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How genetic disorders are inherited

Please read the Genetics Introduction section first

Genetic disorders are usually caused by alterations to a chromosome (or part of a chromosome) or to a gene. At the time of cell division in a fertilized egg, sections of chromosomes can become detached. This detached part may be lost completely (deletions), it may re-attach the wrong way around (inversions) or it may become associated with material from another chromosome (various types of insertion and translocation). A structural change to a chromosome will usually affect the function of a number of genes.  

The Ectodermal Dysplasia syndromes are more often caused by alterations within a gene – spelling mistakes in the DNA code (language) of four letters. Each gene has a set of rules for using a sequence of the 4 letters A, G, T and C using groups of three letter words to make meaning. A change in the sequence of the letters may mean the genetic instruction no longer makes sense.

It may be worth remembering that a chromosome is a long structure made of a chemical known as deoxyribonucleic acid (DNA) as well as many proteins. Chromosomes contain many genes.

Genes are segments of DNA

Genes are contained within – along the length of – chromosomes

A chromosome contains hundreds to thousands of genes

Alterations in the DNA sequence of the gene may be inherited from a parent or normal genes may become altered – they may change (mutate) – at the time the egg or sperm are made or even afterwards, during the course of the child’s development.

It takes two to make a baby, so chromosomes and the genes each parent carry come in pairs; one set of 23 from the mother and one set of 23 from the father, making 46 chromosomes in total. It is the chromosomes in the egg and the sperm that carry the genetic information from one generation to the next.  One of each chromosome pair is contained in the developing sperm, and likewise for the egg, so each has a single set of 23 chromosomes.

When the egg and sperm come together at the time of fertilisation, the correct number of 46 chromosomes meet up, ready for the baby’s development.

However, sometimes this process goes wrong and an egg or sperm may end up with too many chromosomes or not enough chromosomes.  At the time of fertilization, the baby could receive an extra chromosome or have a missing chromosome resulting in 47 or 45.  The most common chromosomal abnormality of this type is Down’s syndrome, usually due to the egg having both copies of chromosome 21 so the affected child ends up with three, making 47 in total. This extra gene action from chromosome 21 disturbs the genetic control of fetus development.

Twenty-two of the chromosome pairs are the same in males and females, but the 23rd pair is different. The chromosome pairs that are the same in both sexes, numbered 1 to 22, are called autosomes to distinguish them from the sex chromosome pair, X and Y. Females have two X-chromosomes, whilst males have one X and one Y chromosome. This means that the chromosomes in each sperm are one of two types; half will have an X chromosome and half a Y chromosome. If an X-carrying sperm fertilises the egg, the baby will be a girl, whilst a Y-carrying sperm makes a boy. The father’s contribution, the sperm, determines the sex of the baby.

There are three simple patterns of inheritance. The first two involve one of the pairs of genes on chromosomes 1 to 22 and are referred to as autosomal dominant and autosomal recessive inheritance. The third, sex-linked inheritance, involves one of the genes on the X-chromosome and for this reason is also called X-linked inheritance.

Which chromosome we get from each pair is completely random. This means that different children in the same family will each get a different combination, which explains why children in the same family look a little like each other and a little like each parent but are not identical to them.

These ‘Mendelian’ patterns of inheritance were originally observed by Gregor Mendel and are illustrated in the section “An introduction to genetic testing” by way of simple chromosome diagrams showing a single chromosome pair and just one gene upon it.

Statistical probabilities are all very well when being reassured by comfortingly low odds (often the case after genetic counselling), but it is not much help to those facing a high chance of an affected child. People often want to know whether they are a carrier or not; whether the developing baby is affected or not. All else is just agonising uncertainty. It has only been through the advances of molecular genetics that most carrier testing and early prenatal diagnosis have become possible.

Some Ectodermal Dysplasia syndromes can be passed down from generation to generation. However, the chances that a man and woman will have an affected child will depend on the inheritance pattern of the type of Ectodermal Dysplasia in that family.

The Ectodermal Dysplasia Society strongly advises individuals who have been diagnosed with an Ectodermal Dysplasia syndrome, as a carrier of an ED syndrome, and/or parents of children affected by an ED syndrome to seek genetic counselling. On this website we can provide a brief overview of the most common types of inheritance patterns but as each syndrome and inheritance pattern will be specific to each family, you are advised to speak with a Genetic Counsellor or Clinical Geneticist about the probability of passing Ectodermal Dysplasia onto future generations.

If you would like more information on how to find a Genetic Counsellor or help to find the answers to the questions you are seeking, please contact us info@edsociety.co.uk or telephone 01242 261332.