Several autosomal mutations had been found in which the facets of the compound eye are disarranged. One that was sex-linked appeared in February 1914. Under the low power of the binocular microscope the facets are seen to be irregular in arrangement, instead of being arranged in a strictly regular pattern. The ommatidia are more nearly circular than hexagonal in outline, and are variable in size, some being considerably larger than normal. The large ones are also darker than
the smaller, giving a blotched appearance to the eye. The short hairs between the facets point in all directions instead of radially, as in the normal eye. The irregular reflection breaks up the dark fleck which is characteristic of the normal eye. The shape of the eye differs somewhat from the normal; it is more convex, smaller, and is encircled by a narrow rim destitute of ommatidia.
Facet arose in a back-cross to test the independence of speck (second chromosome) and maroon (third chromosome). One of the cultures produced, among the first males to hatch, some males which showed the facet disarrangement. None of the females showed this character. The complete output was that typical of a female heterozygous for a recessive sex-linked character: not-facet ♀ ♀ (2), 112; not-facet ♂ ♂ (1), 57; facet ♂ ♂ (1), 51.
Of the three characters which were shown by the F2 males, one, facet, is sex-linked, another, speck, is in the second chromosome, and maroon is in the third chromosome. All eight F2 classes are therefore expected to be equal in size, and each pair of characters should show free assortment, that is, 50 per cent. The assortment value for facet speck is 48, for speck maroon 52, and for facet maroon 48, as calculated from the F2 males of table 58.
Table 58.—P1 speck maroon ♂ × wild ♀ ♀. B.C. F1 wild-type ♀ × speck maroon ♂.
| Reference. | F2 females. | F2 males. | ||||||||||
| Speck maroon. | Wild- type. | Speck. | Maroon. | Facet. | Speck maroon. | Facet speck maroon. | Wild- type. | Facet maroon. | Speck. | Facet speck. | Maroon. | |
| 66 | 31 | 30 | 26 | 25 | 14 | 14 | 14 | 10 | 11 | 17 | 12 | 17 |
LINKAGE OF FACET, VERMILION AND SABLE.
In order to determine the location of facet in the first chromosome, one of the facet males which appeared in culture 66 was crossed out to vermilion sable females. Three of the wild-type daughters were back-crossed to vermilion sable males. The females of the next generation should give data upon the linkage of vermilion and sable, while the males should show the linkage of all three gens, facet, vermilion, and sable. The offspring of these three females are classified in table 59.
The cross-over fraction for vermilion sable as calculated from the females is 19/194. The cross-over value corresponding to this fraction is 10 units, which was the value found in the more extensive experiments given in the section on sable.
It will be noticed that the results in the males of culture 150 are markedly different from those of the other two pairs. While the sable males are fully represented, their opposite classes, the gray males, are