GENETICS: THE HERITABLE TRANSMISSION OF CATALYSTS 161 



The two mating types or sexes of this mold are morphologically 

 indistinguishable, but they may be cultured indefinitely by asexual 

 multiplication if kept separately, though when put together under 

 suitable conditions mating and sexual reproduction occur. Unlike 

 the higher plants and animals which are diploid (i.e., their chromo- 

 somes occur in pairs), Neurospora is a haploid organism (i.e., its seven 

 chromosomes are singletons), which simplifies matters genetically. 



B. 



A. 



D. 



E. 



Figure 22. A. Normal wild-type female Drosophila, gray body color, red eyes, 

 long wings, absence of speck at base of wings. B. Male Drosophila having four 

 linked, recessive characters, viz., black body color, purple eyes, vestigial wings, and 

 speck at the base of the wings. C. Fly showing the mutant character "Curly." (The 

 wings curl up at the ends and are held somewhat apart.) D. Eye of female fly homo- 

 zygous for "Bar" (eye is bar-shaped). E. Fly showing the mutant character "Lobe." 

 (The eyes are small and protruding.) (Courtesy "The Theory of the Gene" by 

 Thomas Hunt Morgan, Nobel Laureate. Yale Univ. Press, 1926.) (From paper in 

 "Colloid Chemistry," Vol. 5, by J. Alexander, Reinhold Publishing Corp., N. Y.) 



When a sexual diploid is formed by the fusion of two individuals of 

 opposite sexes, it does not multiply as such, but immediately forms 

 haploid spores which are the equivalent of gametes. As Beadle 

 states: 1 ". . . all the products of the nuclear divisions which reduce the 

 chromosome number from the double number 14 to the single number 

 7 can be recovered in sexual spores and cultured. The result of this 

 is that the offspring of parents different by one gene will occur in a 

 mechanically fixed ratio of one-to-one. We can omit the phrase 'on 

 the average' which has to be applied to genetic ratios observed in an 

 organism like man." 



