Dwarf mutant of Xiphophorus helleri is used as the female parent to cross normal body type male,showing that dwarf individual can be identified in the offspring and such character is inheritable.The body type pattern of offspring exhibits a successive distribution,similar to male or female parent,in addition to intermediate type.It is less likely to discriminate the individual phenotype by using a clear definition.Dwarf mutants of Xiphophorus hellerimanifest as short bloated body,clumsy swimming,and significantly altered trunk lengthheight ratio.This mutation retards fish growth and compromises the reproductivity.The self-cross and back-cross of F₁ progenies also result in a successive distribution in body type among various crosses.The dwarf characteristic in X.helleri is not inherited in a simple alternative manner,which affects a group of characters rather than a single character.The consistency with primary feature of quantitative character suggests that such mutation may be controlled by multiple quantitative character genes.The skeletal morphological analysis indicates that the spinal length of X.helleri mutant is significantly shortened,whereas the number of vertebrae is comparable to that of normal individual,ranging from 26 to 29.The dramatically shortened vertebral body in the mutant impairs the longitudinal development of spine.The neural spine and hemal spine are in a tight arrangement,whose development is also significantly retarded.Therefore,the dwarf mutation in X.helleri is caused by the developmental defect in vertebral column,and the vertebral malformation underlies the body type alteration anatomically.As fish is representative of lower-ranking vertebrate,the dwarf swordtail mutant is reproducible,and the character is inheritable and closely associated with spinal development.Such mutant is a potential fish model for the systemical research.