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THE AMERICAN NATURALIST 



[Vol. LIV 



aract is known in some mammals other than man, but little is 

 known regarding its transmission. Hurst (8) stated that liabil- 

 ity to cataract-blindness in horses is a Mendelian character. 



The data forming the basis of this paper arose through the 

 circumstance that a registered Holstein-Friesan bull, E. T. H. 

 (Holstein-Friesan Herdbook No. 62924) transmitted desirable 

 economic dairy characters to a marked degree, and consequently 

 attempts were made to fix his characters by inbreeding. A num- 

 ber of cataractous offspring resulted from these close matings. 

 The simultaneous occurrence of several cataracts in the progeny 

 of a single bull could not be attributed to chance environmental 

 or intrauterine conditions; hence the pedigrees of all animals 

 involved were carefully studied. There was no record of cat- 

 aract in any of the ascendants. The original bull, E. T. H., had 

 been mated to a large number of unrelated cows and produced 

 93 normal F 1 offspring. Thirty-two of these F 1 daughters were 

 mated to an F x son and produced 63 F 2 calves, of which 55 were 

 normal and 8 showed well-defined congenital cataracts of the 

 stellate type. 8 The ophthalmological aspects of these cases have 

 already been described by Small (8). 



Cataract is evidently recessive in cattle and if it is a simple 

 Mendelian recessive then the original progenitor, E. T. H., was 

 heterozygous, Nn, where N = normal and n = cataractous. 

 Mated to unrelated normal females, NN, we should expect the 93 

 Fi offspring to be perfectly normal, but of two genetic types in 

 equal numbers, NN + Nn. In selecting any F x son to breed to 

 the Fi daughters, it was equally probable that he would be either 

 a homozygous normal, NN, or a heterozygous normal, Nn. A 

 single F, son, also a registered Holstein-Friesan bull, V. H. (Hol- 

 stein-Friesan Herdbook No. 158293), was chosen and he proved 

 to be heterozygous. Since half of the F t daughters were homo- 

 zygous and half were heterozygous, the former would produce 

 gametes, N-j-N, and the latter would produce gametes, N-f-n. 

 The total population of F x daughters would therefore produce 

 three times as many normal as cataractous gametes. In Men- 

 delian notation the F t matings were as follows : 



%N -f y 4 n = gametes from F t females, 



