A monogenic disease has a recessive autosomal hereditary transmission when the
gene involved is located on one of the autosomes and the pathological condition occurs if both of gene copies (recessive trait) are mutated; for example the cystic fibrosis is caused for a mutation in both of CFTR gene copies, or mutation in the two copies of the Cx26 gene cause a not syndromic genetic deafness. In the population an individual with only one alterated copy is defined heterozygous for a specific mutations and because of he not manifests the disease (asymptomatic) is a “healthy carrier”. Affected individuals generally have two healthy carrier parents.
In the Fig. 1 for a specific gene the normal allele is point out with A, the mutated allele with a; "healthy carrier" individuals produce gametes (oocytes and spermatozoa) with a normal allele A or mutated allele a. So two healthy carrier individuals (heterozygous) can generate:
- with a probability of 25% children with no one of mutated alleles (homozygous individuals AA) they not transmit the mutated allele because they have the normal gene copies.
- with a probability of 50% children with one normal gene copy and one mutated gene copy (heterozygote individuals Aa); they are healthy carrier (how their parents) and so they have the same probability to transmit the mutated allele a.
- with a probability of 25% children with both copies of the mutated gene (homozygous individuals aa) show the disease.
Both male and female individuals can show autosomal recessive diseases.
|Fig.1- Recessive autosomal hereditary transmission|