SUMMABY Cockerels singly or doubly heterozygous (tl/ + , t 2 / + and tl/t2, respectively; collectively heterokaryotypic, HTK) for two different Z-autosome translocations (tl, t2) produce an array of chromosomally balanced and unbalanced spermatozoa owing to adjacent segregation and nondisjunction at the first meiotic division. This study addresses the developmental capacity of embryos derived from matings of control ( + / + ) and HTK (tl/ + , t 2 / + , tl/t2) cockerels with hens bearing normal

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... hromosome complements (+ /W). Estimates of the hatchability of fertile eggs sired by + / + , t l / + , t2/ + and t l / t 2 cockerels were 82-8 %, 43-0 %, 41-3 % and 10-9 %, respectively. Approximately 75 % of the mortality observed in embryos sired by HTK cockerels occurred by four days of incubation. Developmental arrest generally occurred earlier in embryos sired by t 2 / + and t l / t 2 cockerels than in those by t l / + cockerels. These differences reflect variation in the degree of embryonic chromosome unbalance expected among embryos of the different sire groups. The pattern of mortality after three days was similar in embryos sired by HTK cockerels. In control embryos, mortality was highest on days 20-21 of incubation. Chromosome analysis of 16-18 h embryos, day 1-5 embryos and hatched chicks sired by HTK cockerels revealed that most, if not all, chromosomally unbalanced embryos died during development; the majority before three days of incubation. Partial monosomy for chromosome one was found to be more deleterious to embryonic development than partial trisomy.