Inheritance of dominant genes with variable penetrance: An evolutionary aspect

Developmental mosaicism may explain spontaneous reappearance of the Axin(Fu) mutation in mice

The Axin(Fu) mutation is caused by an IAP insertion. In a small percentage of mice, the Axin(Fu) mutation disappears and is not observed in the next generation. In these mice, [Axin(Fu)]/+, the IAP is absent and Axin(Fu) has reverted to the wild allele. Concomitantly with the loss of IAP, there is widespread reorganisation of numerous microsatellite loci across the surrounding region of chromosome. In rare cases, spontaneous reappearance of the mutation can be observed in progeny of [Axin(Fu)]/+ mice. It is revealed here that reappearance of Axin(Fu) was associated with restoration of the IAP insertion. In such mice, alleles of the surrounding microsatellite loci were identical to the alleles observed on chromosomes that carried Axin(Fu). Developmental mosaicism can potentially explain spontaneous reappearance of the Axin(Fu) mutation. Mosaicism can also explain other observations including postnatal changes at the Axin locus, unusual segregation in progeny, and the appearance of [Axin(Fu)]/+ mice that were phenotypically mutant. 2001.

Gametic imprinting and the manifestation of the fused gene in the house mouse

The phenomenon of gametic imprinting in mammals has raised developmentally relevant questions concerning the manifestation and inheritance of genes with variable penetrance. The dominant fused (Fu) gene located on chromosome 17 is one of the few good cases demonstrating the phenomenon in mice. The Fu mutation has a maternal effect. We have previously shown that the t12 haplotype significantly lowers the penetrance of Fu in female Fu/t12 offspring. Results of reciprocal matings of the heterozygotes for Fu indicated that the Fu of maternal origin has a lowered level of penetrance. The dominant suppressors located outside chromosome 17, in contrast to t12 residing in it, had stronger effects on the manifestation of Fu, decreasing its penetrance to 8-17%. Experimental evidence is presented that the pathway via which Fu passes to the zygote nucleus during gametogenesis through successive generations has a marked effect on its penetrance. Based on this evidence, patterns of genetic imprinting are described. A survey of genetic imprinting allowed us to distinguish two developmental phases, gametic and zygotic. The hypothesis for the gametic phase of the development of multicellular organisms suggests that it proceeds from initialization, a process thought to ensure the freeing of chromosomes from redundant epigenetic information and their preparation for the consecutive developmental cycle.

Paternal inheritance of egg traits in mice: a case of genomic imprinting

Eggs from reciprocal hybrids between the C57BL/6By and BALB/cBy strains were tested for their susceptibility to attack by hyaluronidase and pronase. There were significant reciprocal differences between the F1 females in the responses of their unfertilized eggs to both enzymes. The F1 hybrids from BALB mothers showed the increased susceptibility characteristic of C57BL whilst the F1 hybrids with C57BL mothers were more resistant to both enzymes, like BALB mice. Eggs from the four kinds of reciprocal F2 hybrid females also showed patroclinous patterns of susceptibility. A patroclinous difference was found between reciprocal crosses of the CXBD and CXBE recombinant inbred strains but not in crosses between recombinant inbred strains with similar phenotypes. Cross fostering did not alter the phenotypes of the C57BL and BALB females or those of their reciprocal F1 hybrids. The findings are interpreted in terms of differential genomic imprinting of paternally inherited information. The possible general usefulness of patroclinous differences between reciprocal F1 females in revealing differences in imprinting is noted.