XY C57BL/6J (B6) mice harboring a Mus musculus domesticus-type Y chromosome (Y ^POS ), known as B6.Y ^POS mice, commonly undergo gonadal sex reversal and develop as phenotypic females. In a minority of cases, B6.Y ^POS males are identified and a proportion of these are fertile. This phenotypic variability on a congenic B6 background has puzzled geneticists for decades. Recently, a B6.Y ^POS colony was shown to carry a non-B6-derived region of chromosome 11 that protected against B6.Y ^POS sex reversal. Here. we show that a B6.Y ^POS colony bred and archived at the MRC Harwell Institute lacks the chromosome 11 modifier but instead harbors an ∼37 Mb region containing non-B6-derived segments on chromosome 13. This region, which we call Mod13, protects against B6.Y ^POS sex reversal in a proportion of heterozygous animals through its positive and negative effects on gene expression during primary sex determination. We discuss Mod13's influence on the testis determination process and its possible origin in light of sequence similarities to that region in other mouse genomes. Our data reveal that the B6.Y ^POS sex reversal phenomenon is genetically complex and the explanation of observed phenotypic variability is likely dependent on the breeding history of any local colony.

中文翻译：

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小鼠 13 号染色体上的变异区域可防止 XY 性腺性别逆转。

携带家鼠型 Y 染色体 (Y ^POS ) 的 XY C57BL/6J (B6) 小鼠（称为 B6.Y ^POS小鼠）通常会发生性腺性别逆转并发育为表型雌性。在少数情况下，B6.Y ^POS男性被识别出来，其中一部分具有生育能力。这种同源 B6 背景上的表型变异一直困扰着遗传学家数十年。最近，B6.Y ^POS菌落被证明携带 11 号染色体的非 B6 衍生区域，可防止 B6.Y ^POS性逆转。这里。我们表明，在 MRC Harwell 研究所培育和存档的 B6.Y ^POS菌落缺乏 11 号染色体修饰符，而是在 13 号染色体上包含一个 ∼37 Mb 的区域，其中包含非 B6 衍生片段。该区域（我们称为 Mod13 ）保护通过其在初级性别决定过程中对基因表达的积极和消极影响，对抗部分杂合动物中B6.Y ^{POS性别逆转。}我们根据与其他小鼠基因组中该区域的序列相似性讨论了Mod13对睾丸确定过程的影响及其可能的起源。我们的数据表明，B6.Y ^POS性别逆转现象在遗传上很复杂，观察到的表型变异的解释可能取决于任何当地群体的繁殖历史。