Gaping lids, gp, a mutation on centromeric Chromosome 11 that causes defective eyelid development in mice

In mammals, during fetal development, the eyelids grow and flatten over the eyes and temporarily fuse closed. Failure of this normal developmental process in mice leads to the defect, open-eyelids-at-birth. Nearly all newborns of the GP/Bc strain, homozygous for the spontaneous recessive mutation, gaping lids (gp), have bilateral open eyelids at birth, with essentially no fusion between the upper and lower eyelids. Histological sections and scanning electron microscopy of GP/Bc eyes during the normal period of eyelid growth and fusion indicate that gp/gp mutant fetuses have deficient upper and lower eyelids; surface periderm cells that appear to have some role in eyelid growth and fusion are present, but lack a normal ``streaming' pattern toward the fusion zone. No other defects due to the gaping lids mutation were detected. A genetic analysis based on outcrosses of GP/Bc to various linkage marker stocks and to CBA/J and ICR/Bc normal strains was done. Penetrance in F₂ segregants, but not in BC1 segregants, was usually significantly less than 100%, was strongly affected by the identity of the normal strain used, ranging from 44% to 92%, and indicated a potential complexity of modifiers. Forty-one affected F₂ and 120 BC₁ segregants from the outcross of GP/Bc to CBA/J, and 23 affected F₂ segregants from the outcross to ICR/Bc, were used to map gp to proximal Chr 11 between the centromere and D11Dal1 (Camk2b), an interval previously defined as less than 1 cM. Sets of whole F₂ litters from the crosses to CBA/J (n = 106) and ICR/Bc (n = 65) strains were typed for informative SSLPs near gp (D11Mit62 and D11Mit74, respectively) and demonstrated that the segregation ratios in the region are Mendelian. The known genes in the interval, Nf2 and Lif, do not seem to be obvious candidate genes for gp. An Egfr-null allele was used to confirm the previously reported map position of the potential candidate locus, Egfr, to a more distal interval, between D11Mit62/226 and D11Mit151, from which gp had been excluded. Tests for allelism showed that the Egfr mutation and the gp mutation complement each other, and therefore also indicate that they are at different gene loci. Open-eyelids-at-birth is associated with several mutations at other loci with variable penetrance owing to modifiers and in other more complex genetic liabilities in inbred strains, and the genetics of this trait is a model for other genetically complex developmental threshold traits. The gaping lids mutation identifies a previously unknown locus on proximal Chromosome (Chr) 11 that has a strong role in fetal eyelid growth. Received: 13 January 2000 / Accepted: 23 February 2000     

Strain distribution pattern in AXB and BXA recombinant inbred strains for loci on murine Chromosomes 10, 13, 17, and 18

Strain distribution patterns (SDPs) of selected loci previously mapped to murine Chromosomes (Chrs) 10, 13, 17, and 18 are reported for the AXB, BXA recombinant inbred (RI) strain set derived from the progenitor strains A/J (A) and C57BL/6J (B). The loci included the simple sequence length polymorphisms (D10Nds1, D10Mit2, D10Mit10, D10Mit14, D13Mit3, D13Nds1, D13Mit10, D13Mit13, D13Mit7, D13Mit11, D17Mit18, D17Mit10, D17Mit20, D17Mit3, D17Mit2, D18Mit17, D18Mit9, and D18Mit4), the restriction fragment length polymorphisms Pdea and Csfmr, and the biochemical marker AS-1. These loci were chosen because they map to genomic regions that had few or no genetic markers in the AXB, BXA RI set. Several of these loci also were typed in backcross progeny of matings of the (AXB)F₁ to strain A or B. The strain distribution patterns for chromosomes 10, 13, 17, and 18 are reported, and the gene order and map distances determined from the backcross data. The addition of these markers to the AXB, BXA RI strain set increases the genomic region over which linkage for new markers can be detected.     

