10 Mutations and Evolution. 



mutations. The type of mutation involving an extra chromosome 

 differs from this in the following points. (1) There are present 

 three representatives of the extra chromosome instead of a pair. 

 A Mendelian mutation, on the other hand, will first appear usually 

 as a heterozygous individual if the new character is dominant, or as 

 an individual homozygous for the new character if it is recessive : 

 that is, a pair of chromosomes only will be involved, one or both of 

 which carry the new factor. (2) As we shall see later, the presence 

 of the extra chromosome determines, at least in large measure, the 

 hereditary behaviour of the form possessing it. (3) Chromosome 

 duplication, viewed as a germinal change, arises through, or at 

 least in connection with, a rearrangement of the chromatic material, 

 resulting from an irregular distribution or non-disjunction of a 

 pair of chromosomes in the heterotype mitosis, while when a 

 Mendelian character arises one must assume it is by an alteration 

 in one element of a chromosome. 



Thus while the bearing of the extra chromosome in (Enothera 

 lata upon the mutation concept is clear in that, as we have already 

 pointed out, there is a constant relation between the lata characters 

 and the additional chromosome, yet the relation of this type to the 

 numerous other forms now known to have 15 chromosomes is by 

 no means so clear, and raises a number of intricate problems. 

 These forms and the problems connected with them will be 

 discussed later. For the moment we are concerned in pointing out 

 some of the other bases of present concepts regarding mutations. 



The gigas and semigigas series of forms, having respectively 

 28 and 21 chromosomes, again clearly justify the concept of a 

 mutation as a cell change involving a new nuclear structure 

 perpetuated by mitosis. This type of change is now known, 

 through parallel mutations, in several species of (Enothera : but 

 tetraploidy exists in a wide range of wild species, and a doubling 

 of the chromosome series is also known to occur under various 

 experimental conditions which will be referred to later. The 

 forms, such as lata and semigigas, with an unbalanced chromosome 

 number, will seldom if ever breed true, and such forms can only 

 occupy a somewhat temporary and aberrant place in evolutionary 

 descent. But tetraploidy is a condition of evolutionary significance, 

 as the cytological study of many genera and families already shows. 

 It gave the first hints towards the construction of a phytogeny of 

 the chromosomes, and ultimately it is quite certain that such a 

 phytogeny must be constructed. 



