Monosomies 415 



gametophytes usually do not survive; animals homozygous for a 

 deficient whole chromosome do not survive. 



Probably the best-known monosomic is the haplo-IV type of 

 Droso'phila melanogaster. This type is deficient one of the mem- 

 bers of the very small pair of chromosomes known as chromo- 

 some IV. Since this chromosome is exceedingly small, and since 

 it is no longer than some of the missing segments in flies deficient 

 for parts of the three other chromosomes, haplo-IV types survive. 

 Since these diploid organisms do not have two complete genomes, 

 they are not so robust or so healthy as their normal brothers and 

 sisters and they are a little slower to develop. They also have 

 bristles which are shorter and eyes which are more roughened 

 than those of normal, wild-type flies. 



We pointed out in the last chapter that homozygous deficien- 

 cies produced a lethal effect except rarely where the missing seg- 

 ment was very small. Since the fourth chromosome, small as it 

 is, greatly exceeds such a segment in length, no flies have been 

 found which have no fourth chromosome. Similarly, large dele- 

 tions may be lethal even if heterozygous. Probably for that 

 reason no flies have been found which are haplo-II or haplo-III. 



In diploid plants large deletions usually do not survive the 

 gametophyte stage. Similarly, in normal, diploid plants, a miss- 

 ing chromosome is lethal in the gametophyte generation. In 

 polyploids, however, where every chromosome is represented more 

 than twice in the sporophyte and where every chromosomal seg- 

 ment may be present more than once in the gametophyte genera- 

 tion, the loss of one chromosome need not result in the absence 

 of a complete genome. In such plants, monosomic types may 

 exist but, strictly speaking, they are not 2n — 1 but 4n — 1. In 

 fact, Lammerts has pointed out that the discovery of monosomies 

 in a diploid species would be evidence of its polyploid nature. 



Chromosome pairing in a monosomic is just what would be 

 expected from the rule of strict part-by-part pairing at zygo- 

 tene. If one whole chromosome is missing, the homologue has 

 nothing with which to pair, and therefore behaves as a univalent 

 chromosome. Although all the other chromosomes may line up 

 in pairs at first metaphase, the univalent may be found on the 

 equator by itself. At anaphase, then, it may pass apparently at 

 random to either pole, giving theoretically gametes with n and 

 those with n — 1 chromosomes in equal numbers. Such univa- 