An interstitial telomere array proximal to the distal telomere of mouse chromosome 13

Lambda clones of mouse DNA from BALB/c and C57BL/10, each containing an array of telomere hexamers, were localized by FISH to a region close to the telomere of Chr 13. Amplification of mouse genomic DNA with primers flanking SSRs within the cloned DNA showed several alleles, which were used to type eight sets of RI strains. The two lambda clones contained allelic versions of the interstitial telomere array, Tel-rs4, which is 495 bp in C57BL/10 and which includes a variety of sequence changes from the consensus telomere hexamer. Comparison of the segregation of the amplification products of the SSRs with the segregation of other loci in an interspecies backcross (C57BL/6JEi × SPRET/Ei) F₁× SPRET/Ei shows recombination suppression, possibly associated with ribosomal DNA sequences present on distal Chr 13 in Mus spretus, when compared with recombination in an interstrain backcross, (C57BL/6J × DBA/J) F₁× C57BL/6J, and with the MIT F₂ intercross. Analysis of recombination in females using a second interstrain backcross, (ICR/Ha × C57BL/6Ha) F₁× C57BL/6Ha, also indicates recombination suppression when compared with recombination in males of the same strains, using backcross C57BL/6Ha × (ICR/Ha × C57BL/6Ha) F₁. Thus, more than one cause may contribute to recombination suppression in this region. The combined order of the loci typed was D13Mit37–D13Mit30–D13Mit148–(D13Rp1, 2, 3, 4, Tel-rs4)–D13Mit53–D13Mit196–D13Mit77–(D13Mit78, 35). Data from crosses where apparently normal frequencies of recombination occur suggest that the telomere array is about 6 map units proximal to the most distal loci on Chr 13. This distance is consistent with evidence from markers identified in two YAC clones obtained from the region. Received: 24 September 1996/Accepted: 20 January 1997     

Multifactorial genetics of exencephaly in SELH/Bc mice

BACKGROUND: The SELH/Bc mouse strain has 10-30% exencephaly and is an animal model for human neural tube closure defects. This study examined the number of causative genes, their dominance relationships, and linkage map positions. METHODS: The SELH/Bc strain (S) was crossed to the normal LM/Bc strain (L) and frequencies of exencephaly were observed in the F(1), BC(1), and F(2) generations. 102 F(2) males were individually testcrossed by SELH/Bc. The extremes, the 10 highest and 10 zero exencephaly-producing F(2) sires, were typed for 109 SSLP marker loci in a genome screen. Next, the resultant five provisional chromosomal regions were tested for linkage in 31 F(2) exencephalic embryos. Finally, 12 males, SS or LL for the Chr 13 region on an LM/Bc background, were testcrossed by SELH/Bc. RESULTS: The exencephaly frequencies in the F(1) (0.3%), BC(1) (4.4%), and F(2) (3.7%), and the distribution of F(2) males' testcross values (0-15.5%), indicated that the high risk of exencephaly in SELH/Bc is due to the cumulative effect of two or three loci. Linkage studies indicated the location of semidominant exencephaly-risk genes on Chr 13 near D13Mit13 (P < 0.001), Chr 5 near D5Mit168 (P < 0.025), and possibly Chr 11 near D11Mit10 (P < 0.07). The gene on Chr 13, Exen1, and the strong role of other loci were confirmed by the congenic males. CONCLUSIONS: The high risk of exencephaly in SELH/Bc mice is caused by the cumulative effect of two to three semidominant genes. Candidate genes include Msx2, Madh5, Ptch, and Irx1 (Chr 13) and Actb and Rac1 (Chr 5).     

Mapping of simple sequence repeat (SSR) DNA markers in diploid and tetraploid alfalfa

Cultivated alfalfa (Medicago sativa) is an autotetraploid. However, all three existing alfalfa genetic maps resulted from crosses of diploid alfalfa. The current study was undertaken to evaluate the use of Simple Sequence Repeat (SSR) DNA markers for mapping in diploid and tetraploid alfalfa. Ten SSR markers were incorporated into an existing F₂ diploid alfalfa RFLP map and also mapped in an F₂ tetraploid population. The tetraploid population had two to four alleles in each of the loci examined. The segregation of these alleles in the tetraploid mapping population generally was clear and easy to interpret. Because of the complexity of tetrasomic linkage analysis and a lack of computer software to accommodate it, linkage relationships at the tetraploid level were determined using a single-dose allele (SDA) analysis, where the presence or absence of each allele was scored independently of the other alleles at the same locus. The SDA diploid map was also constructed to compare mapping using SDA to the standard co-dominant method. Linkage groups were generally conserved among the tetraploid and the two diploid linkage maps, except for segments where severe segregation distortion was present. Segregation distortion, which was present in both tetraploid and diploid populations, probably resulted from inbreeding depression. The ease of analysis together with the abundance of SSR loci in the alfalfa genome indicated that SSR markers should be a useful tool for mapping tetraploid alfalfa. Received: 10 September 1999 / Accepted: 11 November 1999     

RFLP mapping of the genes controlling hybrid breakdown in rice (Oryza sativa L.)

