Chapter+20+Genetics+NLL

Chapter 20 Genetics

__Chapter 20 Patterns Of Genetic Inheritance__ Genotype: Genes of an individual for a particular trait or traits; often designated by letters, for example, BB, AA. Alleles: Alternative form of a gene; alleles occur at the same locus on homologous chromosomes. Recessive Allele: Allele that exerts its phenotypic effect only in the homozygote; its expression is masked by a dominant allele. Homozygous Dominant: Possessing two identical alleles, such as AA, for a particular trait. Homozygous Recessive: Possessing two identical alleles, such as aa, for a particular trait. Heterozygous: Possessing unlike alleles for a particular trait. Phenotype: Visible expression of a genotype—for example, brown eyes or attached earlobes.
 * 1. Define genotype, alleles, dominant allele, recessive allele, homozygous dominant, and homozygous recessive, heterozygous, phenotype**


 * 2. What is the difference between genotype and phenotype?** The physical appearance of the individual—in this case, unattached earlobes—is called the phenotype. Genotype refers to the genes of the individual.


 * 3. What are the three possible genotypes and the two possible phenotypes for a characteristic that is controlled by two alleles, one being dominant and the other recessive?** The three possible genotypes are homozygous dominant, homozygous recessive, and heterozygous. A dominant allele contributed from only one parent can bring about a particular dominant phenotype. A recessive allele must be received from both parents to bring about the recessive phenotype. The phenotype was attached or unattached earlobes. The phenotype can be any characteristic of the individual, including color blindness or a metabolic disorder such as the lack of an enzyme to metabolize the amino acid phenylalanine.


 * 4. Write down the question and answer for questions 1,2,3 on page 423.**

1. For each of the following genotypes, give all possible gametes. A. WW B. WS, Ws, Ss, WW C. Tt D. tg, gg, Tg E. AB, ab, Ab Ba 2. For each of the following state whether a genotype or a gamete is represented. A. D→Gamete B. Ll→Genotype C. Pw→Gamete D. LlGg→Genotype 3. What is the genotype of the individual from the following crosses? A. EeSs→ unattached earlobes & short fingers B. eeSS→ attached earlobes & short fingers


 * 5. Define the following genetic disorders on pp. 431 to 432 Make sure you describe the function of the gene that is mutated and what happens with the mutated gene.**

At first, it is not apparent that a baby has Tay-Sachs disease. However, development begins to slow down between four and eight months of age, and neurological impairment and psychomotor difficulties them become apparent. The child gradually becomes blind and helpless, develops, uncontrollable seizures, and eventually becomes paralyzed.
 * Tay-sachs disease**: Tay-Sachs disease is well-known autosomal recessive disorder that occurs usually among Jewish people in the United States, most of whom are of central and eastern European descent. Tay-sachs disease results from a lack of the enzyme hexosaminidase A and the subsequent storage of its substrate, a glycosphingolipid, in liposomes. Liposome’s build up in many body cells, but the primary sites of storage are the cells of the brain, which accounts for the onset of symptoms and the progressive deterioration of psychomotor functions.


 * Cystic fibrosis:** Cystic Fibrosis is an autosomal recessive disorder that occurs among all ethnic groups, but it is the most common lethal genetic disorder among Caucasians in the U.S. Research has demonstrated that chloride ions fail to pass through the membrane, sodium ions and water follow. It is believed that lack of water is the cause of abnormally thick mucus in bronchial tubes and pancreatic ducts. In these children, the mucus in the bronchial tubes and pancreatic ducts is particularly thick and viscous, interfering with the function of the lungs and pancreas. To ease breathing, the thick mucus in the lungs has to be manually loosened periodically, but still the lungs become infected frequently. Clogged pancreatic ducts prevent digestive enzymes from reaching the small intestine, and to improve digestion, patients take digestive enzymes mixed with applesauce before every meal.


 * Phenylketonuria**: PKU is an autosomal recessive metabolic disorder that affects nervous system development. Affected individuals lack an enzyme that is needed for the normal metabolism of the amino acid phenylalanine, and therefore, it appears in the urine and the blood. Newborns are routinely tested in the hospital for elevated levels of phenylalanine in the blood. If elevated levels are detected, newborns will develop normally if they are placed on a diet low in phenylalanine, which must be continued until the brain is full developed, around the age of seven, or else severe mental retardation develops. Some doctors recommend that the diet continue for life, but in any case pregnant women with phenylketonuria must be on the diet in order to protect her unborn child from harm.


 * Sickle-cell disease**: Sickle disease is an autosomal recessive disorder in which the red blood cells are not biconcave disks like normal red blood cells; they are irregular. In fact, many are sickle-shaped. The defect is caused by abnormal hemoglobin that differs from normal hemoglobin by one amino acid in the protein globin. The single amino acid changes cause hemoglobin molecules to stack up and form insoluble rods, and the red blood cells become sickle-shaped.


 * Marfan Syndrome:** Marfan syndrome, an autosomal dominant disorder, is caused by a defect in an elastic connective tissue protein, called fibrilin. This protein is normally abundant in the lens of the eye, the bones of limbs, fingers, and ribs; and also in the wall of the aorta. This explains why the affected person often has a dislocated lens, long limbs and fingers, and a caved-in chest. The aorta wall is weak and can possibly burst without warning. A tissue graft can strengthen the aorta, but Marfan patients with aortic symptoms still should not overexert themselves.


 * Huntintongons’s disease:** Huntintongon’s disease is a neurological disorder that leads to progressive degeneration of brain cells. The disease is caused by a mutated copy of the gene for a protein, called huntingtin. Most patients appear normal until they are of middle age and have already had first sign of the disease will appear during the teen years or even earlier. There is no effective treatment, and death come 10 to 125 years after the onset of the symptoms. Researchers found that the gene for Hungtintongon disease was located on the chromosome 4. A test was developed for the presence of the gene, but few people want to know if they have inherited the gene because there is no cure. At least now we know that the disease stems from a mutation that causes the huntingtin protein to have too many copies of the amino acid glutamine.