SOME COMPLICATIONS OF MENDELIAN INHERITANCE 



303 



example of such a gene was found in mice. Repeated experiments showed 

 that when two yellow mice were bred together, the progeny always 

 consisted of 2 yellows to 1 nonyellow, as though 1 individual of a 3:1 

 or a 1:2:1 ratio were persistently missing. Autopsies on pregnant yellow 

 females that had been bred to yellow males showed that 1 embryo out 



pure bred 

 black and splashed while 



Andalusian 

 when crossed produce all 



blue 

 Fl' s 



these F ]' s 



when bred together produce 

 1/4 black 1/4 white 



and 2/4 of 

 the desired blues 



VM>* 



note that the 



blacks breed 

 true 



the blues again give 1/4 1/2 1/4 

 Fig. 20.1. The inheritance of blue as the nondominant monohybrid of black and splashed 

 white in the Andalusian fowl. 



of 4 always died; and genetic experiments proved this embryo to have 

 been the homozygous YY individual. Here, apparently, is a gene that 

 produces yellow coloration as a dominant and some lethal condition as a 

 recessive. Just what this lethal condition may be is unknown, but it is 

 fatal to the developing mouse at a fairly early stage in embryonic exist- 

 ence. Many other lethals are known, some producing death in early 



