A chromosome study in Xenopus laeypsilon;is with five of the six possible male and female sex combinations of the ZZ, ZW and WW genie constitutions furnishes no visible evidence of sex chromosome dimorphism. This conclusion is based on construction and analysis of 71 karyotypes of somatic or spermatogonial cells, of which 31 are of ZZ, 29 of ZW and 11 of WW genetic constitution. The constitution of the twenty donor animals is certified by their breeding records. The morphologic study reveals that the 36 chromosomes of this species can be arranged in a series of 11 submetacentric and another series of 7 subacrocentric chromosomes. The chromosome pairs of all genetic types consist of visually equal partners (homomorphic pairs). Measurement of the overall lengths of all chromosomes (in relative units) and calculation of arm ratios do not reveal differences between genetic types nor furnish any indication of the presence of a heteromorphic Z-W pair. Testicular and ovarian development with complete spermatogenesis and ovogenesis proceed normally with any of the three genie types.
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