212 bulletin: museum of comparative zoology. 



iu the case of other crosses of a hybrid with a recessive form, namely, 

 D (R) c? + L> (H) 9 +i.i S + I'^ ? > '^vhich is not the result obtained 

 iu this case. 



Hence to explain the exceptional results before us we must assume 

 two exceptional occurrences, (1) a partial coupling, among the gametes 

 of the hybrids, of the male sex-character with the dominant (lugeus) 

 form-character, (2) possession of sexual dominance by the gametes of 

 the hybrid parent, when that pai'ent is crossed Avith a recessive. But 

 when two hybrids are intercrossed, as in Generation IV. [2] and Gen- 

 eration V. [4] and [5], we should not expect to find sexual dominance 

 possessed uniformly by the gametes of either parent, since both are 

 hybrids. If, on the other hand, coupling occurs among all the gametes 

 of both hybrid parents, only hybrid offspring will be produced and in 

 the normal sex-i)roportion, approximately an equality. See Table IV. 

 For each parent will produce only gametes D ^ and R 9? f^"d when 

 opposite sex-characters meet, the zygote formed must always be D R. 

 The result will be the same whether sexual dominance is possessed ex- 

 clusively by the gametes of one parent, or is shared equally by those 

 of both. The fact that in all of the three matings indicated a certain 

 number of recessive offspring occurs, shows conclusively that coupling 

 between the male character and the lugens character does not occur in 

 all possible cases. In Generation IV. [2], the total number of recessive 

 offspriug is even greater than it should be if no coupling occurred, and 

 I am at a loss for an explanation of the discrepancy, unless one parent 

 furnished considerably more than the theoretical number (one-half) of 

 recessive gametes. But in the two similar crosses of Generation V., the 

 total number of recessive offspring, on the supposition that no coupling 

 occurs, is less than half the theoretical. In all three cases the sex- 

 projwrtion among the offspring, both dominants and recessives, ap- 

 proximates that which would result from chance combinations of gametes 

 of two hybrid parents on the suppositions: (1) that there occurs a 

 coupling of the male character with the lugens character and of the 

 female with the tau character in approximately one-third of all cases, and 

 (2) that when coupled gametes meet uncoupled ones in fertilization, the 

 sex of the former always dominates in the zygote. On these two hypoth- 

 eses, each hybrid parent will furnish gametes in the proportions 2 D ^J 

 + D 9 -fR^ + 2R9, of which one of the two D <? s and one of the 

 two R 9 s will be coupled. If all possible matings occur and the coupled 

 gametes are sexually dominant over uncoupled ones, the distribution of 

 the offspring will be 8 D J : G D 9 : R ^J : 3 R 9- Ou this basis 