 Complementary recessive genes hwd1 and hwd2 controlling hybrid breakdown (weakness of F₂ and later generations) were mapped in rice using RFLP markers. These genes produce a plant that is shorter and has fewer tillers than normal plants when the two loci have only one or no dominant allele at both loci. A cultivar with two dominant alleles at the hwd1 locus and a cultivar with two dominant alleles at the hwd2 locus were crossed with a double recessive tester line. Linkage analysis was carried out for each gene independently in two F₂ populations derived from these crosses. hwd1 was mapped on the distal region of rice genetic linkage map for chromosome 10, flanked by RFLP markers C701 and R2309 at a distance of 0.9 centiMorgans (cM) and 0.6 cM, respectively. hwd2 was mapped in the central region of rice genetic linkage map for chromosome 7, tightly linked with 4 RFLP markers without detectable recombination. The usefulness of RFLP mapping and map information for the genes controlling reproductive barriers are discussed in the context of breeding using diverse rice germplasm, especially gene introduction by marker-aided selection.     

TheCmv1Host Resistance Locus Is Closely Linked to theLy49Multigene Family within the Natural Killer Cell Gene Complex on Mouse Chromosome 6

Natural killer (NK) cells play important roles in controlling tumor cells and against a range of infectious organisms. Recent studies of mouse NK cell surface receptors, which may be involved in the specificity of NK cells, have shown that many of these molecules are encoded by theLy49andLy55(Nkrp1) multigene families that map to distal mouse chromosome 6. Also mapping to this NK cell gene complex (NKC) is the resistance locus,Cmv1,which is involved in genetically determined resistance to murine cytomegalovirus (MCMV). The aim of this study was to localizeCmv1more precisely in relation to other NKC loci by generating a high-resolution genetic map of the region. We have analyzed 1250 backcross mice comprising panels of 700 (BALB/c × C57BL/6J)F₁× BALB/c and 550 (A/J × C57BL/6J)F₁× A/J progeny. A total of 25 polymorphic genes or microsatellite markers were analyzed over a region of 10 map units fromD6Mit134toD6Mit59.TheCmv1phenotypes of mice recombinant in this interval were tested by infection with MCMV. The results obtained indicate that the functionally important NKC region is a tightly linked cluster of loci spanning at least 0.4 map units. Furthermore,Cmv1maps distal to, but very closely linked to, theLy49multigene family (<0.2 map units), suggesting that MCMV resistance may be conferred by MHC class I-specific NK cell receptors.     

Refined genetic and comparative physical mapping of the canine copper toxicosis locus

Recently, the copper toxicosis (CT) locus in Bedlington terriers was assigned to canine chromosome region CFA10q26, which is homologous to human chromosome region HSA2p13-21. A comparative map between CFA10q21-26 and HSA2p13-21 was constructed by using genes already localized to HSA2p13-21. A high-resolution radiation map of CFA10q21-26 was constructed to facilitate positional cloning of the CT gene. For this map, seven Type I and eleven Type II markers were mapped. Using homozygosity mapping, the CT locus could be confined to a 42.3 cR₃₀₀₀ region, between the FH2523 and C10.602 markers. On the basis of a partial BAC contig, it was estimated that 1-cR₃₀₀₀ is equivalent to approximately 210 kb, implying that the CT candidate region is therefore estimated to be about 9 Mb. Received: 16 December 1999 / Accepted: 23 February 2000     

Genetics of the hemolymph esterases ofLucilia cuprina (Diptera: Calliphoridae)

When hemolymph from adults ofLucilia cuprina was partitioned on native polyacrylamide gels, nonspecific esterase staining demonstrated 10 bands with up to six bands in an individual. The bands derive from alleles at two loci,E _HA (five alleles) andE _HB (four alleles).E _HA is located on chromosome 4, 16.3 map units fromsv (singed vibrissae) and 22.1 map units fromra (radial vein gaps).E _HB is located on chromosome 5, 34.0 map units fromto ² (topaz² eyes) and 7.2 map units frommv (M1-veinless).     

