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CHAPTER 26: GENETIC COUNSELING LECTURE OUTLINE 26.1 Counseling for Chromosomal Disorders Couples seek genetic counseling to determine the risk of a chromosomal or genetic mutation in their family. Various human disorders result from abnormal chromosome number or structure. Such disorders often result in a syndrome, a group of symptoms that always occur together. Karyotyping A karyotype is a visual display of the chromosomes arranged by size, shape, and banding pattern. Cells for karyotype analysis can be obtained from a fetus by amniocentesis or chorionic villi sampling. The Karyotype Cells in metaphase are killed, stained, and photographed. Today a computer is used to arrange the chromosomes in pairs. Changes in Chromosome Number Sometimes individuals are born with either too many or too few chromosomes, most likely due to nondisjunction during meiosis. Nondisjunction occurs during meiosis when either the homologous pair or the sister chromatids fail to separate. Down Syndrome The most common autosomal trisomy seen among humans is trisomy 21, also called Down syndrome. This syndrome has particular characteristics that are easily recognized. Changes in Sex Chromosome Number An abnormal sex chromosome number is the result of inheriting too many or too few X or Y chromosomes. Turner Syndrome Females with Turner syndrome have only one sex chromosome, an X. Klinefelter Syndrome In Klinefelter syndrome, a male inherits two X chromosomes and one Y chromosome. Poly-X Females A poly-X female has more than two X chromosomes. Jacobs Syndrome XYY males, who have Jacobs syndrome, can only result from nondisjunction during spermatogenesis. Changes in Chromosomal Structure A mutation is a permanent genetic change. A change in the structure of a chromosome that can be detected microscopically is a chromosomal mutation. Chromosomal mutations occur when chromosomes break. Deletions and Duplications A deletion occurs when a part of the chromosome is lost. In a duplication, a chromosomal segment is repeated. Translocation A translocation is the exchange of chromosomal segments between two nonhomologous chromosomes. Inversion An inversion occurs when a segment of a chromosome is turned 180 degrees. 26.2 Counseling for Genetic Disorders A counselor needs to know the medical history of the family in order to construct a pedigree. Family Pedigrees A pedigree is a chart that shows the inheritance of a particular genetic trait through successive generations of a family. Males are represented by squares and females by circles. An individual whose symbol is colored displays the genetic trait under consideration. Pedigrees for Autosomal Disorders In an autosomal recessive pedigree, a child that is affected can have two normal parents. The parents are called carriers because they are unaffected but are capable of having a child with the genetic disorder. In an autosomal dominant pedigree, two affected parents can have a normal child. Pedigrees for Sex-Linked Disorders X-Linked Disorders Sons inherit an X-linked recessive allele from their mothers because their fathers gave them a Y chromosome. Y-Linked Disorders A few genetic disorders are carried on the Y chromosome. Unusual Inheritance Patterns A mother and not a father passes mutated mitochondrial genes to her offspring. Genetic Disorders of Interest The following are some of the better-known disorders caused by single-gene mutations. Autosomal Recessive Disorders Examples of autosomal recessive disorders include Tay-Sachs disease, cystic fibrosis, phenylketonuria, and sickle cell disease. Autosomal Dominant Disorders Examples of autosomal dominant disorders include Marfan syndrome and Huntington disease. Incompletely Dominant Disorders An example of an incompletely dominant disorder is familial hypercholesterolemia. X-Linked Recessive Disorders Examples of X-linked recessive disorders include color blindness, Duchenne muscular dystrophy, and hemophilia. Testing for Genetic Disorders The testing method depends on the particular genetic disorder. In some instances it is appropriate to test for a specific protein, and in others to test for the mutated gene. Testing for a Protein Some genetic mutations lead to disorders caused by a missing enzyme. Babies with Tay-Sachs disease lack an enzyme and it is possible to test for the quantity of that enzyme in a sample of cells. Testing the DNA Genetic Markers Testing for a genetic marker is similar to the traditional procedure for DNA fingerprinting. DNA Probes A DNA probe is a single-stranded piece of DNA that will bind to complementary DNA. For the purpose of genetic testing, the DNA probe bears a genetic mutation of interest. A DNA chip is a new technology that consists of a very small glass square containing several rows of DNA probes. The chip allows testing for many genetic disorders at one time. Testing the Fetus, Embryo, or Egg Some serious fetal abnormalities can be detected by ultrasound. DNA of fetal cells, the embryo, and the egg can also be obtained and tested for genetic defects. Ultrasound An ultrasound probe scans the mother’s abdomen, while a transducer transmits high-frequency sound waves that are transformed into a picture on a video screen. Testing Fetal Cells For testing purposes, fetal cells may be obtained by amniocentesis, chorionic villi sampling, or from the mother’s blood. Testing the Embryo Following in vitro fertilization, it is possible to test the embryo without affecting normal development. Testing the Egg Polar bodies can be tested to determine if they carry the normal or mutated allele. The egg would then have the other allele. 26.3 Genomics Genomics is the study of genomes—our genes and the genes of other organisms. Sequencing the Bases We now have a working draft of our genome due to the Human Genome Project. Genome Comparison The genomes of many other organisms are also in the final-draft stage. Researchers can compare the human genome with the genomes of other organisms. The HapMap Project The goal of the HapMap project is to catalog common sequence differences that occur in human beings. Proteomics and Bioinformatics Proteomics is the study of the structure, function, and interaction of cellular proteins. Bioinformatics is the application of computer technologies to the study of the genome. Expanding Present-Day Genomics Investigators now believe that the human genome is three-dimensional. The sections of DNA that encode proteins are but one dimension. The second layer is composed of DNA sequences that interrupt and separate the genes. The third layer consists of the proteins that surround and adhere to DNA. Gene Therapy Gene therapy is the insertion of genetic material into human cells for the treatment of genetic disorders and various other human illnesses. Ex Vivo Gene Therapy In one form of gene therapy, bone marrow stem cells are removed from the blood and infected with an RNA retrovirus that carries a normal gene for a missing enzyme. The cells are then returned to the patient. In Vivo Gene Therapy Genes needed to cure diseases are injected into the body.