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Background on Inheritance Patterns
In the 19th century, Gregor Mendel discovered that traits in living things are inherited according to specific patterns. He gave these patterns names like dominant and recessive. In Mendel's day, genes had not yet been discovered; the way in which traits are passed from parent to offspring was unknown. Mendel gave them the working name "factors".
The modern understanding of genetics is that for most genes, every person has two versions in their DNA. There are a limited number of possible versions, which are called alleles. Terms like "dominant" and "recessive" describe the outcome when a person has two different alleles of the same gene.
In casual speech, a trait showing a Mendelian inheritance pattern may be called a dominant or recessive gene, or a dominant or recessive allele. Technically, however, a gene or allele cannot be (for example) "dominant" as an essential characteristic; these terms instead refer to relationships among alleles of a gene, and the way the resulting trait is expressed. The most accurate way to describe them is with terms like dominant or recessive inheritance.
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A Trait With Autosomal Recessive Inheritance
The disease cystic fibrosis shows a recessive inheritance pattern. This means that (practically speaking) there are only two alleles on the gene behind CF, called CFTR. The allele that causes the disease is recessive to the non-disease allele; this is the same as saying the non-disease allele is dominant to the disease allele.
This description tells only how the disease is passed from parents to child, and can be used to calculate the odds of a couple having a child with CF. It does not tell us anything about the underlying gene itself: what it does, and how it goes wrong.
In the case of CF, the non-disease allele is the normal, fully functional one for CFTR. Only one functional copy of this allele is needed. The disease-causing allele is a "broken" version of the normal one, and it essentially does not function at all. A functional copy can mask the "nothing" of the disease copy. This is the reason the disease shows recessive inheritance: both alleles must be broken for a person to lack the necessary function of the CFTR gene and therefore suffer from CF.
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A Trait With Autosomal Dominant Inheritance
Marfan syndrome shows a dominant inheritance pattern. The gene responsible, FBN1, has a number of different alleles associated with various diseases, but for this example, only two are important: the non-disease allele and the Marfan-causing allele. The Marfan-causing allele is dominant to the non-disease allele; likewise, the non-disease allele is recessive to the Marfan-causing allele.
Because the inheritance of Marfan syndrome follows a dominant pattern, the likelihood of a sufferer passing the disease to his or her children is predictable: 50% (assuming the other parent does not suffer from the syndrome). Again, the inheritance pattern does not provide information about the gene that causes it—such as the fact that it has other alleles that cause different syndromes.
Unlike CF, in which the "broken" allele does nothing, the "broken" Marfan-causing allele does do something, and it is harmful. For this reason, the syndrome shows dominant inheritance—just one copy of the Marfan-causing allele is enough to generate the problems associated with the syndrome.
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Traits With Other Inheritance Patterns
Many genes have more than two alleles. The human ABO bloodtype is controlled by genes with three possible alleles, and describing their interaction in Mendelian terms can be confusing. A and B are both dominant to O, but neither is dominant or recessive to the other; they are said to be co-dominant. The underlying gene activity shows that the A- and B-alleles each code for a distinct protein, while the O allele does nothing.
Some traits appear with a hierarchical pattern of inheritance. In cats, the TYR gene controls different degrees of albinism, or lack of dark pigment in the coat. There are at least four alleles. In a cat that is genetically black, the alleles are associated with these traits (phenotypes): normal (black with yellow eyes), Burmese (brown with yellow eyes), Siamese (brown and cream with blue eyes), and albino (white with blue eyes). Each of these alleles is dominant to the one following it, and recessive to the one before it. The way the TYR gene works, or fails to work, is not apparent from the inheritance pattern alone.