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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.