A genomic clone containing a telomere array maps near the centromere of mouse Chromosome 6

A lambda clone of mouse DNA containing a short array of telomere hexamers has been localized by FISH to a region close to the centromere of Chromosome (Chr) 6. Amplification of DNA with primers flanking an SSR showed that most inbred strains carry one of two alleles, although five other alleles were found among the inbred strains and 11 other alleles were found in wild-derived mice. Analysis of the DNA from four Robertsonian translocations suggests that the amplified sequence is still present in these chromosomes. The finding of two fragments associated with the Sig mutant suggests that the clone lies within a congenic region created when the mutant, obtained in a (C3H x 101)F₁, was back-crossed to C57BL/6J. This region might include all or part of the centromere. Comparison of the segregation of the amplification product with the segregation of centromeric heterochromatin in an interspecies backcross, (C57BL/6 x M. spretus)F₁ x M. spretus, (BSS) shows 1/72 recombinants with the centromeric heterochromatin, while 1/62 recombinants occurred in a BSB backcross. Analysis of other loci at the proximal end of Chr 6 gives the combined map Hc6-0.73-D6Mit86-0.73-D6Rp2-2.2-D6Mitl-2.2-Wnt2-3.0-Cpa. Data from a third cross show that Cola2 lies between D6Mit82 and D6Rp2. The portion of the telomere array, Tel-rs3, that has been sequenced contains only 13/31 repeats of the consensus sequence. A variety of sequence changes from the consensus hexamer suggests that this array has been removed for a long time from evolutionary pressures to retain the TTAGGG sequence.     

Isolation and characterization of microsatellites in the Asian walking catfish Clarias macrocephalus

Microsatellite loci were characterized in the walking catfish, Clarias macrocephalus, random clones from a small genomic library using a (GT)₁₅ probe. Primers for DNA amplifications using polymerase chain reaction (PCR) were designed and synthesized for 23 loci. Twelve loci were polymorphic, with the number of alleles ranging from two to 13 alleles per locus. Developed microsatellite primers should prove useful for population studies and genetic mapping of the walking catfish.     

A detailed linkage map of subtelomeric murine Chromosome 12 region including the situs inversus mutation locus IV

A mouse model is an invaluable tool to tackle genesis of human congenital diseases that have so far eluded human studies. Homozygote for the iv mutation, the murine Si/Col strain presents a left-right lateralization defect of thoracic and abdominal organs and heart defects very similar to human ones. This iv mutation has been mapped to the region between the Aat and Igh-C loci, suggesting the presence of an equivalent human gene in the human syntenic 14q3 region. A precise linkage map of the region is, there-fore, of great interest since it will contribute to the genetic approach of the iv gene. Analysis of 242 backcross progeny from Mus musculus (MAI) or spretus strains of mice and SI/Col mice has allowed mapping of the iv gene to a linkage group of eight markers. It includes four genes: Aat (1-antitrypsin), Ckb (creatine kinase, brain form), Crip (cysteine-rich intestinal protein), and Igh-C (immunoglobulin heavy chain constant region complex); three murine microsatellites: D12Mit6, D12Mit7, and D12Mit8; and one new marker, D12Mtpl, defined by a minisatellite human probe, pYNZ2. After analysis of the data by the LINKAGE program, the following multilocus map has been constructed: centromere-D12Mit6-6.9 cM-D12Mit7-1.7 cM-D12Mtp1-2.6 cM-Aat-5.0 cM-(Ckb, Igh-C)-0.4 cM-D12Mit8-0.4 cM-Crip-11.2 cM-iv-telomere. This map differs from the previous map in placing iv locus telomeric to Igh-C. D12Mit6 and D12Mit7 are now precisely mapped centromeric to the locus Aat. In addition, a new locus D12Mtp1 is located between Aat and D12Mit7.     

