3i8 CHROMOSOMES IN HEREDITY: PHYSIOLOGICAL 



It might be expected from these observations that the progeny 

 of triploids would for the most part receive from their parents 

 approximately ja;/2 chromosomes. 



Table 47 

 Distribution of Chromosomes at Meiosis in Triploids 



Nos. of chromosomes 



Hyacinthus (pollen 



grain) AT = 8. 



Belling, 1925 ; D. 



1926 . 

 Tradescantia (pollen 



grain) x = 6. 



D., 1929 c . 

 Petunia (second 



metaphase) x = j. 



Dermen, 1931 



16 



Note. — The slight excess of the numbers below ^xjz is due to loss of chromo- 

 somes at meiosis, cf. Levan, 1933 a, 1935 b. The chance is calculated as 10 

 per cent, in Hyacinthus (D., 1926), 11-5 per cent, in Narcissus (Nagao, 1935). 



The zygote progeny, however, do not correspond at all with the 

 gametic proportions. They show a different range according to 

 whether the triploid is the male or the female parent, or both, and 

 whether the opposite parent is diploid or tetraploid. In all these 

 types of cross there is elimination of gametes and zygotes having 

 extra chromosomes. In consequence triploids show a sterility 

 which increases with the number of chromosomes in the haploid set, 

 other things being equal. This elimination depends, first, on a 

 failure of growth of abnormal pollen grains, which is sometimes 

 complete and sometimes merely a failure to compete with normal 

 ones — or with better suited ones, since the diploid pollen of a 

 triploid (Enothera grows better on the tetraploid stigma and the 

 haploid pollen on the diploid stigma (Table 49). In two cases in 

 Solanum only the diploid gametes of triploids have functioned on 

 selfing (Huskins, 1934 ; Janaki Ammal, 1934). Secondly, it depends 

 on competition between zygotes in the embryonic stage ; this 

 applies equally to the seed of Solanum and the larva of Drosophila. 



