Recessive autosomic transmission

A genetic disease is known as of recessive autosomic transmission when

  • the abnormal Gène is on a nonsexual chromosome (neither X, nor Y)
  • the presence of two Gène S abnormal is essential so that the disease exprime.
One of two abnormal genes is transmitted by the father, the other by the mère.
During the Gamétogénèse, the male or female cells pass from 2n Chromosome S (cell Diploïde) to N Chromosome S (cell Haploïde). The abnormal gene is thus present in half of the gamètes.
The characteristics of a genetic disease with recessive autosomic transmission are the following ones:
  • It reaches as much the Homme S as the Femme S
  • It reaches the descendants of a generation and saves the descendants of the following generation (it jumps of a generation to another).
  • the Consanguinité increases the risque.
But to be carrying gene inevitably does not mean to be sick, the demonstrations of a genetic disease depend on its Pénétrance and the Variabilité of its expression

Outline general of the recessive autosomic transmission

In the diagrams, the gamétes carrying normal gene are in blue, the gamètes carrying abnormal gene are in red. The sick people are in red, the nonsick people but carrying gene are purple, the people noncarrying gene are in blue.

Union of a carrier and nona carrier

Like this diagram shows it, with each pregnancy, this couple has a risk:
  • 1/2 to have a child carrying abnormal gene
  • 1/2 to have a child noncarrying abnormal gene
Thus the transmission of a recessive autosomic genetic disease can stop (it is enough that no carrying child is born from the union).

Union of two carriers

Like this diagram shows it, with each pregnancy, this couple has a risk:
  • 1/4 to have a child carrying normal gene
  • 1/2 to have a child carrying abnormal gene
  • 1/4 to have a child carrying two abnormal genes thus malade
One can thus calculate the probability of which has occurred of a recessive autosomic disease by knowing the incidence of carrier of the abnormal allele in the general population. Thus if an allele is present at a subject on 70, the probaility which an unspecified couple has a child reached by the disease of 1/70 is multiplied per 1/70 multiplied by 1/4 is 1/19  600 if the penetrance of this disease is of 100%
This simple statistical calculation explains for example why, in the case of the hémochromatose, in spite of the fact that 10% of the French population either carrying abnormal gene or a probability of 1/10 multiplied per 1/10 multiplied by 1/4 or 1/400 with each pregnancy to have a child homozygote the prevalence of the disease is weaker because of a rate of penetrance bas.
In the case of the Mucoviscidose, the prevalence of carrier of a change of the gene CFRT responsible for 1 out of 20 either the theoretical probability of 1/20 multiplied per 1/20 is multiplied by 1/4 or 1/1600 but the prevalence of the serious mucoviscidose is much weaker because of the very diverse demonstrations of this disease.

Union of nona carrier and a patient

Like this diagram shows it, all the children of this couple will be carrying abnormal gene

Union of a carrier and a patient

Like this diagram shows it, with each pregnancy, this couple has a risk:
  • 1/2 to have a child carrying abnormal gene
  • 1/2 to have a child carrying two abnormal genes thus sick

Union of two patients

All the children are sick

General base of the genetic council in the diseases with recessive autosomic transmission

In genetics, the proposer nominates the person reached of the genetic disease from which one makes the genetic Conseil.

Relative of a proposer

Phratry of a proposer

Descent of a proposer

Sources

See too

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