Genetic Disorders

Genetic Disorders Getting the Idea Most traits organisms inherit are either neutral or helpful. For example, having blood type A or B is neither an advantage nor a disadvantage to a human, whereas some fur colors may help an animal hide and avoid predators. Other inherited traits are harmful. Those traits increase an organism's risk of illness or death. Inherited Diseases Recall that many organisms, including humans, have two types of chromosomes: autosomes and sex chromosomes. Autosomes are chromosomes that do not play a role in determining the gender of an organism. Most traits an organism inherits are autosomal traits, traits that are carried on the autosomes. Humans have 22 pairs of autosomes. The sex chromosomes are the pair of chromosomes that determine the organism's gender. Humans have one pair of sex chromosomes. A sex chromosome can be either an X or a Y chromosome. If one of an individual's sex chromosomes is a Y chromosome (genotype XY), that individual is male. If both of an individual's sex chromosomes are X chromosomes (genotype XX), that person is female. As you learned in Lesson 22, a trait that is inherited on the sex chromosomes is called a sex-linked trait. A disease is anything other than an injury that interferes with the life processes of an organism. Some diseases in organisms are genetic, meaning that they are passed down from one generation to the next. An inherited disease is a disease that is transmitted in the genes an organism inherits from its parents. Many inherited diseases result from mutations. Recall that mutations are changes in an organism's DNA. A mutation is passed on to every cell that develops from the mutated cell. When a mutation is present in a gamete, the altered genes and DNA that contain the mutation can be passed from parent to offspring. Many inherited diseases in humans are autosomal diseases, meaning they are carried on autosomes. Three examples of inherited autosomal diseases are cystic fibrosis, Huntington disease, and sickle cell disease. Other inherited diseases are sex-linked, meaning that the disease is carried on the sex chromosomes. Cystic Fibrosis Cystic fibrosis (CF) is a disease that affects the glands of the body that secrete sweat and mucus. People with this disease have too many chloride ions outside their cells, so they have difficulty maintaining a proper salt balance within their cells. CF commonly affects the lungs, where the extra chloride ions cause the mucus that coats the lungs to be thicker and stickier than normal. The thick mucus makes breathing very difficult. It also supports the growth of bacteria that can cause serious lung infections. CF can also affect other organs of the body, including the pancreas, liver, and sex organs. CF is an autosomal recessive disease, meaning that it is caused by a recessive trait that is carried on an autosome. Because the trait is recessive, only people who inherit a recessive gene for the disease from each parent have CF. Huntington Disease Huntington disease is a degenerative disease of the nervous system. It is a progressive disease that worsens over time, eventually causing the breakdown of the nerve cells in the brain. As the disease progresses, it causes uncontrollable movements, mental deterioration, and premature death. Huntington disease is an autosomal dominant disease, meaning that an individual need inherit only one gene for the disease in order to develop the disease. A child of a parent with Huntington disease has a 50 percent chance of inheriting the gene for the disease. If no gene for the disease is passed to the child, the child will not develop Huntington disease. Such children will not pass the disease to their future offspring. Sickle Cell Disease Sickle cell disease (sometimes called sickle cell anemia) changes a person's hemoglobin. The variant hemoglobin causes red blood cells to become distorted and fold into a sickle shape. The sickle-shaped cells cause blockages within the person's blood vessels that disrupt blood flow. This disease causes weakness and pain, and it damages the brain, heart, and spleen. It can be fatal. Sickle cell disease is an autosomal recessive disease with incomplete dominance. Recall that in incomplete dominance, neither allele is dominant over the other, resulting in a blend of two forms of a trait. In the case of sickle cell disease, a person who inherits two alleles for the disease will have the disease. A person who inherits only one allele for the disease may show symptoms of the disease, including sickle-shaped red blood cells, under certain conditions. However, the symptoms will usually be less severe than in someone with two copies of the sickle cell allele. Individuals who inherit only one allele for sickle cell disease are described as having sickle cell trait. Sickle cell trait appears to increase a carrier's chance of surviving a malaria infection because the abnormal red blood cells do not provide a favorable environment for the parasite that causes malaria. Therefore, the trait is useful in places where malaria is common. This is why sickle cell disease and sickle cell trait are more frequent in persons of African descent. Hemophilia Hemophilia is a disease in which the body is unable to produce a protein that is needed for normal blood clotting. Individuals with hemophilia bruise easily and can die of what appear to be minor cuts because they do not stop bleeding. Hemophilia is most commonly a sex-linked genetic disease that is transmitted on the X chromosome. Most individuals who have the disease are males who inherit a single X chromosome that carries the allele for hemophilia. Females do not develop hemophilia unless