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30 Cards in this Set
- Front
- Back
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Potential parents are becoming aware that many illnesses are caused by abnormal chromosomal inheritance or by gene mutations.
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Therefore, more couples are seeking genetic counseling, which is available at many major hospitals as a means to determine the risk of inherited disorders in a family.
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A karyotype
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is a visual display of the chromosomes arranged by size, shape, and banding pattern. Any cell in the body except red blood cells, which lack a nucleus, can be a source of chromosomes for karyotyping
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Amniocentesis
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is a procedure for obtaining a sample of amniotic fluid from the uterus of a pregnant woman. Blood tests and the age of the mother are considered when determining whether the procedure should be done. The risk of spontaneous abortion increases by about 0.3% due to amniocentesis, and doctors use the procedure only if it is medically warranted. Amniocentesis is not usually performed until the fourteenth to the seventeenth week of pregnancy.
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Chorionic villus sampling (CVS)
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is a procedure for obtaining chorionic cells in the region where the placenta will develop. This procedure can be done as early as the fifth week of pregnancy. A long, thin suction tube is inserted through the vagina into the uterus
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n humans, only a few variations in chromosome number, such as Down syndrome, Turner syndrome, and Klinefelter syndrome, are typically seen. Changes in chromosome structure, however, are much more common in the population.
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Syndromes that result from changes in chromosome structure are due to the breakage of chromosomes and their failure to reunite properly. Various environmental agents—radiation, certain organic chemicals, and even viruses—can cause chromosomes to break apart. Ordinarily, when breaks occur in chromosomes, the segments reunite to give the same sequence of genes. But their failure to do so results in one of several types of mutations: deletion, duplication, translocation, or inversion.
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A deletion occurs when a single break causes a chromosome to lose an end piece, or when two simultaneous breaks lead to the loss of an internal chromosome segment.
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An individual who inherits a normal chromosome from one parent and a chromosome with a deletion from the other parent no longer has a pair of alleles for each trait, and a syndrome can result.
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Williams syndrome occurs
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when chromosome 7 loses a tiny end piece (Fig. 13.4). Children who have this syndrome look like pixies, with turned-up noses, wide mouths, a small chin, and large ears. Although their academic skills are poor, they exhibit excellent verbal and musical abilities. The gene that governs the production of the protein elastin is missing, and this affects the health of the cardiovascular system and causes their skin to age prematurely.
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he colored bands (see Fig. 13.2) are not the natural color of the chromosomes. Instead, using a procedure called fluorescent immunohistochemistry in situ hybridization (FISH for short), short pieces of DNA are labeled with a fluorescent marker.
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These pieces of DNA bind to their complementary sequences on the chromosomes. When exposed to specific wavelengths of light, the markers emit different colors of light. This helps counselors more easily identify the chromosomes. A tagged piece of DNA can also be developed for a specific gene (see the accompanying figure), which helps researchers identify the chromosomal location of a gene of interest.
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Cri du chat (cat’s cry) syndrome
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ccurs when chromosome 5 is missing an end piece. The affected individual has a small head, is mentally disabled, and has facial abnormalities. Abnormal development of the glottis and larynx results in the most characteristic symptom—the infant’s cry resembles that of a cat.
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In a duplication, a chromosome segment is repeated, so that the individual has more than two alleles for certain traits
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An inverted duplication is known to occur in chromosome 15. (Inversion means that a segment joins in the direction opposite from normal.) Children with this syndrome, called inv dup 15 syndrome, have poor muscle tone, mental disabilities, seizures, a curved spine, and autistic characteristics that include poor speech, hand flapping, and lack of eye contact (Fig. 13.5).
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A translocation
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s the exchange of chromosome segments between two nonhomologous chromosomes. A person who has both of the involved chromosomes has the normal amount of genetic material and a normal phenotype, unless the chromosome exchange breaks an allele into two pieces or fuses two genes together. The person who inherits only one of the translocated chromosomes will no doubt have only one copy of certain alleles and three copies of other alleles.
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In 5% of Down syndrome cases,
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a translocation that occurred in a previous generation between chromosomes 21 and 14 is the cause. As long as the two chromosomes are inherited together, the individual is normal. But in future generations a person may inherit two normal copies of chromosome 21 and the abnormal chromosome 14 that contains a segment of chromosome 21. In these cases, Down syndrome is not related to parental age but instead tends to run in the family of either the father or the mother.
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An inversion
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occurs when a segment of a chromosome is turned 180°. You might think this is not a problem because the same genes are present, but the reverse sequence of alleles can lead to altered gene activity if it disrupts control of gene expression.
