INHERITANCE—IMP RO VEMENT B I ' BREEDING 1 "j t 



is not blue. If it is not blue, it may be white, cream, or some CHAP, 

 other colour; if it is not tall, it may be short or medium- V ' 

 sized. These facts have led to the concept of pairs of opposite 

 unit-characters, known as allelomorphic pairs, either one, but 

 not both of which, can be present in any gamete. Any one 

 allelomorphic pair consists only of its two allelomorphs, i.e. 

 characters which will not unite in the same gamete ; but a 

 gamete may include any number of characters which are not 

 allelomorphic, i.e. not opposed to one another. The zygote, on 

 the other hand, can carry both, since it is composed of two 

 gametes each of which may contain either one of the two 

 allelomorphs. But where each of the two parents possesses 

 the opposite one of an allelomorphic pair of characters (e.g. 

 blackness and absence of blackness), both of these allelomorphs 

 cannot appear in the F, heterozygote with the same strength 

 with which they appear in the parents ; it is found that one of 

 the two unit-characters always appears (Fig. 72c) to the ex- 

 clusion of the other, e.g. when we crossed yellow- and white- 

 grained types of maize the resulting grain was yellow. 



The one unit-character which appears in the first filial (¥{) 

 generation is called dominant, because when present it masks 

 the other ; the other is called recessive because it always re- 

 cedes, or gives place to the dominant, when the latter is 

 present. Bateson (1) has pointed out that dominance of cer- 

 tain characters is often an important but never an essential 

 feature of Mendelian heredity ; it is only a subordinate incident 

 of special cases. 



We sometimes meet with cases in which two colours ap- 

 pear to merge in the F 1 progeny, e.g. when blue- and yellow- 

 grained breeds of maize are crossed together, the result is 

 green ; this latter is probably not a case of " incomplete 

 dominance," but the temporary merging of two dominants 

 belonging to different allelomorphic pairs. 



From the examples cited in preceding paragraphs we 

 learn that : (a) the characters of both parents are transmitted 

 to the offspring, even though the two parents are very dis- 

 similar ; (b) if the parents are dissimilar their immediate 

 progeny cannot be like each of them in all particulars ; ' 



1 Other experiments show that this also holds true when two similar parents 

 are mated, if they are heterozygous (IT 127), i.e. themselves the progeny of dis 

 similar parents. 



