Ap Bio Diseases Research

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CHAPTER 14
1. Norm of reaction: the phenotypic range that a genotype is associated with due to environmental influences. 2. Multifactorial: characters that have many factors, both genetic and environmental, collectively influence phenotype. 3. Pleiotropy: when a gene controls multiple phenotypic effects 4. Epistasis: When a gene at one locus alters the phenotypic expression of a gene at a second locus 5. Carrier: a person who is heterozygous for a recessive disease and therefore does not display the phenotype (disease). They are called carriers because although they are phenotypically normal with regard to the disorder, they can transmit the recessive allele to their offspring. They are heterozygotes and normal (Rr) DISEASES

a. Recessively Inherited Disorders
Tay-Sachs disease
* Inherited disorder in humans where the brain cells of a child with the disease cannot metabolize certain lipids because a crucial enzyme does not working properly. * As the lipids accumulate in the brain cell, the child beings to suffer seizures, blindness, and degeneration of motor & mental performance and dies within a few years. * Only children who inherit two copies of Tay-Sachs allele (HOMOZYGOUS) have the disease. Therefore at the organismal level, the Tay Sachs allele qualifies as recessive * The intermediate phenotype observed at the biochemical level is characteristic of incomplete dominance of either allele * At the molecular level, the normal allele and the Tay-Sachs allele are codominant because a person heterozygous for Tay Sachs disease does not have the disease symptoms, but the individual still produces equal number of normal and dysfunctional enzyme molecules Cystic Fibrosis

* Most common lethal genetic disease in the U.S, strikes one out of every 2,500 people of European descent. * The normal allele for this gene codes for a membrane protein that functions in the transport of chloride ions between certain cells and the extracellular fluids. * A person who has the disease has two recessive alleles. * Two recessive alleles result in defective chloride transport channels in their plasma membrane. * Therefore they have a high conc. Of extracellular chloride which makes mucus thicker and sticker and it builds up in the pancreas, lungs, digestive tract, etc. leading to multiple (pleiotropic) effects. This includes poor absorption of nutrients from intestines, chronic bronchitis, recurrent bacterial infections, and disablement of a natural antibiotic made by some body cells.

Sickle-Cell Disease
* Most common inherited disorder among people of African descent, affects 1/400 of A.A. * Caused by the substitution of a single amino acid in the hemoglobin protein of red blood cells. * A person with the disease has 2 recessive alleles.

* When the O2 content of an affected person’s blood is low, the sickle cell hemoglobin molecules aggregate into long rods that dorm the normal circular red blood cells in a shriveled sickle shape. * Sickled cells can clump blood vessels which = physical weakness, pain, organ damage, and even paralysis. * The multiple effects of a two recessive sickle-cell allele are another example of pleiotropy. * At the organismal level, the normal allele is incompletely dominant to the sickle-cell allele. * Heterozygotes of the Sick Cell disease have one sickle-cell allele and another normal allele. They are said to have the sickle-cell trait * Heterozygotes of the disease have a single copy of the sickle cell allele, and it reduces the frequency and severity of malaria attacks, especially among young children. * Therefore, about 1/10 A.A have the sickle cell trait because in tropical Africa, where infection with malaria parasite is common, the sickle-cell allele is more common as well because A.A’s with the sickle cell trait have a higher survival rate. * The presence of heterozygous amounts of sickle-cell hemoglobin...
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