Chapter 15 cont.

1. Chapter 15 cont. Biology

3. A. Karotypes shows chromosomes paired according to size, shape, and appearance in metaphase. can be used to diagnose chromosomal abnormalities.

4. B. Nondisjunction Causes Abnormalities failure of one or more chromosomes to separate. Study of spontaneous abortions suggests many trisomies and nearly all monosomies are fatal.

8. C. Down Syndrome 1. Most common autosomal trisomy, involves chromosome 21. 2. Most often, Down syndrome is due to nondisjunction during gametogenesis.

9. C. Down Syndrome a. In 23% of cases, the sperm had the extra chromosome 21. b. In 5% of cases, there is translocation where chromosome 21 is attached to chromosome 14.

10. C. Down Syndrome 3. Chances of a woman having a Down syndrome child increase with age. 4. Chorionic villi sampling or amniocentesis and karyotyping detects a Down syndrome child.

13. C. Down Syndrome 5. Down syndrome child has tendency for leukemia, cataracts, faster aging, and mental retardation. 6. Gart gene, located on bottom third of chromosome 21, leads to high level of purines; is associated with mental retardation; future research may lead to suppression of this gene.

15. How do children with Down syndrome develop? Children with Down syndrome are usually smaller physical and mental developments are slower mild to moderate range of mental retardation, however, some children are not mentally retarded at all motor development is slow and instead of walking by 12 to 14 months as other children do, children with Down syndrome usually learn to walk between 15 to 36 months. Language development is also markedly delayed.

16. What are the physical features of a child with Down syndrome? flattening of the back of the head slanting of the eyelids small skin folds at the inner corner of the eyes depressed nasal bridge slightly smaller ears, small mouth decreased muscle tone, loose ligaments, and small hands and feet About fifty percent of all children have one line across the palm, and there is often a gap between the first and second toes

17. How may chromosome subtypes are observed in Down syndrome? 1. 95 percent have an extra 21 chromosome. Instead of the normal number of 46 chromosomes in each cell, the individual with Down syndrome has 47 chromosomes. This condition is called trisomy 21. 2. 3 - 4 percent of the individuals with Down syndrome have a translocation since the extra 21 chromosome is attached, usually on chromosome 14, 21 or 22. I 3.Another chromosome problem, called mosaicism, is noted in about 1 percent of individuals with Down syndrome. In this case, some cells have 47 chromosomes and others have 46 chromosomes. Mosaicism is thought to be the result of an error in cell division soon after conception.

18. Can Down syndrome be medically treated? Although many medications and various therapies have been touted as treatment for people with Down syndrome, there is no effective medical treatment available at the present time. However, recent advances in molecular biology make it feasible now to examine the genetic basis for Down syndrome. Once we identify the genes on chromosome 21 ( many already have been discovered) and once we find the mechanism of how these genes interfere with normal developmental sequences, and if one could counteract these specific actions, a rational approach to medical therapy could emerge.

19. What health concerns are often observed in people with Down syndrome? 1.Sixty to 80 percent of children with Down syndrome have hearing deficits. 2.Forty to 45 percent of children with Down syndrome have congenital heart disease. 3.Intestinal abnormalities also occur at a higher frequency in children with Down syndrome. For example, a blockage of the esophagus, duodenum, and at the anus are not uncommon in infants with Down syndrome. 4. 3 percent of infants with Down syndrome have cataracts. Other eye problems such as cross-eye (strabismus), near-sightedness, far-sightedness. 5. Obesity is often noted during adolescence and early adulthood. 6.Skeletal problems have also been noted at a higher frequency in children with Down syndrome, including kneecap subluxation (incomplete or partial dislocation), hip dislocation, and atlantoaxial instability. 7.Other important medical aspects in Down syndrome, including immunologic concerns, leukemia, Alzheimer disease, seizure disorders, sleep apnea and skin disorders, may require the attention of specialists in their respective fields.

20. What is the cause of Down syndrome? Although many theories have been developed, it is not known what actually causes Down syndrome. Some professionals believe that hormonal abnormalities, X-rays, viral infections, immunologic problems, or genetic predisposition may be the cause of the improper cell division resulting in Down syndrome. It has been known for some time that the risk of having a child with Down syndrome increases with advancing age of the mother; i.e., the older the mother, the greater the possibility that she may have a child with Down syndrome. However, most babies with Down syndrome (more than 85 percent) are born to mothers younger than 35 years. Some investigators reported that older fathers may also be at an increased risk of having a child with Down syndrome. It is well known that the extra chromosome in trisomy 21 could either originate in the mother or the father. Most often, however, the extra chromosome is coming from the mother.

21. D. X and Y Numbers Also Change 1. XYY males with Jacob syndrome have two Y chromosomes instead of one. a. Usually taller than average; suffer from persistent acne; tend to have lower intelligence. b. Earlier claims that XYY individuals were likely to be aggressive are not correct.

22. D. X and Y Numbers Also Change 2. Turner (XO) syndrome females have only one sex chromosome, an X. a. Turner females are short, have a broad chest and webbed neck. b. Ovaries of Turner females never become functional; therefore, do not undergo puberty.

