Autosomal Dominant Inheritance

Inheritance refers to how genetic information is passed down from parent to child. Mendelian inheritance, or “classic” inheritance is one of the foundations of genetics. Gregor Mendel, who is often called the founder of modern genetics, performed many experiments on pea plants and was able to make several important observations. Those observations led to “rules” that are still followed to this day, and allowed Mendel to define the basic inheritance patterns.

Each of the inheritance patterns follows certain rules:

• Each child inherits half of their genetic information from their mother and half from their father. In the case of human beings, a child inherits 23 chromosomes from their mother and 23 from their father, for a total of 46 chromosomes (23 pairs).

• The chromosomes carry our genes. Genes come in pairs, just like chromosomes come in pairs. For the most part, everyone has two copies of every gene. Since a child inherits only one chromosome of each pair from a parent, the child also inherits only one copy of a gene from each parent. Each parent contributes one copy of every gene, so the child ends up with two copies of each gene.

• Genes for different traits are inherited separately from one another. For example, the “gene for eye color” is not linked to the “gene for height.” A child may have his mother's eyes, but not her height. For the most part, each trait is inherited separately.

The basic patterns of inheritance follow all of these rules, as well as some of their own. Autosomal dominant inheritance is one of the major basic patterns.

Autosomal dominant inheritance refers to a dominant allele being passed on from parent to child on any chromosome other than the X or the Y. An allele is a “version” of a gene. Everyone has two copies of each gene. Those copies may be the same, or they may be different versions of the gene. A dominant allele is one which is expressed even if only one copy is present. It “dominates” its partner allele. If the dominant allele has a mutation, the result of that mutation will be expressed, even if the partner allele does not have a mutation. Autosomal dominant conditions can be passed on in this manner.

If a person has an autosomal dominant condition, it means that at least one allele in a certain gene pair has a mutation. Since autosomal dominant conditions are rare, it is unlikely that a person would be homozygous (have a mutation in both copies) for a dominant allele; it is much more likely for a person to be an affected heterozygote (have only one mutation). If a person is indeed a heterozygote, it is possible to predict how likely it is that person will have children with the same autosomal dominant condition.

In order to determine the chance that the child of an affected person will also be affected, one has to take into account the person's partner. Let's take the example of a father who is affected and a mother who is not:

The green and yellow shapes represent chromosomes. The red markers represent an allele with a mutation. The blue markers represent an allele without a mutation.

The green and yellow shapes represent chromosomes. The red markers represent an allele with a mutation. The blue markers represent an allele without a mutation.

In this picture, we see that one of dad's chromosomes has an allele without a mutation (shown with a blue marker), and one of his chromosomes has an allele with a mutation (shown with a red marker). Both of mom's chromosomes have alleles without mutations. Looking at the picture, we can see it does not matter which of mom's chromosomes is passed on: she will always pass on an allele without a mutation. However, dad can pass on either an allele with a mutation or an allele without a mutation. If a child inherits an allele with a mutation, that child will be affected with the autosomal dominant condition. Therefore, if one parent is affected with an autosomal dominant genetic condition and the other is not, there is a 2 in 4 (50%) chance with every pregnancy a child will be affected, and a 2 in 4 (50%) chance the child will not be affected.

If the situation is a little different and both parents are affected, our picture changes:

In this picture, mom and dad each have one chromosome with a mutated allele, and one chromosome with a non-mutated allele. A child could inherit an allele with a mutation from dad, and an allele without a mutation from mom, and would be affected with the condition. A child could inherit an allele without a mutation from dad, and an allele with a mutation from mom, and would be affected. A child could inherit two alleles without mutations, and would be unaffected. Or a child could inherit two alleles with mutations, and would be affected. If this was the case, there is variability as to how a child would be affected, depending on the genetic condition. Often, there appears to be some kind of a “double-dose” effect, in which having two of the mutated alleles results in a more serious form of disease. Overall, if both parents are affected with an autosomal dominant genetic condition, there is a 2 in 4 (50%) chance with every pregnancy a child will be affected with the condition, a 1 in 4 (25%) chance the child will be unaffected, and a 1 in 4 (25%) chance the child will receive a “double dose,” the effects of which are different from condition to condition.

It is worth nothing should that no one has the ability to choose which chromosomes and which alleles are passed on . A parent is not able to examine their alleles, and choose one to be inherited over the other. If a child inherits an allele with a mutation from an affected parent, it is by random chance. If a child inherits an allele without a mutation, that is also by random chance. In either case , no one is to blame and it is no one's fault .

There are several “exceptions to the rule” for autosomal dominant inheritance, though most autosomal dominant conditions seem to follow this pattern of inheritance. However, as more and more is learned about genetic inheritance, it is possible that these rules may have to bend.

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