Low resolution mapping around the flavivirus resistance locus (Flv) on mouse Chromosome 5

Although the phenomenon of innate resistance to flaviviruses in mice was recognized many years ago, it was only recently that the genetic locus (Flv) controlling this resistance was mapped to mouse Chromosome (Chr) 5. Here we report the fine mapping of the Flv locus, using 12 microsatellite markers which have recently been developed for mouse Chr 5. The new markers were genotyped in 325 backcross mice of both (C3H/HeJxC3H/ RV)F₁xC3H/HeJ and (BALB/cxC3H/RV)F₁xBALB/c backgrounds, relative to Flv. The composite genetic map that has been constructed identifies three novel microsatellite loci, D5Mit68, D5Mit159, and D5Mit242, tightly linked to the Flv locus. One of those loci, D5Mit159, showed no recombinations with Flv in any of the backcross mice analyzed, indicating tight linkage (<0.3 cM). The other two, D5Mit68 and D5Mit242, exhibited two and one recombinations with Flv (0.6 and 0.3 cM) respectively, defining the proximal and distal boundaries of a 0.9-cM segment around this locus. The proximal flanking marker, D5Mit68, maps to a segment on mouse Chr 5 homologous to human Chr 4. This, together with the previous data produced by our group, locates Flv to a region on mouse Chr 5 carrying segments that are conserved on either human Chr 4, 12, or 7, but present knowledge does not allow precise identification of the syntenic element.     

Integration of the feline radiation hybrid and linkage maps

The recent development of genome mapping resources for the domestic cat provides a unique opportunity to study comparative medicine in this companion animal which can inform and benefit both veterinary and human biomedical concerns. We describe here the integration and order comparison of the feline radiation hybrid (RH) map with the feline interspecies backcross (ISB) genetic linkage map, constructed by a backcross of F₁ hybrids between domestic cat (Felis catus) and the Asian leopard cat (Prionailurus bengalensis). Of 253 microsatellite loci mapped in the ISB, 176 equivalently spaced markers were ordered among a framework of 424 Type I coding markers in the RH map. The integration of the RH and ISB maps resolves the orientation of multiple linkage groups and singleton loci from the ISB genetic map. This integrated map provides the foundation for gene mapping assessments in the domestic cat and in related species of the Felidae family. Received: 10 July 2000 / Accepted: 01 February 2001     

Identification of a New Chemically Induced Allele (Lp) at the Loop-Tail Locus: Morphology, Histology, and Genetic Mapping

Loop-tail (Lp) is a semidominant mutation that affects neurulation in mice. Heterozygous animals are characterized by a looped-tail appearance (pig tail) and wobbly head movements while homozygous embryos exhibit a neural tube closure defect that extends from the caudal midbrain to the tip of the tail. The Lp gene has been finely mapped to the distal part of chromosome 1, and a positional cloning strategy has been initiated to isolate the defective gene. This study represents the characterization of a new Lp allele (Lp^m1Jus) induced by N-ethyl-N-nitrosurea mutagenesis. Lp^m1Jus/+ mice have a looped-tail appearance, and both Lp^m1Jus/Lp^m1Jus homozygotes and Lp/Lp^m1Jus compound heterozygotes fail to initiate neural tube closure along most of the embryonic axis. These data indicate that the Lp^m1Jus allele causes a neural tube defect and overall phenotype similar to that of the original Lp allele. Segregation analysis of 90 (Lp^m1Jus/+ × C57BL/6J)F₁ × C57BL/6J looped-tail mice with seven markers that define the Lp genetic map (D1Mit455/D1Mit146/D1Mit148/D1Mit270–1 cM–D1Mit113–0.4 cM–Lp–0.2 cM–D1Mit149–0.8 cM–D1Mit115) showed significant linkage between Lp^m1Jus and all loci analyzed (P < 0.0001). Eight crossovers were detected with the proximal cluster of D1Mit455, D1Mit146, D1Mit148, and D1Mit270, indicating a recombination rate higher than expected in this region, and a single recombinant was encountered with the distal markers D1Mit149 and D1Mit115. Based on these phenotypic and genetic data, Lp^m1Jus is most likely allelic to Lp, thereby representing a valuable additional tool for the positional cloning of the Lp gene and its subsequent molecular characterization.     

