiseshell cats Some genes involved in coat color are X­linked Figure 15.8 There are approximately 20,000­25,000 genes in humans But, there are only 23 homologous pairs of chromosomes Therefore, there must be many genes on any given chromosomes (called linked genes) Linkage – tendency for groups of genes on same chromosome to be inherited together Chromosomes are inherited as units Pair and separate during meiosis Linked genes tend to be inherited together in successive generations (they are not acting independently) Violates Mendel’s principle of independent assortment (2  law) Unlinked genes Let A and B be unlinked genes Independently assorting On different chromosomes P                     AABB x aabb P gametes        AB       ab F1                   AaBb F1 gametes      ¼ AB ¼ Ab ¼ aB ¼ ab Linked genes Complete linkage 2 genes are so close together on same chromosome that they are inherited as unit Let B and D be linked P                     BBDD x bbdd P gametes        BD        bd
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