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Genetic Disorders and Genetic
Testing
Medical Interventions
© 2010 Project Lead The Way, Inc.
Objectives and Understandings
• Analyze genetic testing results to predict phenotype
• Be able to answer the question – What is genetic
testing?
Genetic Testing
• The use of molecular methods to determine if:
• someone has a genetic disorder
• will develop one
• is a carrier of a genetic illness
• Involves sampling a person’s DNA and examining
the chromosomes or genes for abnormalities
Biology DNA/Genetics Review
• DNA has 4 base pairs
• Adenine – Thymine
• Cytosine – Guanine
• DNA has an alternating
phosphate-sugar
(deoxyribose) backbone
• DNA has a slightly
negative charge
Biology DNA/Genetics Review
• Gene: a segment of DNA that contains the instructions for
the production of a specific protein.
• Trait: a genetically determine characteristic
• Genotype: the actual allele combinations – the genes that
determine the proteins produced
• The DNA code
• Phenotype: the physical (visual) representation of a trait
• What is seen
Biology DNA/Genetics Review
• If:
• R = protein produced allows person to roll tongue
• r = protein produced doesn’t allow person to roll tongue
• Then:
• What are the possible genotypes of a person who can roll
their tongue?
Biology DNA/Genetics Review
• Key terms:
• Dominant – will always produce a certain phenotype
• Recessive – phenotype can only be seen when dominant
gene is absent
• Autosomes – all chromosomes that are not the sex
chromosomes
• Sex – Linked – all genes/traits located on the sex
chromosomes
Biology DNA/Genetics Review
• If W = widow’s peak and w = no widow’s peak:
• What would the genotype be for:
• Homozygous dominant
• Homozygous recessive
• heterozygous
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.
• 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 non-chromosomal DNA of
mitochondria.
• Mitochondrial 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.
Chosen Child
• 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 quickly identify
affected newborns in order to provide treatment and
care from the start.
Activity 2.1.1
Genetic Counselor Case File Project
• 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.