Summary
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1.
The diploid chromosome number forEquus caballus is 64, that forEquus asinus is 62, and that for the mule, 63.
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2.
The chromosome complements ofE. caballus andE. asinus are completely different morphologically.E. caballus has 19 pairs of metacentric autosomes and 18 pairs of acrocentrics, whileE. asinus has 19 pairs of metacentric autosomes and 11 pairs of acrocentrics. The morphology of the sex chromosomes is also dissimilar between the species.
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3.
While the disparity between parental chromosomes satisfactorily accounts for sterility in male hybrids, it does not explain the occasional fertile female able to produce several offspring with noE. asinus traits. Apparently morphological disparity between parental chromosomes is only a small part of the relatively complex matter of female gametogenesis.
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This work was supported by grants from the National Science Foundation (C-17601) and from the National Cancer Institute, U. S. Public Health Service (C-5138).
The authors gratefully acknowledge the editorial assistance ofPatricia A. Ray.
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Trujillo, J.M., Stenius, C., Christian, L.C. et al. Cromosomes of the horse, the donkey, and the mule. Chromosoma 13, 243–248 (1962). https://doi.org/10.1007/BF00577041
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DOI: https://doi.org/10.1007/BF00577041