536 THE PURE LINE HYPOTHESIS. 



It is assumed that a so-called unit-character exhibits the 

 following peculiarities : ( i ) It is essentially indivisible, normally 

 being inherited only as a whole; (2) it is supposed to be repre- 

 sented in a perfectly fixed and constant manner as a factor or 

 group of factors in the fertilised egg-cell, and according to 

 Morgan and others, the factors are lodged in a particular pair of 

 chromosomes, and possibly in a definite place in the chromosomes ; 

 (3) it is frequently capable of being replaced by a second unit- 

 character of a different nature. 



The above features of a unit-character, with the exception^ 

 of the supposed agency of the chromosomes, were demonstrated 

 by the original experiments of Mendel. The two mutually re- 

 placeable characters mentioned under (3) are the dominant and 

 recessive characters of the Mendelians, and are known as allelo- 

 morphs. 



-3. Chromosomes and the Mendelian Relationships. 



Recent cytological research and Morgan's* striking experi- 

 ments with the fruit-fly Drosophila appear to show that the 

 ordinary cells of the body, the non-maturated sex-cells and the 

 fertilised egg-cells contain a nucleus in which the chromosomes 

 are arranged in homologous pairs, one of each pair being derived 

 from the nucleus of the matured eg^, and one from the nucleus 

 of the sperm. Morgan has endeavoured to demonstrate that the 

 factors governing characters are located in definite places in par- 

 ticular chromosomes of the fertilised egg-cell. When the factors 

 are similar in the two chromosomes of an homologous pair, the 

 individual which arises is said to be hovnozygoits ; while if one 

 chromosome of the pair contains the factor of a certain character, 

 and the other chromosome contains the factor of the alternative 

 character, or allelomorph, the individual which arises is said to 

 be lieterosy(^ous. 



In the maturation of the eo;o; or of the sperm in the last 

 mitotic division the individual chromosomes of the homologous 

 pairs separate from each other, and in the matured e^^. or in the 

 spermatozoon, there remains only one chromosome of each pair, 

 and accordingly a unit-character, or its allelomorph, is repre- 

 sented by a single factor only, and not by a pair of homologous 

 factors, as in the fertilised egg-cell. This separation of the pairs 

 of homologous factors has been called the secfreqation of the 

 factors (see plate). 



The heterozygous condition arises by crossing two indi- 

 viduals in wdiich jointly both factors for the two alternative or 

 non-blending characters are present. The ordinary results that 

 may be expected by various crossings of homozygous and hetero- 

 zvgous individuals can be readily 'calculated theoretically, and 

 the predictions frequently tally remarkably well with the results 

 of breedine experiments with resnect to non-blendine characters. 



Mendelian heredity necessarilv involves the notion of unit- 



* Moro-an. T. H.. and others : " The IMechanism of Mendelian Here- 

 dity." New York, 19x5. 