23. D. X and Y Numbers Also Change 3. Klinefelter syndrome males have one Y chromosome and two or more X chromosomes. a. Affected individuals are sterile males; testes are underdeveloped. b. Individuals have large hands and feet and long arms and legs.

25. D. X and Y Numbers Also Change 4. Triplo-X females have three or more X chromosomes. a. There is no increased femininity; most lack any physical abnormalities. b. May experience menstrual irregularities, including early onset of menopause.

27. E. Fragile X Syndrome 1. X chromosome is nearly broken; most often found in males. 2. As children: hyperactive or autistic; delayed speech.

28. E. Fragile X Syndrome 3. As adults: large testes, unusually protruding ears. 4. Occurs in females, but symptoms are less severe. 5. Passes from symptomless male carrier to grandson.

30. D. Pedigree Charts 1. Pedigree charts show pattern of inheritance within a family.

32. D. Pedigree Charts 3. A carrier is a heterozygous individual who has no apparent abnormality but can pass on an allele for a recessively inherited genetic disorder.

33. D. Pedigree Charts 4. Autosomal dominant and autosomal recessive alleles have different patterns of inheritance.

36. C. Some Disorders Are Dominant

38. 1. Neurofibromatosis a. This is an autosomal dominant disorder that affects one in 3,000 people. b. Affected individuals have tan skin spots at birth, which develop into benign tumors.

40. 1. Neurofibromatosis c. Neurofibromas are comprised of nerve cells or other cell types. d. Most case symptoms are mild, patients live a normal life; sometimes symptoms are severe:

41. 1. Neurofibromatosis 1) skeletal deformities, including a large head; 2) eye and ear tumors that can lead to blindness and hearing loss; and 3) learning disabilities and hyperactivity.

42. 1. Neurofibromatosis e. Gene that codes for neurofibromatosis is huge; includes three smaller nested genes. 1) It is a tumor-suppressor gene active in controlling cell division. 2) When it mutates, a benign tumor results.

44. 2. Huntington Disease a. This is also an autosomal dominant disorder that affects one in 20,000 people. b. It leads to progressive degeneration of brain cells, which in turn causes severe muscle spasm, personality disorders, and death in 10-15 years from onset.

45. 2. Huntington Disease e. Gene contains many repeats of base triplet CAG; normal persons have 11- 34 copies; affected persons have 42- 120 or more copies. f. Severity and time of onset of associated disorders depend on number of triplet repeats.

46. C. Some Disorders Are Recessive

47. 1. Tay-Sachs Disease a. Usually occurs among Jewish people in the U.S. of central and eastern European descent.

48. 1. Tay-Sachs Disease b. Symptoms are not initially apparent; infant's development begins to slow at 4-8 months, neurological and psychomotor difficulties become apparent, child gradually becomes blind and helpless, develops seizures, eventually becomes paralyzed, dies by age of three or four.

49. 1. Tay-Sachs Disease c. Results from lack of enzyme hexosaminidase A (Hex A) and subsequent storage of its substrate, glycosphingolipid, in lysosomes.

50. 1. Tay-Sachs Disease d. Primary sites of storage are cells of the brain; accounts for progressive deterioration. e. No treatment or cure; prenatal diagnosis is by amniocentesis and chorionic villi sampling.

52. 2. Cystic Fibrosis a. This recessive autosomal disease is most common lethal genetic disease in Caucasians in U.S. b. About 1 in 20 Caucasians is a carrier, and about 1 in 2,500 births has this disorder.

53. 2. Cystic Fibrosis c. Involves production of viscous form of mucus in the lungs and pancreatic ducts. 1) Resultant accumulation of mucus in the respiratory tract interferes with gas exchange. 2) Digestive enzymes must be mixed with food to supplant the pancreatic juices.

54. 2. Cystic Fibrosis d. New treatments have raised average life expectancy to 28 years. e. Research has demonstrated chloride ions (Cl-) fail to pass plasma membrane proteins.

55. 2. Cystic Fibrosis f. Since water normally follows Cl- , lack of water in the lungs causes thick mucus. g. Cause is mutated gene on chromosome 7; attempt to insert gene into nasal epithelium has had limited success. h. Genetic testing for adult carriers and fetuses is possible.

56. 3. Phenylketonuria (PKU) a. PKU occurs 1 in every 5,000 births; it is most common inherited disease of nervous system. b. Lack of enzyme needed to metabolize amino acid phenylalanine results in accumulation of the amino acid in nerve cells of the brain; this impairs nervous system development.

57. 3. Phenylketonuria (PKU) c. PKU is caused by a mutated gene on chromosome 12. d. Now newborns are routinely tested in hospital for high levels of phenylalanine in the blood. e. If infant has PKU, child is placed on diet low in phenylalanine until brain is fully developed.

58. E. Other Inheritance Patterns

59. 1. Polygenic Traits a. Polygenic inheritance occurs when one trait is governed by two or more sets of alleles. b. Dominant alleles have a quantitative effect on the phenotype: each adds to the effect.

