Abstract
A pair mating in Drosophila obscura (Fallén) produced a sex ratio of 106 females to 22 males. This was about a 1:4 ratio and indicated a case of two sex-linked lethals, or lethals at two loci on the X-chromosome. Both lethals (l1 and l3) appeared simultaneously in the same culture from a female whose mother did not carry a sex-linked lethal, as was shown by the normal sex ratio produced by her; and the father could not have carried such a lethal and lived.
Three daughters inherited both lethals in the same chromosome with a sex-linked gene producing the character “short” wing veins. These three females produced a total of 352 females to 40 normal and 50 “short” males. Such a count suggested that the gene for “short” was between the two lethals, which were far enough away from it for each to segregate almost independently from it, that is, giving approximately 50 per cent. crossing over between each lethal and “short.” This shows more crossing-over than is known to occur in the X-chromosome of other species of Drosophila and may correspond to the greater length of the X-chromosome in obscura.
Further breeding tests showed the presence of two separate sex-linked lethals and agreed with the original assumption as to their loci. One (I2) was found to be very close to the locus for “I beaded” and gave 10 cross-overs with it in 155 males. “Beaded” was already known to be far enough from short to show no appreciable linkage to it. The other lethal (I3) was independent of “beaded” but showed some linkage to “short” by a ratio of 21 “short” to 34 “not-short” males, while the number of “beaded” and “not-beaded” males was equal.
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