Fanconi Anemia . . .

Genetic Cross Tables

First, let's review the notation for these genetic cross tables. "A" represents a dominant gene--in our case, this would be a non-disease causing Fanconi gene.  "a" represents a recessive gene--in our case, a disease causing Fanconi gene. You only need one dominant gene to avoid the disease--a person who is "Aa" (heterozygous) is disease-free, but carries the disease causing gene and can pass it on to offspring.  Disease only manifests when a person is "aa" (homozygous recessive), having two genes that cause the disease. It almost goes without saying that a person who is "AA" (homozygous dominant) is also disease-free and cannot pass the disease on to offspring. 

Note that an "aa" offspring inherits a diease causing gene from each parent.  These genes must be the same gene--for example, the FANCA gene. If one parent carries a pathogenic FANCA gene, and the other parent carries a pathogenic FANCC gene, then the offspring will not have the disease. An offspring has to inherit two pathogenic FANCA genes, or two pathogenic FANCC genes, etc. to be affected by Fanconi Anemia.

Cross tables

Aa x Aa

  This is the case of two carriers mating. We saw this on the prior page.  I repeat it here for completeness:

Result:
  1/4: AA, not affected
  2/4: Aa, not affected, carriers of pathogenic gene
  1/4: aa, affected

AA x Aa

This is the case of a non-carrier mating with a carrier:

Result:
  2/4: AA, not affected
  2/4: Aa, not affected, carriers of pathogenic gene

Aa x aa

  This is the case of a carrier mating with an affected person:

Result:
  2/4: Aa, not affected, carriers of pathogenic gene
  2/4: aa, affected

AA x aa

   This is the case of a non-carrier mating with an affected person:

Result:
  4/4: Aa, not affected, carriers of pathogenic gene

XY x Xx

  This is the case for complementation group B, which is X-linked. Usually the father (XY) will not be affected with FA-B; it would usually be passed by a heterozygous mother (Xx).

Result:
1/4: XX, not affected girl, not a carrier
1/4: Xx, not affected girl, carrier of pathogenic gene
1/4: XY, not affected boy, not a carrier
1/4: xY, affected boy

Note that the girls have a 50% chance of being a carrier, and the boys have a 50% chance of being affected.

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Last updated: 01 Sep 2005