| By nature, everyone is born with an inheritance: 23 chromosomes from each parent that arrange themselves into 23 pairs in each cell. Chromosomes are made up of genes, the body’s blueprint. A gene can change (mutate), and the altered gene can be passed on within a family. These altered genes, alone or in a pair (one from each parent) cause the body to function differently, often in an imperceptible way, but sometimes with serious results. Also, many genes have common variations called polymorphisms, which can increase the risk of certain diseases. When the inheritance pattern is dominant, only one flawed gene in a pair is necessary for a condition to occur. A person who has an autosomal dominant disorder like neurofibromatosis 1, which causes tumors to grow along the nerves, may have a parent with the condition. The chances are 50-50 that a parent will pass along an autosomal dominant disorder to a child. Autosomal Dominant Disorder | | | In autosomal dominant disorders, if one affected parent has a disease-causing gene that dominates its normal counterpart, each child in the family has a 50% chance of inheriting the disease-causing gene and the disorder. Image and caption provided by Science + Literacy for Health, a project of the AAAS Directorate for Education and Human Resources. | An autosomal recessive pattern is different: both parents “carry” one altered gene. There’s a 50 percent chance that a child will also be a carrier; a 25 percent chance that a child will have received one altered gene from the mother and one altered gene from the father, and thus have the disorder; and a 25 percent chance that a child will neither be a carrier nor have the disorder. Sickle cell anemia, a blood disorder, is an example of an autosomal recessive condition. The mother and father of a child with sickle cell anemia themselves each have one altered sickle cell gene, but because they each also have an unaffected gene, they do not have the condition. Autosomal Recessive Disorder | | | In diseases associated with altered recessive genes, both parents -- although disease free themselves -- carry one normal gene and one altered gene. Each child has one chance in four of inheriting two altered genes and developing the disorder, one chance in four of inheriting two normal genes, and two chances in four of being a carrier like both of the parents. Image and caption provided by Science + Literacy for Health, a project of the AAAS Directorate for Education and Human Resources. Image and caption provided by Science + Literacy for Health, a project of the AAAS Directorate for Education and Human Resources. | X-linked recessive disorders are inherited through the X chromosome, which is passed on by the mother. X-linked diseases almost always occur in males. Males have only one X chromosome, so a single recessive gene on that X chromosome will cause the disease. The Y chromosome, present in all males, does not protect the male from the recessive disorder. Females inheriting one affected x chromosome become carriers. It is extremely rare for a female to inherit an x-linked recessive disorder. In that case, the mother must be a carrier of the gene and the father must be affected with the disease; the female would then have a 25% chance of inheritance. X-linked recessive disorders can be quite serious, such as in the case of hemophilia and Duchenne muscular dystrophy. X-Linked Recessive | | | In this example, an unaffected woman carries one copy of a gene mutation for an X-linked recessive disorder. She has an affected son, an unaffected daughter who carries one copy of the mutation, and two unaffected children who do not have the mutation. Image and caption provided by Science + Literacy for Health, a project of the AAAS Directorate for Education and Human Resources. Image and caption provided by Science + Literacy for Health, a project of the AAAS Directorate for Education and Human Resources. | | 
Inheritance Patterns
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