Genetictraitscan be passed from parent to child in different ways. As you will see, people can carry agenebut not be affected directly by it themselves. These patterns help to explain why a condition can seem to skip a generation or be more common in boys than in girls. Making a family health portrait, as described inHow Do I Collect My Family History?, can help to uncover these patterns.

Ourgenesare grouped into collections calledchromosomes. Most people have 46 chromosomes, in 23 pairs. One of the pairs is the sex chromosomes, called X and Y. Your sex chromosomes carry the genes that make you male or female. Women have two X chromosomes, and men have an X and a Y. The rest of your chromosomes are calledautosomalchromosomes. Let's see what happens when you have a gene that does not work the way it is supposed to on these chromosomes.

Autosomal Inheritance Patterns

Autosomal dominant

Autosomal dominant means that only one copy of the gene that does not work correctly is needed for someone to have the condition.

If one parent has an autosomal dominant condition, they have one functional copyof the gene and one copy that does not work properly. If the other parent has two copies of the gene that work correctly:

Autosomal dominant conditions, such as Huntingtons disease, affect males and females equally.

Autosomal recessive means that a person needs two copies of a gene that do not work properly to have the condition. In this pattern, people with one working copy of the gene and one copy of the gene that does not function correctly are called carriers. Carriers do not have any signs or symptoms of the condition, but they can still pass on the gene that does not function properly to their children. Usually, parents of children with anautosomal recessivecondition are carriers.

If both parents are carriers of a condition:

Autosomal recessive conditions, such as cystic fibrosis, affect males and females equally.

Your sex chromosomes carry the genes that make you a male or female. A female has two X chromosomes. A male has oneX chromosomeand oneY chromosome. If a gene for a condition is carried on the sex chromosomes, we say it is X-linked. X-linked patterns are not as simple as autosomal patterns, because they show up differently in males and females.

X-linked dominantinheritanceoccurs when a gene that does not work correctly on a single X-chromosome results in a condition. Conditions caused by X-linked dominance are rare, and the same condition can vary considerably in severity, especially among women.

The odds of passing down a condition that is X-linked dominant are different depending on whether the mother or father has the gene that does not function properly and on the sex of the child.

If a father has the condition:

If a mother has one working copy of the gene and one copy of the gene that does not work correctly:

Males are often more seriously affected than females by disorders inherited through X-linked dominance. Sometimes, even if a female inherits the gene change on one of her X chromosomes, she will not show symptoms or her symptoms will be less severe. It is thought that if a female has a working copy of the gene on one X-chromosome in addition to the altered copy on the other X-chromosome, the effects of the condition may be dampened. This has led some scientists to suggest that X-linked inheritance should not be described in terms of dominant and recessive, but rather simply be explained as X-linked inheritance.

Incontinentia pigmentiis an X-linked dominantdisorderthat affects multiple systems, but especially the skin.

X-linked recessive means that if there is one working copy of the gene, a person will not have the condition. The gene for these conditions is on the X chromosome. X-linked recessive conditions affect males more often than females. If a male has a copy of the gene that does not function the way it should on his only X chromosome, then he will be affected by the condition.

Some forms of hemophilia are X-linked recessive conditions.

If a father has an X-linked recessive condition:

If a female has two copies of the gene that do not function correctly, then she will be affected by the condition. If she has a working copy on one X chromosome and a copy of the gene that does not work the way it should on her other X chromosome, then she is called a carrier. Carriers are not affected by the condition, but they can still pass the gene that does not work correctly on to their children.

If a mother has an X-linked recessive condition, then she has two copies of the gene that do not function properly:

If a mother is a carrier of an X-linked recessive condition, she has one functional copy of the gene and one copy that does not function correctly:

If the mother is a carrier and the father has the condition, then there is a 1 in 2 chance (50%) that a daughter would be affected. She would always get the gene that does not work properly from her father, but she might get a working gene from her mother.

Most of our genes are stored in our chromosomes, which sit in each cells headquartersthe nucleus. We also have some genes in small structures in the cell called mitochondria. Mitochondria are sometimes called the power plants of the cell: they work on molecules to make them ready to give us the energy we need for our body functions. The mitochondrial genes always pass from the mother to the child. Fathers get their mitochondrial genes from their mothers, and do not pass them to their children.

Mitochondrial inheritance, also called maternal inheritance, refers to genes in the mitochondria. Although these conditions affect both males and females, only mothers pass mitochondria on to their children.

Diabetes mellitus and deafness, a rare form of diabetes, follows the mitochondrial inheritance pattern.

Check outGenetics Home Referencefor more about genetic conditions and inheritance.

See more here:
Main Inheritance Patterns | Genes in Life

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