Recessive genetic conditions are a result of an individual having 2 faulty copies of a recessive condition-causing gene. This occurs when the individual inherits one faulty copy from their mother and one from their father. This person is affected by the genetic condition.
An individual with one faulty copy of a gene that causes a recessive condition is a carrier of the condition and has a 50:50 chance of passing the fault onto their child. Carriers are not affected physically in any way.
- Genetic carrier screening for recessive genetic conditions in someone who is healthy determines whether the person is a carrier.
A baby receives one copy of each gene from each parent, thus being a “mixture” of genetic information from his/her parents, see figure 1 and 2.
- Figure 1: When two unaffected carriers of the gene fault become parents, then for each pregnancy:
- There is a 1 in 4 (25%) chance that the child will inherit the normal gene copy from both parents. They will be completely free of the gene fault and will not be affected by the disorder (figure A).
- There is a 2 in 4 (50%) chance that the child will inherit both a normal copy and a faulty copy of the gene. The child will be a carrier of the faulty gene, and will be UNAFFECTED by the disorder, just like his/her parents (figures B and C).
- There is a 1 in 4 (25%) chance that the child will inherit the faulty gene copy from both parents. This child WILL BE AFFECTED by the disorder (figure D).
- Figure 2 shows that if only one parent is a carrier, there is no chance that they will have a baby affected with the disorder, however, there is a 50% chance that each of their children will be carriers of the faulty gene.
Several recessively inherited diseases occur at a much higher rate in the Ashkenazi Jewish community in comparison to the general community in Australia. The Community Genetics Program NSW (CGP) screens for conditions of this type. The conditions include:
- Tay-Sachs disease
- Canavan disease
- Fanconi anaemia
- Familial dysautonomia
- Cystic fibrosis
- Bloom syndrome
- Niemann-Pick disease
- Glycogen storage disease type 1a
- Mucolipidosis type 4
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