FANCB is the one exception to FA being autosomal recessive, as this gene is on the X chromosome. These genes are involved in DNA repair.The carrier frequency in the Ashkenazi Jewish population is about one in 90. Genetic counseling and genetic testing are recommended for families who may be carriers of Fanconi anemia.Because of the failure of hematologic components—white blood cells, red blood cells, and platelets—to develop, the bodys capabilities to fight infection, deliver oxygen, and form clots are all diminished. Pathogenesis
Clinically, hematological abnormalities are the most serious symptoms in FA. By the age of 40, 98% of FA patients will have developed some type of hematological abnormality. However,  a few cases have occurred  in which older patients have died without ever developing them. Symptoms appear progressively, and often lead to complete bone marrow failure. While at birth, blood count is usually normal, macrocytosis/megaloblastic anemia, defined as unusually large red blood cells, is the first detected abnormality, often within the first decade of life (median age of onset is 7 years). Within the next 10 years, over 50% of patients presenting haematological abnormalities will have developed pancytopenia, defined as abnormalities in two or more blood cell lineages. This is in contrast to Diamond–Blackfan anemia, which affects only erythrocytes, and Shwachman–Diamond syndrome, which primarily causes neutropenia. Most commonly, a low platelet count (thrombocytopenia) precedes a low neutrophil count (neutropenia), with both appearing with relative equal frequencies.
With a crippling mutation in any FA protein in the complex, DNA repair is much less effective, as shown by its response to damage caused by cross-linking agents such as cisplatin, diepoxybutane and Mitomycin C. Bone marrow is particularly sensitive to this defect. In another pathway responding to ionizing radiation, FANCD2 is thought to be phosphorylated by protein complex ATM/ATR activated by double-strand DNA breaks, and takes part in S-phase checkpoint control. This pathway was proven by the presence of radioresistant DNA synthesis, the hallmark of a defect in the S phase checkpoint, in patients with FA-D1 or FA-D2. Such a defect readily leads to uncontrollable replication of cells and might also explain the increase frequency of AML in these patients. Spermatogenesis
In humans, infertility is one of the characteristics of individuals with mutational defects in the FANC genes. In mice, spermatogonia, preleptotene spermatocytes, and spermatocytes in the meiotic stages of leptotene, zygotene and early pachytene are enriched for FANC proteins. This finding suggests that recombinational repair processes mediated by the FANC proteins are active during germ cell development, particularly during meiosis, and that defects in this activity can lead to infertility. Neural stem cell homeostasis
Microphthalmia and microcephaly are frequent congenital defects in FA patients. The loss of FANCA and FANCG in mice causes neural progenitor apoptosis both during early developmental neurogenesis and later during adult neurogenesis. This leads to depletion of the neural stem cell pool with aging. Much of the Fanconi anemia phenotype might be interpreted as a reflection of premature aging of stem cells.