Inversions usually do not cause problems, but may lead to an increased occurrence of abnormal chromosomes during sexual reproduction. Crossing-over between an inverted chromosome and the noninverted homologue can lead to recombinant chromosomes that have both duplicated and deleted segments. |
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Do we know what genes cause Down syndrome?
Since individuals who inherit Down syndrome by a translocation event receive only a portion of chromosome 21, it has been possible to determine which genes on chromosome 21 are contributing to the symptoms of Down syndrome |
One of the more significant genes appears to be GART, a gene involved in the processing of a type of nucleotide called purines. Extra copies of GART are believed to be a major contributing factor in the symptoms of mental retardation shown in Down syndrome patients. Another gene that has been identified encodes for the protein collagen (COL6A1), a component of connective tissue. Too many copies of this gene cause the heart defects common in Down syndrome.
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A pedigree
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is a chart of a family’s history with regard to a particular genetic trait.
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Some traits are carried on the autosomes (nonsex chromosomes) and are called
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autosomal traits;
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other traits are carried on the sex chromosomes and are, therefore, called
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sex-linked traits.
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Methemoglobinemia
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is a relatively harmless disorder that results from an accumulation of methemoglobin, an alternative form of hemoglobin, in the blood. Since methemoglobin is blue instead of red, the skin of people with the disorder appears bluish-purple in color
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Cystic fibrosis
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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 (Cl−) fail to pass through a plasma membrane channel protein in the cells of these patients.
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Black urine disease, or alkaptonuria
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is a rare genetic disorder that follows an autosomal recessive inheritance pattern. People with alkaptonuria lack a functional copy of the homogentisate oxygenase (HGD) gene found on chromosome 3. The HGD enzyme normally breaks down a compound called homogentisic acid.
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Sickle cell disease
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is an autosomal recessive disorder in which the red blood cells are not biconcave disks like normal red blood cells, but are irregular in shape
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Marfan syndrome
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an autosomal dominant disorder, is caused 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 the wall of the aorta. Thus, the affected person often has a dislocated lens, tall stature, long limbs and fingers, and a caved-in chest. The aorta wall is weak and can burst without warning. A tissue graft can strengthen the aorta, but affected individuals should not overexert themselves.
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Huntington disease
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s a dominant neurological disorder that leads to progressive degeneration of neurons in the brain (Fig. 13.15). The disease is caused by a single mutated copy of the gene for a protein called huntingtin. Most patients appear normal until they are middle-aged and have already had children, who may also have the inherited disorder. There is no effective treatment, and death usually occurs 10 to 15 years after the onset of symptoms.
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Color Blindness
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Color blindness is a common X-linked recessive disorder. About 8% of Caucasian men have red-green color blindness. Most of them see brighter greens as tans, olive greens as browns, and reds as reddish-browns. A few cannot tell reds from greens at all; they see only yellows, blues, blacks, whites, and grays.
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Duchenne muscular dystrophy
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is an X-linked recessive disorder characterized by wasting away of the muscles. The absence of a protein, now called dystrophin, is the cause of the disorder. Much investigative work determined that dystrophin is involved in the release of calcium from the sarcoplasmic reticulum in muscle fibers. The lack of dystrophin causes calcium to leak into the cell, which promotes the action of an enzyme that dissolves muscle fibers. When the body attempts to repair the tissue, fibrous tissue forms (Fig. 13.16), and this cuts off the blood supply, so that more and more cells die.
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As an example, consider that individuals with sickle cell trait or Huntington disease have an abnormality in a gene’s base sequence.
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This abnormality in sequence is a genetic marker.
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e. For example, it is now possible to place thousands of known disease-associated mutant alleles onto
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a DNA microarray—a small silicon chip containing many DNA samples—in this case, the mutant alleles
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If gene therapy becomes routine in the future, it’s possible that an egg will be given genes that control traits desired by the parents, such as musical or athletic ability, prior to IVF.
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Such genetic manipulation, called eugenics, carries many ethical concerns.
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Gene therapy
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is the insertion of genetic material into human cells for the treatment of a disorder. It includes procedures that give a patient healthy genes to make up for faulty genes, as well as the use of genes to treat various other human illnesses, such as cardiovascular disease and cancer.
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On the other hand, sometimes the gene is injected directly into a particular region of the body. This section discusses examples of
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ex vivo gene therapy (the gene is inserted into cells that have been removed and then returned to the body) and in vivo gene therapy (the gene is delivered directly into the body).
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