15S 



( ii \r i i i' II 



t 



TETRAPLOID AN 



#• ##? 



2N I 2N 2 



IGlobel 



4N I 4N 2 4N 3 



FIGUR] 11-8. /.//t'<7 upon f/ie capsule of 

 Datura of the presence of one or more extra 

 "Globe" chromosomes. 



In comparison, the tetraploid (4N) indi- 

 vidual is phenotypically almost like the dip- 

 loid, since chromosomal balance is undis- 

 turbed. The tetraploid which has one extra 

 Globe chromosome (4N -+- 1, making it a 

 pentasomic tetraploid) deviates from the 

 tetraploid in the same direction as the 

 2N -f- 1 deviates from 2N, but does so less 

 extremely. Hexasomic tetraploids (4N + 2) 

 de\ iate from 4N just about as much as 

 2N + 1 deviates from 2N. It is clear, 

 therefore, that adding a single chromosome 

 to a tetraploid has less phenotypic effect 

 than its addition to a diploid, since the shift 

 in balance between chromosomes is relatively 

 smaller in the former than in the latter. 

 Thus, polyploids can stand whole chromo- 

 some additions and subtractions better than 

 diploids can. 



Since crosses between tetraploid Datura 

 produce fertile seed in amounts sufficient to 

 maintain a tetraploid race, the question 

 arises, can a tetraploid race of Drosophila 

 be produced? As mentioned, the gametes 

 of a tetraploid Drosophila female contain 

 complete genomes more often than do those 

 of triploids. Since it produces many diploid 

 eggs, the tetraploid female presents no diffi- 

 culty for the continuity of a tetraploid race. 



To be o\' normal sex, a tetraploid male has 

 to carry 2X + 2Y + 4 sets of A (Chapter 

 8). But the X's (and Y's) in such a male 

 usually synapse with each other during meio- 

 sis so that after meiosis each sperm carries 

 IX and I Y in addition to the 2A sets. In 

 fertilizing eggs (from tetraploid females) 

 containing 2X + 2 sets of A. sperm of this 

 type produce zygotes with 3X -f- 1Y + 4 

 sets of A which develop as sterile inter- 

 sexes. Thus, a self-maintaining tetraploid 

 race of Drosophila cannot be established. 

 In fact, we can conclude that any species 

 containing a heteromorphic pair of sex chro- 

 mosomes (such as X and Y) cannot form 

 polyploid races, since the correct balance 

 between sex chromosomes and autosomes is 

 upset by the meiotic divisions. This factor 

 probably explains why polyploid races and 

 species are rarer among animals than among 

 plants whose sexuality (as in monoecious 

 forms) is not associated with heteromorphic 

 homologs. 



3. /// Man 



Down's syndrome, or mongolism, in hu- 

 man beings is sometimes the result of a 

 trisomic diploid chromosomal constitution. 

 In this case, the trisomic is number 21 the 

 third smallest of all human chromosomes 

 (the smallest being the Y) (Figures 11-9, 

 1 1-10). Trisomies for several other of the 

 smaller autosomes are also known, each pro- 

 ducing its own characteristic set of congenital 

 abnormalities. Trisomy for the largest auto- 

 somes is apparently lethal before birth, prob- 

 ably due to the imbalance of too many 

 genes. The very severe phenotypic defects 

 observed among the least affected auto- 

 somals trisomic individuals makes it a rea- 

 sonable expectation that the monosomic 

 condition of any autosome is lethal before 

 birth — in accordance with the view that 

 chromosome subtraction is even more detri- 

 mental than chromosome addition. 



