Mutational Mosaic Coat Patterns of the Guinea Pig.
TABLE OF CONTENTS PAGE INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 The Color Factors of Guinea Pigs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334 A Somatic and Germinal Mosaic (Ccd+Cdcd). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336 Two Intense-dilute Mosaics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342 An Agouti-black Mosaic ( A A +aa) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344 Two Intense-dilute Mosaics ( F f + j ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 An Intense Brown-pale Brown Mosaic (Pp+pp). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347 DISCUSSIOV.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348 LITERATURE CITED.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351
Threshold phenomena versus cell heredity in the manifestation of sex-linked genes in mammals.
The mode of action of sex-linked genes in mammals has been the subject of much discussion in recent years. This has centred round the concept that in the mammalian female, either the maternal or the paternal X-chromosome is randomly and irreversibly inactivated during development, with the result that in the female, as in the male, one X-chromosome only is active in a given cell (Lyon, 1966, and earlier papers). The Lyon Hypothesis (l.h.) is based, in part, on the mottled or striped phenotypes of heterozygotes for certain sex-linked genes affecting the coat of the mouse and other mammals: these phenotypes are regarded as a mixture of clones descended from cells in which either one or the other X-chromosome has been inactivated. As Baker (1967) rightly insists, the interpretation of a pattern as clonal must be based on known cell lineages, as without such a basis one simply postulates what should be put to the test.
A Dwarf Mutation in Portulaca Showing Vegetative Reversions.
I n 1914 Mr. B. T. AVERY, who was then my gardener assistant a t the CONNECTICUT AGRICULTURAL COLLEGE, called my attention to a single dwarf individual in a bed of red-flowered plants from commercial seed of a variety of Portulaca grandijora. This plant formed a low compact pin-cushion-like growth in striking contrast to the long straggling habit of the normal plants near it. It is the origin of a series of dwarfs which we now have in a variety of colors, both singles and doubles. Commercial florists are on the lookout for dwarf plants for bedding purposes and such a cushion form as ours would likely have been noticed and propagated commercially by trained florists. So far as we have been able to ascertain, however, no drawf form of Portulaca is known to the trade. I t is not unlikely, therefore, that the plant discovered by us was the first of its kind to be observed in this species.