Genetics

Genetic counselling and inheritance

Most short stature disorders are largely genetically originated. One main question that arises is “Will the same condition occur in the family?”

Genetic counselling is the process by which information is given regarding the genetic risks of a disorder. It involves being certain of the particular diagnosis calculating the risks of recurrence and putting this information clearly so it can be understood by those seeking advice. As bone disorders are an especially complex group it is usual for counselling to be given at a clinic with experience in these disorders. Genetic counselling clinics are held in several Australian hospitals.

Often the diagnosis is reassessed by obtaining information on family members and a family tree is drawn up upon which all the relevant details are shown. A physical examination is usually required to confirm the diagnosis.

The particular risks of passing the disorder can then be discussed. These are usually given as “Odds”, e.g. a 10% chance or 1 in 10 that a child may be affected. A genetic counselling clinic does not tell people whether or not they should have a child. Most involved in genetic counselling feel that this must be the decision of the individual couple, that genetic counselling is to provide information and support.

Patterns of inheritance

Most bone growth disorders show one of the following three types of inheritance.

1. Autosomal Dominant Inheritance

Achondroplasia is a condition which shows autosomal dominant inheritance. Either sex may be affected and either may pass it on to their children. A child with achondroplasia born to average-stature parents is a spontaneous mutation in the genetic material of the child at the time of conception. The odds of this happening to any child is approximately 1 in 20,000.

Once a mutation has occurred the effect is permanent and an individual having achondroplasia can pass the condition on to their children. A child born of one achondroplastic parent and one average-stature parent will have a 50% chance of havibg achondroplasia themselves.

When two people with achondroplasia decide to have children, the chances of them passing the condition on to their children are increased. Such couples have a 50% chance that a child would inherit achondroplasia from either parent and there is a 25% chance that a child would inherit achondroplasia from both parents. A child who inherits two sets of achondroplasia genes may have serious medical problems. This situation only applies to achondroplasia. There is also a 25% chance the child will be average height. With a couple whose types of short stature each have a different disorder, the pattern of inheritance will be determined by the inheritance characteristics of their individual conditions.

2. Autosomal Recessive Inheritance

In this instance an abnormal gene has to come from both parents. The disorder only occurs when the child inherits the same growth restricted gene from each of them. The parents are usually healthy because they also carry the complimentary normal gene which can compensate for, or mask, the growth restricting gene.

The pattern of recessive inheritance differs from the dominant type in that a child of an affected parent and a normal parent will be of normal appearance. However, all the children will be “carriers”. Only if both parents were affected by the same recessive disorder would all the children be affected also.

3. X-linked (sex-linked) Inheritance

Here only males are usually affected but the condition is passed on through healthy “carrier” women to their sons. This form of inheritance is uncommon but is important to recognise because of the risks for female relatives transmitting the disorder. A well-known example is haemophilia, and a short stature condition showing this type of inheritance is the Tarda type Spondylo-epiphyseal Dysplasia.

The mechanism of inheritance is as follows. Females have two X sex chromosones, and if one has a gene abnormality, the normal gene on the second chromosone can compensate for it. Males have one X and one Y sex chromosone, so any restriction on their single X chromosone becomes apparent.

Pregnancy

Women of short stature when contemplating pregnancy will be anxious about difficulties. Only a few short-statured women would be advised against having children because pregnancy could risk their health, e.g. In a woman with chest deformities that causes breathing difficulties, pregnancy can further complicate their breathing. Most women of short stature have normal fertility and can safely carry a pregnancy full-term.

In most growth disorders, the pelvis will be too small for normal labour, necessitating delivery by caesarean section. A choice of anaesthetic may need to be considered such as an epidural anaesthetic injection directly into the spine. Wiith an achondroplastic woman where the spine is abnormal a general anaesthetic is recommended. It is recommended that all couples consult an obstetrician before starting a family.

Prenatal diagnosis

Prenatal tests can now identify some of the most serious disorders of growth. This can especially help in a situation where risk of such a condition recurring is high.

The methods used include:

(1) The non-invasive ultrasound scan. Major abnormalities can be seen at 20 weeks.

(2) Amniocentesis test which examines the womb fluid for enzyme or chromosone abnormalities at 16 weeks.

(3) Chorionic villus biopsy test where the foetal tissue is investigated. This test is carried out around the 9th week.

Generally tests for many types of short stature are not reliable. Tests may identify abnormalities at a stage of the pregnancy when termination is possible.

Adoption

Adoption or fostering may be a possible alternative. It is a fact that some children are rejected or circumstances are such that a child is placed for fostering or adoption. Couples where one or both partners are of short stature, may feel that through their own experiences they have something to offer such a child.

(Source: Information Guide to Persons of Short Stature, edited by Stephen Pinnell, pp. 24-26)

© 2014 SSPA.