There are many factors that play a role in the health of your developing baby: nutrition, the age of the mother at the time of birth, environmental factors, and of course, genetics. These factors may also play a role in the possibility that your baby may inherit a genetic disorder.
Along with genetic counseling, prenatal genetic testing can provide more information on the likelihood of your child carrying or inheriting genetic abnormalities. For couples who suspect that they have a family history of a genetic disorder, it is a good idea to be aware of how a disorder might be passed on so you can better understand the chances of passing these traits on to your children and decide if prenatal tests should be considered.
There are two types of chromosomes, autosomes and sex chromosomes. Autosomes are numbered 1 – 23 and are the chromosomes that are not associated with sex determination. Sex chromosomes are labelled as X and Y and determine the sex of the child. Each chromosome pair is made up of two chromosomes, one given from each parent. Inheritance patterns occur in either dominant or recessive forms. If a gene is said to be dominant, only one of the affected chromosomes needs to be present in the pair in order for the genetic disorder to be inherited. If a gene is characterized as recessive, then both chromosomes in the pair will need to be carrying this gene in order for the disorder to manifest.
A carrier is an individual who has the mutated gene but is unaffected. This gene can be passed down for generations without consequence until this individual has a child with another carrier of the same mutated gene. This is important when considering recessive disorders.
In autosomal dominant disorders the mutated gene is located on an autosome. Because these disorders are characterized as dominant, only one affected chromosome needs to be present in order for the disorder to be inherited. A parent carrying the affected chromosome will have a 50% chance of passing this disorder on to their offspring.
In autosomal recessive disorders, the mutated gene is located on an autosome. In order for a recessive genetic disorder to be inherited, both chromosomes carrying the mutated gene will need to be inherited. If only one parent is a carrier, there will be no chance of inheriting the disorder. If both parents are carriers of this gene, there will be a 25% chance of inheritance.
In X-linked recessive disorders, males are more affected than females. If a female has one X chromosome with the mutated gene, the other healthy X chromosome will compensate for the mutation. If a male carries an X chromosome with the mutated gene, he does not have a second X chromosome to step in and is consequentially affected. A woman with a mutated gene on an X chromosome has a 50% chance of passing the gene to her child. If a male contains an X chromosome, his Y chromosome is unaffected, and poses no risk to a son. If he has a daughter, he will pass on his affected X chromosome, who will then become a carrier.>
In X-linked dominant disorders, females are more affected than in X-linked recessive disorders. The mutated gene is dominant, or “stronger”, and the second healthy X chromosome will not be able to fully compensate for the mutation in this case.
If you believe that you may have a family history of a genetic disorder, you can speak about your options in detail with a genetic counselor regarding prenatal DNA testing.