114 Outline of Genetics 



appearance of an extra piece of chromosome which dupHcates in 

 its gene content a known region of one of the normal chromosomes. 

 An organism in this condition is really triploid with respect to a 

 part of one of the chromosome sets. 



2. Non-disjunction. — ^This phenomenon, made fa- 

 mous through the classic work of Bridges on the sex 

 chromosomes of the fruit fly (chap, xiii), may prove to 

 be a fairly common occurrence. In an irregular reduc- 

 tion division, one of the chromosomes fails to '' disjoin" 

 properly from its mate. As a result, one or two gametes 

 are formed with an extra chromosome, and others which 

 lack this chromosome. The latter fail to function, but 

 a mating of the former with a normal gamete would 

 produce a zygote with an extra chromosome. Blakes- 

 LEE, Belling, and Farnham (2) have discovered this 

 phenomenon in the jimson weed. Datura. The normal 

 diploid number of chromosomes in this form is twenty- 

 four. Twelve different '^ mutants" have been discovered 

 with twenty-five chromosomes each. This seems to 

 indicate that each of the twelve chromosomes (haploid) 

 has failed to disjoin at least once in history. These 

 twelve new forms are abnormal in their vegetative fea- 

 tures, notably low in fertility, and tend to revert to 

 the normal diploid ancestor. 



3. Tetraploidy. — A hurried or incomplete mitosis 

 will sometimes result in the simultaneous duplication of 

 all of the chromosomes. This phenomenon has been 

 observed several times in culture, and there are indica- 

 tions that it has taken place frequently in the past. 

 A general survey of the chromosome counts in our exist- 

 ing plants and animals emphasizes the fact that the 

 haploid number is much more frequently an even number 



