CHAPTER IX 



Chaeacteristics of Pedigrees for the Different Breeds. 

 Homozygosis Resulting from Different 

 Types of Matings 



The reader will recall that an individual which is homozygous 

 for a large number of factors will tend to have offspring more like 

 himself than will an individual which is heterozygous for a wide 

 number of factors. A sire homozygous for dominant factors will, 

 consequently, practically always reproduce his kind, or be prepotent. 

 The degree of homozygosis of a sire or cow is consequently of con- 

 siderable importance to the breeder. The degree of homozygosis 

 also has a further significance, in that a method of breeding which 

 rapidly increases homozygosis in each generation tends to do two 

 things — lessen the vigor of the offspring as measured by weight, 

 fertility, and health, and second, increase the uniformity of the 

 stock and consequently the prepotency in outside crosses. 3 Wright 

 has indicated the methods by which the degree of homozygosis of 

 an individual may be determined. The results give the probable 

 average homozygosis as a per cent based on the assumption that 

 the original ancestors were themselves random bred. 



REFERENCES 



In case the reader has not already done so it would be of interest for him 

 to read: 



Wright. 1921. Systems of mating 1 to o, Genetics, vol. 6, pp. 111-178. 

 Wright. 1922. Coefficients of inbreeding and relationship, American Natura- 

 list, vol. 56, pp. 330-338. 



As an illustration of the method, I take the pedigree of King 

 Walker 40358, a Holstein-Friesian bull (see page 56). 



1 A word of comment should be added, while these effects are the average 

 results of increase in homozygosis they may be avoided by careful selection 

 to isolate the superior lines of the stock in each generation. This result brings 

 in another agent, selection, which must be rigorously used on many characters 

 to result in superior lines of homozygous stock. 



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