VI 



ABNORMAL INHERITANCE 



The characters non-agouti, pink-eye dilution, blue dilution, 

 brown, recessive spotting, waltzer, rodless, shaker, and the 

 albino series of allelomorphs seem to be recessive unit- 

 characters and to behave according to the rule for simple 

 Mendelian inheritance. Varieties characterized by these all 

 breed true because they are homozygous. Yellow-belly 

 behaves as a dominant unit-character and breeds true only 

 when homozygous. Yellow and dominant spotting do not 

 breed true but produce only 50 per cent of offspring possess- 

 ing the characters in question, because they are dominant 

 lethals and so cannot be made homozygous. 



Normal Overlaps. Silver behaves as a recessive unit- 

 character but seemingly does not always breed true because 

 some silvered animals are with difficulty recognized as such. 

 Silvered mice throw a certain percentage of animals of such a 

 low grade of silver that they may not be distinguished read- 

 ily from non-silvered animals, although they are genetically 

 silvers. Such animals which are genetically of a mutant type 

 but somatically are capable of classification as of the normal 

 type are known as normal overlaps. 



Dominant Lethals. Two of the unit-characters of mice, 

 namely yellow and dominant spotting, are dominant lethals. 

 By this we mean that animals bearing one factor for the 

 character exhibit the character, but that animals bearing 

 two factors for the character are non-viable. Thus, all yellow 

 mice carry a non-yellow factor as a recessive. When two 

 yellows are mated together they produce on the average one 

 homozygous yellow which dies, two yellows heterozygous 

 like themselves, and one non-yellow animal, as was first 

 pointed out by Cuenot (££). 



The same is true for dominant spotting. Two dominant- 

 spotted animals produce on an average one homozygous 



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