Genetic disorders are diseases whose root cause is a mutation in the genome. The cause of genetic disorders may be from:
- a single point mutation (monogenic inheritance disorder)
- multiple genes with point mutations (multifactorial inheritance disorder)
- a combination of both inherited and environmental mutations
- chromosomal mutations (chromosomal disorder)
We will focus on some of the more well-known monogenic and chromosomal disorders in this lesson. You are responsible for knowing the basic information that would characterize each disorder as well as what type of genetic error causes it.
Dominant Allele Disorders
Achondroplasia: a dominant condition in which the long bones do not develop properly resulting in unusually short arms and legs, a large head, and a relatively long torso. Most cases of achondroplasia occur spontaneously as the result of a new mutation in an egg or sperm of one of the normal- appearing parents. Only 1/8th of the children born with this condition inherited it from a parent.
Huntington’s Disease: a progressive, degenerative disease that causes certain nerve cells in your brain to waste away. As a result, you may experience uncontrolled movements, emotional disturbances and mental deterioration. Huntington’s disease is inherited as a dominant allele. If one parent has this disease, the offspring have a 50% chance of inheriting it.
Recessive Allele Disorders
Cystic Fibrosis: an inherited recessive disorder resulting from a mutation of the gene that regulates the secretory glands, including mucus and sweat glands. The characteristics of cystic fibrosis are salty tasting skin, normal appetite but poor growth and poor weight gain, excess mucus production, frequent chest infections and coughing/shortness of breath. Both parents must carry the allele for the child to inherit the condition.
Tay-Sachs Disease: a fatal genetic lipid storage disorder in which harmful quantities of a fatty substance build up in tissues and nerve cells in the brain. Infants with Tay-Sachs disease appear to develop normally for the first few months but as fatty material builds up, a relentless deterioration of mental and physical abilities occurs. The child becomes blind, and deaf. Muscles atrophy and they can no longer swallow. Children with Tay-Sachs disease usually die by age 4. Both parents must carry the allele for a child to inherit the condition.
Albinism: a recessive disorder that occurs when one of several genetic defects makes the body unable to produce or distribute melanin, a natural substance that gives color to your hair, skin, and iris of the eye. In one type of albinism, people have white or pink hair, skin, and iris color, as well as vision problems. Another type of albinism, called ocular albinism type 1, affects only the eyes while the person’s skin and eye colors are usually in the normal range. However, an eye exam will show that there is no coloring in the back of the eye.
Hemophilia: a disorder that impairs the body’s ability to control blood clotting which is used to stop bleeding. Because it is X-linked, only males typically exhibit symptoms.
Muscular Dystrophy: refers to a group of X-linked inherited diseases marked by progressive weakness and degeneration of the skeletal, or voluntary, muscles that control movement. Duchenne muscular dystrophy is the most common and severe childhood form of the disease.
Color Blindness: an X-linked condition that results in the inability to perceive differences between some of the colors that others can distinguish. There are several types that enable some individuals to perceive different colors than other affected individuals. Some cannot distinguish between red and green while others have inability to distinguish between blue and yellow.
Klinefelter Syndrome: a condition that occurs in men who have an extra X chromosome in most of their cells (XXY). The syndrome can affect different stages of physical, language and social development. The most common symptom is infertility. Because they often don’t make as much of the male hormone testosterone as other boys, teenagers with Klinefelter’s syndrome may have less facial and body hair and may be less muscular than other boys.
Turner Syndrome: a rare disease that only affects females. It is caused by an abnormality in one of a girl’s X chromosomes, leaving her with only one fully functional X chromosome. As a result, girls with Turner syndrome tend to be sexually underdeveloped and infertile. Other common symptoms of Turner syndrome include a short stature and a webbed neck.
Down Syndrome: children with down syndrome have some of the following physical traits: short stature, weak muscles, a short, wide neck with excess fat and skin. Small, low-set ears and irregularly shaped mouth and tongue and mild to moderate mental disability or retardation are also characteristic of Down syndrome. An extra 21st chromosome is responsible for this disorder.
Genetic Testing for Disorders Discussion
Genetic testing (whole genome sequencing) can be used to determine if a child has (or will have) specific genetic disorders, either before or after birth. Watch the following video about one family’s struggle to find out how to help their children. Then, answer the questions that follow.
- What are the benefits of genetic testing for disorders?
- What are the drawbacks?
- Would you want to know if one of your own children were likely to have a genetic disorder ahead of time? Why? If so, how early (before birth, after birth, later at symptom onset) would you want to know?