Meaning of Dominant and Recessive 



Confusion has arisen in some quarters as to the meaning 

 of the terms dominant and recessive. For example, it has 

 been suggested that "a dominant character is the result of a 

 long process of selective breeding." This idea of course is 

 groundless. 



Again, it has been stated more frequently that the 

 older of the two members of a pair of characters is domin- 

 ant. This theory holds good in some cases but it breaks 

 down in others. 



Technical Terms. — In the great extension of our know- 

 ledge of heredity during the last few years certain terms 

 have been introduced to express more clearly and accurate- 

 ly the new and complex situations that have arisen. In- 

 stead of the term "character," "determiner" is used as be- 

 ing that element or condition within the gamete which 

 determines the development of a given somatic character. 

 Synonymous terms are factor and gene. (See also page 109). 



"Dominant" and "recessive" characters are now fre- 

 quently explained by the ''Presence and Absence Theory/' 

 which assumes that a determiner for any character either is, 

 or is not, present, 



If both of the pairing gametes contain the same deter- 

 miner, the zygote or offspring will be homozygous or will have 

 a double, or duplex, dose of the character. If only one of 

 the pairing gametes contains the determiner, the zygote will 

 be heterozygous or will have a single, or simplex, dose of the 

 character. If absent from both gametes the zygote is said 

 to be nulliplex. 



Organisms that have identical determiners in the germ- 

 plasm belong to the same genotype, while those that appear 

 alike without reference to germ plasm belong to the same 

 phenotype. (See also Glossary, page 177). 



Exercise. — Determine the probable nature of the progeny 

 when 



(1) Duplex x duplex = 



(2) Duplex X Simplex = 



(3) Simplex x Simplex = 



(4) Duplex X NuUiplex = 



(5) Simplex x Nulliplex = 



(6) Nulliplex X Nulliplex = 



95 



