Genetic Disorders and Genetic Testing Medical Interventions © 2010 Project Lead The Way, Inc.

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 (Chromosome Pairs 1 – 22) Autosomal recessive Sex linked (Chromosome Pair 23) 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.

Karyotype

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.