Genetic Disorders and
Genetic Testing
What are Genetic Disorders?
• Both environmental and genetic factors play a
role in the development of disease.
• A genetic disorder is a disease caused by
abnormalities in an individual’s genetic material.
– In this course, we will consider four different types of
genetic disorders:
• Single-gene
• Multifactorial
• Chromosomal
• Mitochondrial
Single Gene Disorders
• Single gene disorders are caused by changes or
mutations that occur in the DNA sequence of
one gene.
• Remember that a gene, a segment of DNA,
contains instructions for the production of a
protein.
• Diseases and disorders result when a gene is
mutated resulting in a protein product that can
no longer carry out its normal job.
Single Gene Disorders
• Single gene disorders are inherited in
recognizable patterns:
– Autosomal dominant
– Autosomal recessive
– Sex linked
• Genetic testing looks at genotype to
determine if someone has a genetic
disorder, will develop one, or is a carrier.
Review Question
• A man and a woman are both carriers for
sickle cell disease, an autosomal
recessive trait. What is the risk of their
having an affected child?
Answer
• Each parent is heterozygous for the condition
and has the genotype Ss (where “s” is the
recessive sickle cell allele). Each parent passes
one allele to the child, so there is a 25% chance
that the child will have sickle cell disease.
• Think back to PBS – what is sickle cell disease
and what is life like for a person affected with
this disorder?
Multifactorial Disorders
• Multifactorial disorders are caused by a
combination of environmental factors and
mutations in multiple genes.
– Development of heart disease is associated with
multiple genes, as well as lifestyle and environmental
factors.
– Different genes that influence breast cancer
development have been found on chromosomes 6,
11, 13, 14, 15, 17 & 22.
• Many of the most common chronic illnesses are
multifactorial.
Chromosomal Disorders
• Humans have 46 chromosomes in their body
cells.
– 44 autosomes
– 2 sex chromosomes
• Because chromosomes carry genetic
information, problems arise when there are
missing or extra copies of genes, or breaks,
deletions or rejoinings of chromosomes.
• Karyotypes, pictures of the paired chromosomes
of an individual, are important in diagnosing
chromosomal disorders.
Mitochondrial Disorders
• Mitochondria, the organelles in your
cells that convert energy, also contain
DNA.
• A mitochondrial disorder, a relatively
rare type of genetic disorder is caused
by mutations in nonchromosomal DNA
of mitochondria.
• Mitochondiral DNA is unique in that it is
passed solely from mother to child
Types of Genetic Testing
and Screening
Carrier Screening
• Carrier screening determines whether an
individual carries a copy of an altered
gene for a particular recessive disease
even though they do not show the trait
phenotypically.
• Carrier screening is often used if a
particular disease is common in a couple’s
ethnic background or if there is a family
history of the disease.
• Examples of carrier tests include those for
Tay-Sachs disease or sickle cell disease.
Preimplantation Genetic Diagnosis
(PGD)
• PGD is used following in vitro fertilization to
diagnose a genetic disease or condition before
the embryo is implanted in the uterus.
• A single cell is removed from an embryo and
examined for chromosome abnormalities or
genetic changes.
• Parents and doctors can then choose which
embryos to implant.
• Secrets of the Sequence – Chosen Child video
http://www.sosq.vcu.edu/videos.aspx. Type in
“Chosen Child” into Keyword search and view
the “Chosen Child – Screening Genetic Content”
video.
The Process of
Preimplantation
Genetic Diagnosis
Fetal Screening/Prenatal Diagnosis
• Prenatal diagnosis allows parents to
diagnose a genetic condition in their
developing fetus.
• Techniques such as amniocentesis,
chorionic villi sampling (CVS), and regular
scheduled ultrasound allow parents to
monitor the health of the growing fetus.
Newborn Screening
• The most widespread type of genetic screening,
newborn screening is used to detect genetic or
metabolic conditions for which early diagnosis
and treatment are available.
• State tests for newborns typically screen
anywhere from 4 to over 30 genetic or metabolic
disorders.
– Testing protocol and mandates vary from state to
state.
• The goal of newborn screening is to identify
affected newborns quickly in order to provide
quick treatment and care.
Activity 2.1.1
• In this activity, you will assume the role of
a genetic counselor and meet with clients
faced with difficult decisions regarding
genetic disorders and genetic testing.
• Your goal is to effectively explain the
disease or disorder to your clients as well
as to outline potential options for genetic
testing and screening and overall
management of disease.