Heredity and Genetics

Humans have about 50,000 genes. Genes come in pairs, and each pair makes up a very small section on a chromosome. Humans have 46 chromosomes arranged in 23 pairs in every cell in the body, except for the sperm and egg cells.

Twenty-two of these pairs are called autosomes and they are the same for each sex. The twenty-third pair of chromosomes consists of the sex chromosomes. Women have two X chromosomes while men have one X and one Y chromosome. The Y chromosome determines maleness.

Sperm and egg cells contain just one of the chromosomes from each pair, i.e., 23 individual chromosomes. When a sperm and egg come together and conception begins, a new cell is formed that brings the individual chromosomes together into one pair. Thus, every person receives half of his or her genes from the father’s sperm cell, and half from the mother’s the egg cell.

HSP is not one disease but rather a group of similar disorders, each caused by different genes that cause very similar symptoms. The risk of inheriting an HSP disorder depends on the particular features of the gene involved. In other words, it depends on whether the gene lies on an autosome or the X-chromosome, and on whether the gene is dominant or recessive. In some forms of HSP, it can also depend on the sex of the child and/or the parent.

There are three different modes of inheritance: autosomal dominant, autosomal recessive, or X-linked.

Most forms of HSP are autosomal dominant. Autosomal means the HSP gene is located on one of the autosomal chromosomes. The gene can be present in either sex, and it can be passed down from either a mother or a father to a son or a daughter. Dominant means that only one HSP gene is needed to cause the disorder.

Since there is a 50% chance a child will receive the dominant HSP gene mutation from the affected parent, there is a 50% chance the child will inherit the gene and the disorder. This is the same risk for every birth, independent of every other birth.

Note: There are many reasons why someone with no apparent family history may have an autosomal dominant form of HSP. Please see “How can it be HSP when no one else in the family has it?” below.

Some forms of HSP are autosomal recessive. This type of HSP also lies on one of the autosomes, so it can be present in males or females and passed to males or females. Since it is recessive, two copies of the gene are needed to result in the disorder, one from each parent.

In these forms, neither parent has HSP. Instead, they are carriers. They each have one mutant HSP gene and one normal HSP gene. A mutant HSP gene that is recessive can be passed down silently for generations until someone finally inherits the recessive gene from both parents and develops the disorder.

If a mother and father are each carriers for a recessive HSP gene mutation, each of their children has a 25% chance of developing HSP. This is the same risk for every birth. There is a 50% risk the child will be a carrier like the parents. That child would receive one mutant HSP gene from one parent and a normal HSP gene from the other. There is a 25% chance that the child would receive only the normal HSP genes from each parent. This child would not be affected by HSP, nor would he be a carrier.

For individuals with autosomal recessive HSP to have children with the disorder, their spouses have to either have the disorder or be a carrier. This is possible in marriages between cousins in families with this type of HSP.

Download a Fact Sheet that Explains Autosomal Recessive Inheritance

Some HSP genes are found on the X chromosome. Disorders due to genes on the X chromosome are called sex-linked or X-linked since the gene is on one of the sex chromosomes.

The inheritance risks and severity of this type of HSP differ depending on the individual’s sex. Women with an X-linked mutant HSP gene are generally not affected by the disorder; or, if they are, usually have less severe symptoms than males.

Each son of a woman who is a carrier for X-linked HSP has a 50% chance of developing HSP. Each daughter of a woman who is a carrier for X-linked HSP has a 50% chance of being a carrier (female carriers of X-linked disorders often have no symptoms).