The flexed-tail mutation appeared in a stock maintained by Dr. H.R. Hunt at Michigan State College (J:12951). Homozygotes are small at birth and have a transitory hypochromic, microcytic anemia characterized by a large number of siderocytes containing non-heme iron granules. Most homozygotes also have flexed tail and a belly spot, but these are not constant manifestations of the mutant. Because of the anemia there is probably greater postnatal mortality among f/f than among normal mice (J:14979).

The anemia begins on the 12th day of embryonic life when the liver first starts to produce blood cells (J:14979). It is most intense at 15 days of gestation and still severe at birth, but by 2 weeks of age has disappeared. Although adults have normal blood values, their response to hemopoietic stress is defective (J:5439, J:27511).

The results of numerous studies have led to the conclusion that the prenatal deficiency in number of erythrocytes and the defective response of adult erythropoietic cells are due to a delay in maturation of already committed erythroid stem cells, and that earlier uncommitted precursors are unaffected by f (J:5439, J:5654, J:5582).

An additional effect of f in homozygotes is defective heme synthesis, which occurs in fetal reticulocytes but not in adult reticulocytes nor in erythroblasts at earlier stages of maturation. In fetal reticulocytes there is normal uptake of iron but poor incorporation into hemoglobin (J:5439), probably as a result of reduced activity of delta-aminolevulinate synthetase and dehydratase (J:5591).

Fetal erythrocytes of f/f mice have more alpha than beta globin chains. In both f/f and wild-type fetal erythrocytes there is more alpha- than beta-chain mRNA; probably some regulatory mechanism bringing about equal alpha- and beta-chain synthesis exists in wild-type mice but is defective in f/f (J:5827, J:30711).

The tail abnormalities are first noticeable on the 14th day of gestation as abnormal differentiation of the intervertebral discs (J:13090). The possibility that abnormal heme synthesis could cause the tail and pigment defects in f/f mice has been discussed (J:5591).

It was suggested that flexed-tail might be a mutation in the mouse homolog Fancc of the gene defective in human Fanconi anemia, complementation group C, but no mutation in the Fancc gene or abnormalities in Fancc mRNA have been detected in f/f mutants (J:13598). Also, flexed-tail mice are not susceptible to increases in chromosomal aberrations induced by mitomycin C, a characteristic of Fancc mutant mice (J:35839).

This allele arose on a genetically undefined stock in 1927 and was subsequently transferred onto several genetic backgrounds to create the congenic and recombinant inbred lines Je/Le-f/f, FL1/ReJ, WB/ReJ-f/f and C57BL/6J-f/f. The phenotypes listed above might be associated with any of these strains; in most cases it was not specified.