Most of the known genetic disorders are dominant-gene linked; however, the vast majority of dominant-gene linked disorders are not serious or debilitating. For example, the majority of those with Tourette syndrome suffer only minor tics from time to time and can easily control their symptoms. Huntington’s disease is a dominant-gene linked disorder that affects the nervous system and is fatal, but does not appear until midlife. Recessive gene disorders, such as cystic fibrosis and sickle cell anemia, are less common but may actually claim more lives because they are less likely to be detected as people are unaware that they are carriers of the disease. Some genetic disorders are sex-linked: the defective gene is found on the X-chromosome. Males have only one X chromosome so are at greater risk for sex-linked disorders due to a recessive gene, such as hemophilia, colour blindness, and baldness. For females to be affected by these genetic defects, they need to inherit the recessive gene on both X chromosomes, but if the defective gene is dominant, females can be equally at risk. Table 2.1 lists several genetic disorders.
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Recessive Disorders (homozygous): the individual inherits a gene change from both parents. If the gene is inherited from just one parent, the person is a carrier and does not have the condition.
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Cases per Birth |
| Sickle cell disease (SCD) is a condition in which the red blood cells in the body are shaped like a sickle (like the letter C) and affect the ability of the blood to transport oxygen. Carriers may experience some effects but do not have the full condition. |
1 in 2,500 in Canada (more common in people of African ancestry) |
| Cystic fibrosis (CF) is a condition that affects breathing and digestion due to thick mucus building up in the body, especially the lungs and digestive system. In CF, the mucus is thicker and stickier than normal. |
1 in 3,600 |
| Phenylketonuria (PKU) is a metabolic disorder in which the individual cannot metabolize phenylalanine, an amino acid. Left untreated, intellectual deficits occur. PKU is easily detected and is treated with a special diet. |
1 in 12,000 |
| Tay-Sachs disease is caused by enzyme deficiency resulting in the accumulation of lipids in the nerve cells of the brain. This accumulation results in progressive damage to the cells and a decrease in cognitive and physical development. Death typically occurs by age five. |
1 in 4,000; 1 in 27 American Jews in the US is a carrier; 1 in 20 French Canadians is a carrier |
| Albinism is when the individual lacks melanin and possesses little to no pigment in the skin, hair, and eyes. Vision problems can also occur. |
Fewer than 20,000 US cases per year |
| Autosomal Dominant Disorders (heterozygous): In order to have the disorder, the individual only needs to inherit the gene change from one parent. |
Cases per Birth |
| Huntington’s disease is a condition that affects the individual’s nervous system. Nerve cells become damaged, causing various parts of the brain to deteriorate. The disease affects movement, behaviour, and cognition. It is fatal and occurs at midlife. |
1 in 7,000; 1 in 5,500 is at risk of inheriting the disease |
| Tourette syndrome is a tic disorder that results in uncontrollable motor and vocal tics, as well as body jerking. |
0.89 per 1,000 in males and 0.44 per 1,000 in females |
| Achondroplasia is the most common form of disproportionate short stature. The individual has abnormal bone growth resulting in short stature, disproportionately short arms and legs, short fingers, a large heard, and specific facial features. |
1 in 40,000 |
Table 2.1: Genetic Disorders
| Sex-Linked Disorders: When the X chromosome carries the mutated gene, the disorder is referred to as an X-linked disorder. Males are more affected than females because they possess only one X chromosome without an additional X chromosome to counter the harmful gene. |
Cases per Birth |
| Fragile X syndrome occurs when the body cannot make enough of a protein it needs for the brain to grow and problems with learning and behaviour can occur. Fragile X syndrome is caused from an abnormality in the X chromosome, which then breaks. If a female has fragile X, her second X chromosome is usually healthy, but males with fragile X don’t have a second healthy X chromosome. This is why symptoms of fragile X syndrome are usually more serious in males. |
1 in 4,000 males; between 1 in 8,000 and 1 in 11,000 females |
| Hemophilia occurs when there are problems in blood clotting, causing both internal and external bleeding. |
1 in 10,000 males |
| Duchenne muscular dystrophy is a weakening of the muscles, resulting in an inability to move, wasting away, and possible death. |
1 in 3,500 males |
