Genetic diseases happen when a gene changes. These changes, called mutations, prevent people from functioning like they are supposed to. A mutated gene can come from an affected parent or a parent carrier (unaffected). The gene can also spontaneously mutate in the sperm or egg of healthy parents. The unpredictable characteristic of dwarfism is why we say everyone can have a child with short stature. In certain types of dwarfism like achondroplasia, these spontaneous mutations are relatively common. Dwarfism is not hereditary in this case, and because the mutation is often isolated, most likely only one child will be affected. It is possible, however, that the affected child’s children will later inherit the mutated gene. Dwarfism will therefore be hereditary in the second generation.

Types of Heredity

If one parent is a carrier of a mutated gene responsible for dwarfism, they can pass it on to their child. In the vast majority of cases, dwarfism is caused by a mutated gene on an autosome (autosomnal heredity). This means dwarfism affects girls as much as boys. It can be inherited in two ways:

• Autosomnal Dominant Heredity: only one mutated gene is needed for the disease to appear (achrondoplasia, pseudoachondroplasia, spondyloepiphyseal dysplasia, Kniest dysplasia).
• Autosomnal Recessive Heredity : two mutated genes are needed for the disease to appear (diastrophic dysplasia, Morquio’s syndrome, Seckel syndrome). Some people are carriers of a mutated gene without being affected by the disease.

However, sometimes the dwarfism-causing gene appears on a sex chromosome. This is called sex-linked dominance.

Finally, in other cases such as Turner syndrome, it is not a mutated gene but the number of chromosomes that causes the disease.

Autosomnal Dominant Heredity

Let's use a fictional couple for an example. The father has achondroplasia, but the mother doesn't (the reverse is also possible). Their children have a 50% chance of receiving the mutated gene and developing achondroplasia. If the child doesn't receive the mutated gene, they have no chance of passing it on because it isn't part of their genetic make-up.

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In some types of dwarfism, if both parents have the same disease by dominant heredity, each child will have a 25% chance of not surviving (two mutated dominant genes are deadly), 25% chance of being healthy, and 50% chance of having dwarfism. Luckily, a double mutated gene is not a risk in all kinds of dwarfism.

Automsomnal Recessive Heredity

In the case of this couple, each child will have a 50% chance of inheriting the mutated gene but will not have any symptoms because the “good” gene dominates in the pair.

If both parents are carriers of the mutated gene, the children will have a 25% chance of having dwarfism because two recessive genes are necessary to develop the disease. Each child will have a 50% chance of being a healthy carrier of the mutated gene and 25% chance of not receiving the gene.

If one of the parents has two mutated genes, they have the disease. If the other parent does not have the mutated gene, each child will be a carrier of the gene but will not haved dwarfism.

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If one of the parents is a carrier of the mutated gene while the other has the disease, their children will have a 50% chance of developing the disease, and 50% will be carriers.

If both parents have the disease, all of their children will have it too.

Sex-linked Dominance

Sometimes a genetic mutation happens on a sex chromosome instead of an autosome. This is sex-linked dominance. In rare cases such as spondyloepiphyseal dysplasia, only boys are affected, while girls are carriers.

Other Cases

Turner syndrome is a frequent cause of dwarfism that affects almost exclusively women. People with Turner syndrome have 45 chromosomes instead of 46, that is 22 pairs of normal autosomes and only one X chromosome instead of two. Such a mistake in the genetic code is accidental and happens during the first cell divisions after fertilization. If a couple has a girl with Turner syndrome, it is unlikely they will have another with the same disease. Women with this disease do not pass on their chromosome abnormality because they are generally infertile.

In Summary

In light of these explanations, it is important to study each partner's genetic history whenever possible to evaluate the possibility of their child having a chromosomal abnormality.

This information was taken and adapted from Nathalie Boëls' book: Le nanisme: Se faire une place au soleil dans un monde de grands, Ed., CHU Ste-Justine, 2008.

Thank you to Gail Ouellette from Portail québécois des maladies génétiques orphelines.

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