both their X chromosomes carry the allele. However, a female who inherits the allele from only one parent can pass it on to her offspring. You will further explore the transmission of hemophilia from one generation to another later in the lesson. Genetics and Environment When it comes to disease, genetics is not always all or nothing. Some diseases have a genetic component, but the risk of developing the disease is also affected by other factors, such as behavior and exposure to certain substances. For example, some people's genes make them likely to develop type 2 diabetes or heart disease. However, these diseases have also been linked to behavior such as lack of exercise and poor diets that lead to obesity. People with a genetic predisposition to type 2 diabetes may not develop diabetes if they eat healthful diets and exercise regularly. The same is true for people with a family history of heart disease. Skin cancer is another disease affected by both genes and the environment. There is a strong link between amount of exposure to ultraviolet rays from the sun and an individual's risk of developing skin cancer. This risk is often higher in people who have fair skin and light-colored hair and eyes—traits that are determined by genetics. Some diseases that are caused by inherited traits can be controlled or managed by changes in behavior. For example, phenylketonuria (PKU) is an autosomal recessive disease that is caused by a missing enzyme. The lack of this enzyme disrupts metabolism because the body cannot break down an amino acid called phenylalanine. If PKU is not detected and treated, it can interfere with brain development and cause seizures and mental retardation. However, once PKU has been diagnosed, the disease can be managed with a restricted diet. Recall that some environmental factors can be mutagens. Exposure to certain chemicals and forms of radiation can cause mutations that lead to disease. For example, asbestos and chemicals present in smoking tobacco are common causes of lung cancer. Exposure to the chemicals in chewing tobacco has been linked to the development of cancers of the mouth. Similarly, prolonged exposure to UV radiation from the sun may cause skin cancer, even though the skin needs some UV radiation in order to produce vitamin D. This vitamin is essential to healthy bones and has other roles in the body. Although there is a cause-and-effect relationship between genetics and disease, it is important to realize that not everyone who carries a gene for a disease will develop that disease. Similarly, not everyone who is exposed to a potentially harmful factor in the environment will become ill. Pedigrees Scientists use diagrams called pedigrees to study the transmission of traits from one generation to the next. A pedigree is a chart, or family tree, that traces the inheritance of a trait. A person who has at least one copy of an allele is called a carrier, whether or not the trait is expressed. This pedigree shows the inheritance of freckles. Each row represents one generation, with the earliest at the top. The gene for freckles is dominant. It is found on an autosome. The pedigree displays a large amount of information. The mating of a nonfreckled female and a male with one gene for freckles produces three children, two of whom inherit the gene. Those two children have freckles. This is shown on the left side of the diagram. The right side shows the mating of two other people, each of whom has one allele for freckles. They have three daughters, two of whom are homozygous for freckles. The noncarrier male of the family on the left then mates with the noncarrier female of the family on the right. They have two noncarrier children. The pedigree displays both genotypes and phenotypes. Unshaded shapes represent people without freckles, and shapes with total or partial shading represent people with freckles. A pedigree can show how a sex-linked disorder, such as hemophilia, is inherited. The gene for the disorder is recessive and carried on the X chromosome. In the next pedigree, the female in the first generation carries one gene for hemophilia and passes it to some of her descendants. Note that no females shown have the disorder. However, four males have hemophilia. In this family, some, but not all, descendants inherit one recessive gene. Males with one recessive gene have hemophilia. A female would have to inherit two recessive genes to have the disorder. Pedigrees can be useful for tracking any type of trait, whether harmful, such as hemophilia, neutral, such as hair color, or beneficial. A pedigree chart can also be used to show the inheritance of traits, such as human blood types. Focus on Inquiry In their work, scientists make observations and conduct research to help them answer their questions about the natural world. This research may lead a scientist to develop a hypothesis. A hypothesis is a possible answer to a scientific question that is based on research, experience, and reasoning. A hypothesis must be testable. Scientists test their hypotheses with experiments and models. A scientist is studying the passing of albinism, an inherited absence of skin pigment, through three generations of a family. Albinism is an autosomal trait. The scientist collects the following information through interviews. In the first generation, neither parent had albinism. They had four male children. One of those four had albinism. One son who did not have albinism mated with a woman who did not have albinism. They had two daughters, both of whom displayed albinism. Draw a pedigree for the family, using the key below. Use a circle or square with a question mark if you do not know a person's genotype.