432 SECTIONAL TRANSACTIONS.— D. 



In addition there are genes which govern the presence or absence of antho- 

 cyanins, anthoxanthins or plastid pigments, and genes which produce a 

 general or local intensification or dilution of colour. 



The biochemical value of this work lies in the fact that single genes have 

 been shown to control simple chemical reactions, such as oxidation resulting 

 in the introduction of a hydroxyl group, and glucoside formation. In sweet 

 peas two whites crossed together may give coloured progeny, each parent 

 introducing a different gene necessary for anthocyanin formation. Similarly 

 in maize, thirteen dominant genes are necessary for the production of 

 chlorophyll. It seems then that each stage in the synthesis of any plant 

 product is governed by a single gene, and the geneticist is therefore able to 

 separate the metabolic processes into their different stages. This should 

 make it possible for the biochemist to find out what the reactions are and 

 how genes bring them about. 



The second part of the exhibit is designed to show that the differences in 

 flower colour are controlled by single genes which conform to the Mendelian 

 laws of inheritance. Examples are given of the inheritance of alternative 

 pairs of gene-controlled characters : 



1(a) Anthocyanin — no anthocyanin. 

 (b) Anthoxanthin copigment — no copigment. 

 (c) Delphinidin (oxidation) — cyanidin (no oxidation). 

 (d) Higher pH — lower pH. 



I : I ratio 1 



VI Me) Diglycoside — monoglycoside. 



cross .1 



These five show complete dominance of the first character, that is, the 

 heterozygote is indistinguishable from the homozygous dominant — one 

 gene is doing the work of two. Incomplete dominance is shown in the cross 

 red X white Antirrhinum ; the first generation is intermediate (pink), 

 and in the second generation a ratio i red : 2 pinks : i white is obtained. 

 The red and white breed true, but the pinks always throw red, pink and 

 white. 



In the cross purple X scarlet Verbena independent segregation of two 

 pairs of characters is shown, with recombination in the second generation 

 resulting in the production of two new colour types. For example, pelar- 

 gonidin 3 : 5 -diglycoside is produced by bringing together the gene for 

 diglycoside carried by one plant with that for pelargonidin from the other 

 parent. 



In a similar manner it is possible to obtain an anthocyanin different from 

 that of either of the parents. The salmon Streptocarpus carries a gene for 

 methylation, but this gene has no effect when the anthocyanin is derived 

 from pelargonidin. On crossing with a variety containing cyanidin from 

 which the methylation gene is absent, the flowers of the progeny contain 

 a methylated cyanidin derivative. 



The segregation of three pairs of factors is shown in the second generation 

 from the cross blue X salmon Streptocarpus. Of the three genes involved 



M produces malvidin. 

 P produces peonidin. 

 I produces copigmented flavone. 



