Chromosomes are the structures in our cells that contain our genetic information. Humans have two sets of 23 chromosomes, for a total of 46. Chromosomes 1-22 are called autosomes and are numbered 1-22. The last pair of chromosomes is the “sex chromosomes”. There are two types of sex chromosomes: X and Y. Females have two X chromosomes and males have one X and one Y. Nearly all our cells contain the full complement of 46 chromosomes. However egg and sperm cells each only contain one set of chromosomes (23) so that when they meet at fertilization the resulting embryo will have a total of 46 chromosomes.
Genes are the basic units of DNA that provide instructions to our body. Our genes are housed on chromosomes. Although the same set of genes is contained in most of the cells of our body, a particular gene may only play a role in a certain part of the body (for example, the heart). Genes are a series of letters (A, C, G, and T) and are involved in the production of proteins. Proteins perform a variety of functions in our body. If there is a change in the letter sequence of a gene (called a mutation), the protein may not be made or may not work properly.
Genetic Inheritance Patterns
Genetic conditions can be inherited in a variety of different ways. In some cases we know what pattern a particular condition follows. In other cases, we rely on a detailed family history, or pedigree, to assess the pattern of inheritance in a particular family.
Figure 1: Example of a pedigree. Males are represented by squares and females by circles. The darkened shapes indicate affected individuals.
Descriptions of some common inheritance patterns
Autosomal Dominant
Although we have two copies of our genes, in autosomal dominant conditions a mutation in one copy of the gene is enough to cause the disease or susceptibility to the disease. A person who carries a mutation for an autosomal dominant condition has a 50% chance of passing the mutation on to his/her children. If a person does not carry a mutation, it is not possible to pass it on to the next generation. Dominant conditions affect both males and females.
Autosomal Recessive
In an autosomal recessive condition there is a mutation in both copies of the gene. In assessing risk to future generations for a recessive disorder the genetic status of both parents is important. A person affected with a recessive condition will pass on a mutated copy of the gene 100% of the time because both copies of his/her carry a mutation; therefore, all of this individuals’ offspring will inherit a mutation. If the other parent is unaffected there is no chance to have an affected child but all children will be carriers of that condition. In most cases a carrier is clinically unaffected. However, a carrier can pass on the gene carrying the mutation to his/her children.
If one parent is affected and the other is a carrier there is a 50% of having an affected child and a 50% of having a child that is a carrier of the condition.
If only one parent is a carrier it is not possible to have an affected child but there is a 50% to have a child that is a carrier.
If both parents are carriers of the same condition there is a 25% chance of having an affected child, a 50% chance of having a child who is a carrier, and a 25% that the child will be unaffected-inheriting two working copies of the gene.
X-linked
X-linked conditions are due to a mutation in a gene on the X chromosome. X-linked conditions can be either dominant or recessive. The general rules for each are explained below, but there are exceptions:
X-linked recessive
If a condition is X-linked recessive, a female will not show any symptoms or features of the condition because she has a second copy of the gene on her other X chromosome. She will be a carrier of the condition. Just as with autosomal recessive inheritance, carriers typically do not show any signs of the condition but are at risk of passing the mutation on to offspring. This means that sons of a woman carrying a mutation for an X-linked recessive condition have a 50% chance of being affected and daughters have a 50% chance of being a carrier of the condition. Males who inherit a mutation will be affected with condition because they only have one copy of the X chromosome. Affected males will have unaffected male offspring because they pass on their Y chromosome to sons. All their daughters of affected males will be carriers because the only copy if an X that the affected male has to pass on to female offspring carries a mutation.
X-linked Dominant
Both men and women can show features of an X-linked dominant condition, although they may be affected differently. Sons of affected males will be unaffected (they get their Y chromosome from their father-not the X) and all daughters of affected males will be affected since they must get their father’s X chromosome. Sons and daughters of affected females have a 50% chance of inheriting the mutation.
Other points to consider
A mutation can either be inherited from a parent or may arise in an affected person for the first time — this is called a spontaneous or new mutation. However, the spontaneous mutation typically occurs at such an early stage of development that it is present in all the cells, including sperm and egg cells. This means that the mutation can then be passed to future generations. If the mutation occurs later in development it may not be present in all the cells — this is called mosaicism. The risks to future generations are less clear in these situations.
As is the case with many inherited cardiac conditions, there may be a great deal of variation in how people are affected with a condition — even within the same family. This phenomenon is referred to as variable expressivity. For example, some individuals with hypertrophic cardiomyopathy experience very few symptoms of their disease while others may experience progressive heart failure or even sudden death. In some cases a person may carry a disease-causing mutation but never develop any signs or symptoms of the condition. Conditions in which there are individuals with disease-causing mutations that do not develop the disease are said to have reduced penetrance.
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