Time of Death of Lethal Homozygotes in the T (Brachyury) Series of the Mouse. ∗

Abstract

Dunn 1 reported the third lethal that was found in the T (Brachy) series in the house mouse. This lethal (t ¹) was determined by Dunn in a tailless line, which had been isolated by Dobrovolskaia-Zavadskaia and Kobozieff. 2 Tailless mice of this line (29) are Tt ¹. Results of matings of tailless mice of line 29 and tests of their descendants showed that both homozygous combinations TT and t ¹ t ¹ are lethal, but the combination Tt ¹ is viable.

In order to learn more about the lethal embryos t ¹ t ¹, uteri of pregnant females were examined in different stages. The females used were all heterozygous normal-tailed mice (+t ¹) and they were mated to heterozygous normal-tailed males (+t ¹).

Table I shows the results of dissections of pregnant females at the age of 7, 8, 9, 10, and 11 days after fertilization. A total of 40 litters were dissected out, yielding 294 embryos. Of these, 275 were normal and 19 abnormal. Of the abnormal embryos, 13 were resorbed and 6 showed different kinds of abnormalities, but were not resorbed. If the t ¹ t ¹ homozygotes died after implantation, 25% of the implanted embryos would be expected to be of the lethal type. Since the number of resorbed embryos found at dissection is far below expectation, it has to be assumed that death of the homozygous lethal takes place before implantation, and that the death of the resorbed and abnormal embryos found was due to accidental causes. It is highly improbable that death of the t ¹ t ¹ takes place after the 11th day. All embryos examined at 11 days were entirely normal and no indications were found for later death of t ¹ t ¹.

The litter size in the matings is another factor indicating that death of t ¹ t ¹ occurs before implantation.