WHAT IS A GENE? 151 



always to remember the different experimental operations on which the 

 concepts are based. 



The gene was first thought of as a unit of heredity determining a dif- 

 ference in phenotype and detectable if it existed in two or more dif- 

 ferent states, which came to be called alleles, each with a characteristic 

 developmental effect. This definition of the gene states that it is the 

 ultimate unit of function responsible for biochemical activity and pheno- 

 typic difference; it is the "hereditary factor" of Mendel and the "gene" 

 of the Danish biologist, Johannsen, who originated the word. 



When an organism shows an inherited difference in character, we as- 

 sume that this is due to the presence of an altered functional unit. 

 We may distinguish between two nonallelic genes if one is responsible 

 for a difference in one character and the other is responsible for a dif- 

 ference in another. Two recessive mutants, each arising independently 

 from a wild-type strain, are considered to be due to allelic heritable 

 units for the character in question if the homozygous recessive individual 

 in which these units are combined has a mutant phenotype. It is as- 

 sumed that there is a one-to-one correspondence between allelic genes 

 and some basic cell function. If two allelic mutant genes carry out the 

 same mutant function, a heterozygote between them cannot perform the 

 normal wild-type function. 



Functional units usually segregate at sexual reproduction in such a 

 manner as to indicate that they are located on chromosomes. It is these 

 functional units, and not extrachromosomal ones, which are the genes 

 discussed in this chapter. Through crossing over or other exchange 

 mechanisms, different units on a single chromosome may be separated 

 from one another. In general, the closer together two units are, the 

 less frequently will they be separated from one another by exchange. 

 A single unit may or may not be separated during meiosis from other 

 units at different locations on the same chromosome, but it will always 

 be separated from its mate on the homologous chromosome (unless the 

 homologous chromosomes do not disjoin, as is the case with attached 

 X's in Drosophila). Such mated pairs, that always separate at meiosis, 

 are called alleles. This definition of the unit of transmission depends 

 upon our ability to observe the segregation of alleles in meiosis; alleles 

 of the transmission unit always segregate from one another, nonallelic 

 units sometimes segregate. 



The distance between the loci of two nonallelic genetic units is meas- 

 ured in terms of the frequency with which they form recombinants. In 

 practice, recombinants with frequencies as small as 10*^ have been ob- 

 served. But theoretically, recombinants in much smaller frequencies 

 would occur between units located closer together and could be detected 