60. 1. Polygenic Traits c. Result is a continuous variation in phenotypes: a bell-shaped curve. d. A hybrid cross for skin color provides a range of intermediates.

62. 1. Polygenic Traits e. Includes cleft lip, clubfoot, hypertension, diabetes, schizophrenia, allergies and cancers. f. Behavioral traits including suicide, phobias, alcoholism, and homosexuality may be associated with particular genes but are not likely completely predetermined.

63. 2. Multiple Alleles a. Occur where a gene has three or more alternative expressions (alleles). b. The ABO system of human blood type is a multiple allele system.

64. 2. Multiple Alleles 1) Two dominant alleles (A and B) code for presence of A and B antigens on red blood cells. 2) Also includes recessive allele (o) coding for no A or B antigens on red blood cells. 3) As a result, there are four possible phenotypes (blood types): A, B, AB, and O.

66. 3. Sickle-cell disease is a blood disorder controlled by incompletely dominant alleles.

67. 3. Sickle-cell disease With sickle-cell disease, red blood cells are irregular in shape (sickle-shaped) rather than biconcave, due to abnormal hemoglobin that the cells contain.

68. 3. Sickle-cell disease c. Due to irregular shape, sickle-shaped red blood cells clog vessels and break down; results in poor circulation, anemia, low resistance to infection, hemorrhaging, damage to organs, jaundice, and pain of abdomen and joints.

69. 3. Sickle-cell disease d. Persons heterozygous for sickle-cell are usually asymptomatic unless stressed.

70. 3. Sickle-cell disease e. In malaria regions of Africa, infants heterozygous for sickle-cell allele have better chance of surviving; malaria parasite dies as potassium leaks from sickled cells. f. Bone marrow transplants pose high risks; other research focuses on fetal hemoglobin, etc.

72. C. X-Linked Disorders

73. 1. Color Blindness a. Can be X-linked recessive disorder involving mutations of genes coding for green or red-sensitive cone cells, resulting in inability to perceive green or red, respectively.

74. Possible genotypes for color blindness 1) XB XB = a female who has normal color vision; 2) XB Xb = a carrier female who has normal color vision; 3) Xb Xb = a female who is color blind; 4) XBY = a male who has normal color vision; and 5) XbY = a male who is color blind.

76. 2. Muscular Dystrophy a. characterized by wasting away of muscles, eventually leading to death; it affects one out of every 3,600 male births. b. X-linked recessive disease involves a mutant gene that fails to produce protein dystrophin.

78. MDA National Chairman Jerry Lewis MUSCULAR DYSTROPHIES: Duchenne Muscular Dystrophy (DMD) Becker Muscular Dystrophy (BMD) Emery-Dreifuss Muscular Dystrophy (EDMD) Limb-Girdle Muscular Dystrophy (LGMD) Facioscapulohumeral Muscular Dystrophy (FSH) (Also known as Landouzy-Dejerine) Myotonic Dystrophy (MMD) (Also known as Steinert's Disease) Oculopharyngeal Muscular Dystrophy (OPMD) Distal Muscular Dystrophy (DD) (Miyoshi) Congenital Muscular Dystrophy (CMD)

79. MUSCULAR DYSTROPHIES: MOTOR NEURON DISEASES: Amyotrophic Lateral Sclerosis (ALS) (Also known as Lou Gehrig's Disease) INFLAMMATORY MYOPATHIES: DISEASES OF THE NEUROMUSCULAR JUNCTION: Myasthenia Gravis (MG) MYOPATHIES DUE TO ENDOCRINE ABNORMALITIES: DISEASES OF PERIPHERAL NERVE: METABOLIC DISEASES OF MUSCLE:

80. 2. Muscular Dystrophy c. Symptoms (e.g., waddling gait, toe walking, frequent falls, difficulty in rising) soon appear. d. Muscle weakens until individual is confined to wheelchair; death usually occurs by age 20.

82. 2. Muscular Dystrophy e. Affected males are rarely fathers; the gene passes from carrier mother to carrier daughter. f. Lack of dystrophin causes calcium ions to leak into muscle cells; this promotes action of an enzyme that dissolves muscle fibers.

83. 2. Muscular Dystrophy g. As body attempts to repair tissue, fibrous tissue forms and cuts off blood supply. h. Test detects carriers of muscular dystrophy; treatments are under research.

85. 3. Hemophilia About one in 10,000 males is a hemophiliac with impaired ability of blood to clot. Hemophiliacs bleed externally after an injury and also suffer internal bleeding around joints.

86. When was hemophilia first recognized? Hemophilia was recognized, though not named, in ancient times. The Talmud, a collection of Jewish Rabbinical writings from the 2nd century AD, stated that male babies did not have to be circumcised if two brothers had already died from the procedure.

87. 3. Hemophilia Hemorrhages stop with transfusions of blood (or plasma) or concentrates of clotting protein. Hemophiliacs were at high risk of AIDS if receiving blood or using blood concentrate to replace clotting factors.

88. 3. Hemophilia Of Queen Victoria's 26 offspring, 5 grandsons had hemophilia, 4 granddaughters were carriers.

92. The End.