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Linked Genes

Linked Genes

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Epistasis

This is when a gene located at one locus effects a gene on a different locus. An example of this is albinism. There is a gene for both skin color and production of the pigment.  If the pigment is not made the person will be albino regardless of what the skin color gene says. 

Source: Holtzclaw, Fred, and Theresa Holtzclaw. AP Test Prep Series. San Francisco: Pearson Education Inc., 2013. Print.

X-inactivation

This is the process that occurs during embryotic development that results in one of the X-chromosomes in female cell becoming inactive. In mammals, each cell randomly decides which chromosomes will become inactivated and it develops into a Barr body (tightly coiled). When these cells reproduce the same X chromosomes will be inactive in the cells they create. Calico cats are a good example of X-inactivation. Orange and black patches of fur is controlled by the X chromosomes. In the orange patches the orange allele remained on the active chromosome and the black allele remained active in the black patches.

Source: PACK, P. (2013). AP BIOLOGY. (4TH ED., PP. 73-81). BOSTON: HOUGHTON MIFFLIN HARCOURT.

Nondisjunction

Nondisjuction occurs when a pair of chromosomes or chromatids do not separate during cell division. This results in cells with an extra or missing chromosome. If the cell that has an extra chromosome becomes fertilized it will now have three copies of that chromosome which is called trisomy. If the cell missing a chromosome becomes fertilized it will only have one copy of that chromosome which is called monosomy. 

Genetic Disorders

There are many disorders/diseases that result purely from ones genetic makeup. Sickle-cell anemia is one example. This disease is the result of a point mutation in the DNA and it causes the defective hemoglobin to form. The blood cells become sickle shaped when there is low oxygen which prevents them from moving through the blood vessels as easily as they should. This can result in parts of the body not receiving enough oxygen which can greatly damage organs. Individuals with the heterozygous genotype, however, have the advantage of being immune to malaria. 

Tay-Sachs disease is cause by an insertion in the DNA. This disease prevent lysosomes from having the enzyme needed to break down glycolipids. These fats build up around nerve cells in the brain which inevitably kills the cells.

Down syndrome (often called trisomy 21) results from the inheritance of an extra 21st chromosomes. Klinefelter syndrome occurs when a males has an extra X chromosomes (XXY) which results in sterility. Turner syndrome is a form of monosomy where a female only has one X chromosome which also results in sterility.  

 

Source: Holtzclaw, Fred, and Theresa Holtzclaw. AP Test Prep Series. San Francisco: Pearson Education Inc., 2013. Print. PACK, P. (2013). AP BIOLOGY. (4TH ED., PP. 73-81). BOSTON: HOUGHTON MIFFLIN HARCOURT.