Genetic+Inheritance+MV

Molly Vaughn 17 November 2008 Human Biology

Chapter 20 Patterns Of Genetic Inheritance 1. Define genotype, alleles, dominant allele, recessive allele, homozygous dominant, homozygous recessive, heterozygous, phenotype - Genotype: Genes of an individual for a particular trait or traits; often designated by letters, for example, BB or Aa. - Alleles: Alternative form of gene; alleles occur at the same locus on homologous chromosomes. - Dominant allele: Allele that exerts its phenotypic effect in the heterozygote; it makes the expression of the recessive allele. - Recessive allele: Allele that exerts its phenotypic effect only in the homozygote: it expression is masked by a dominant allele. - Heterozygous: Possessing unlike alleles for a particular trait. - Phenotype: Visible expression of a genotype-for example, brown eyes or attached earlobes.

2. What is the difference between genotype and phenotype? - Genotypes are genes of an individual for a particular trait, and a phenotype is when you can actually see the effects of the genotype visibly.

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 the homozygous dominant genotypes, homozygous recessive genotypes, and the heterozygous genotypes.

4. Write down the question and answer for questions 1,2,3 on page 423. - 1. A) A homozygous dominant genotype B) A homozygous dominant and heterozygous genotype C) A heterozygous genotype D) A heterozygous recessive genotype E) A heterozygous genotype - 2. A) Gamete B) A heterozygous genotype C) A heterozygous genotype D) Two heterozygous genotypes - 3. A) Es B) EsEs

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. Tay-sachs disease: Tay-sachs disease is a 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 result s from a lack of the enzyme hexosaminidase A (Hex A) and the subsequent storage of its substrate, a glycosphingolipid, in lysosomes. Lysosomes 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. 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 then become apparent. The child gradually becomes blind and helpless, develops uncontrollable seizures, and eventually becomes paralyzed.

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 United States. Research has demonstrated that chloride ions fail to pass through a plasma membrane channel protein in the cells of these patients. Ordinarily, after chloride ions have passed 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: Phenylketonuria 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 fully 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, a pregnant woman with phenylketonuria must be on the diet in order to protect her unborn child from harm.

Sickle-cell disease: Sickle-cell 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 cause by an abnormal hemoglobin that differs from normal hemoglobin by one amino acid in the protein globin. The single amino acid change causes hemoglobin molecules to stack up and form insoluble rods, and the red blood cells become sickle-shaped. Because sickle-shaped cells can’t pass along narrow capillary passageways as disk-shaped cells can, they clog the vessels and break down. This is why persons with sickle-cell disease suffer from poor circulation, anemia, and low resistance to infection. Internal hemorrhaging leads to further complications, such as jaundice, episodic pain in the abdomen and joints, and damage to internal organs. Sickle-cell heterozygotes have sickle-cell traits in which blood cells are normal unless they experience dehydration or mild oxygen deprivation. Still, at present, most experts believe that persons with sickle-cell trait do not need to restrict their physical activity.

Marfan Syndrome: Marfan syndrome, an autosomal dominant disorder, is cause by a defect in an elastic connective tissue protein, called fibrillin. 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 finger, and a caved-in chest. The aorta wall is week and can possibly burst without warning. A tissue graft can strengthen the aorta, but Marfan patients with aortic symptoms still should not overexert themselves.

Huntington’s disease: Huntington 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 children, who may later also be stricken. Occasionally, the first sign of the disease will appear during the teen years or even earlier. There is no effective treatment, and death comes 10 to 15 years after the onset of symptoms. Several years ago, researches found that the gene for Huntington disease was located on chromosome 4. A test was developed for the presence of the gene, but few people want to know it they have inherited the gene because there is no cure. At least now we know that the disease stems from a mutation that cause the huntingtin protein to have too many copies of the amino acid glutamine. The normal version of huntingtin has stretches of between 10 and 25 glutamines. If huntingtin has more than 36 glutamines, it changes shape and forms large clumps inside neurons. Even worse, it attracts and causes other proteins to clump with it. One of Researchers hope they may be able to combat the disease by boosting CBP levels.