Physical mapping of genes on yeast mitochondrial DNA: Localization of antibiotic resistance loci, and rRNA and tRNA genes

Summary We have physically mapped the loci conferring resistance to antibiotics that inhibit mitochondrial protein synthesis (erythromycin, chloramphenicol and paromomycin) or respiration (oligomycin I and II), as well as the 21s and 14s rRNA and tRNA genes on the restriction map of the mitochondrial genome of the yeast Saccharomyces cerevisiae. The mitochondrial genes were localized by hybridization of labeled RNA probes to restriction fragments of grande (strain MH41-7B) mitochondrial DNA (mtDNA)¹ generated by endonucleases EcoRI, HpaI, BamHI, HindIII, SalI, PstI and HhaI. We have derived the HhaI restriction fragment map of MH41-7B mit DNA, to be added to our previously reported maps for the six other endonucleases.The antibiotic resistance loci (ant ^R) were mapped by hybridization of ³H-cRNA transcribed from single marker petite mtDNA's of low kinetic complexity to grande restriction fragments. We have chosen the single Sal I site as the origin of the circular physical map and have positioned the antibiotic loci as follows: C (99.5-1.Ou)-P(27-36.Ou)-O_II (58.3-62u)-O_I (80-84u)-E (94.4-98.4u). The 21s rRNA is localized at 94.4-99.2u, and the 14s rRNA is positioned between 36.2-39.8u. The two rRNA species are separated by 36% of the genome. Total mitochondrial tRNA labeled with ¹²⁵I hybridized primarily to two regions of the genome, at 99.5-11.5u and 34-44u. A third region of hybridization was occasionally detected at 70-76u, which probably corresponds to seryl and glutamyl tRNA genes, previously located to this region by petite deletion mapping.Supported by USPHS Training Grant T32-GM-07197.Supported by USPHS Training Grant 5-T01-GM-0090-19.The Franklin McLean Memorial Research Institute is operated by the University of Chicago for the U. S. Energy Research and Development Administration under Contract EY-76-C-02-0069.     

Mapping of the genes encoding tum- transplantation antigens P91A,P35B,and P198

Tum ^- comprises a class of genes, mutation of which in P815 tumor cells has led to the acquisition of new cytotoxic T cell-recognized epitopes. The cells carrying the mutant alleles have impaired tumorigenicity compared with their progenitors due to in vivo induction of a cytotoxic T-cell response specific for tum ^- antigens. Two tum ^- genes, P91A and P35B, were found to be single copy loci mapping to chromosomes 11 and 15 respectively. A third, P198, was found to map to chromosome 7 and to be a member of a small gene family with other members on chromosomes 13, 14, and 15. Multiple P198-related sequences were found in other mammalian species suggesting the P198 related gene family is a general feature of mammalian genomes.     

Identification and characterization of a novel powdery mildew resistance gene <Emphasis Type="Italic">PmG3M</Emphasis> derived from wild emmer wheat, <Emphasis Type="Italic">Triticum dicoccoides</Emphasis>

Powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt) is one of the most important wheat diseases worldwide. Wild emmer wheat, Triticum turgidum ssp. dicoccoides, the tetraploid ancestor (AABB) of domesticated bread and durum wheat, harbors many important alleles for resistance to various diseases, including powdery mildew. In the current study, two tetraploid wheat mapping populations, derived from a cross between durum wheat (cv. Langdon) and wild emmer wheat (accession G-305-3M), were used to identify and map a novel powdery mildew resistance gene. Wild emmer accession G-305-3M was resistant to all 47 Bgt isolates tested, from Israel and Switzerland. Segregation ratios of F₂ progenies and F₆ recombinant inbred line (RIL) mapping populations, in their reactions to inoculation with Bgt, revealed a Mendelian pattern (3:1 and 1:1, respectively), indicating the role of a single dominant gene derived from T. dicoccoides accession G-305-3M. This gene, temporarily designated PmG3M, was mapped on chromosome 6BL and physically assigned to chromosome deletion bin 6BL-0.70-1.00. The F₂ mapping population was used to construct a genetic map of the PmG3M gene region consisted of six simple sequence repeats (SSR), 11 resistance gene analog (RGA), and two target region amplification polymorphism (TRAP) markers. A second map, constructed based on the F₆ RIL population, using a set of skeleton SSR markers, confirmed the order of loci and distances obtained for the F₂ population. The discovery and mapping of this novel powdery mildew resistance gene emphasize the importance of the wild emmer wheat gene pool as a source for crop improvement.     

Unravelling the complex genetics of cleft lip in the mouse model

Nonsyndromic cleft lip in ``A' strain mice and humans is genetically complex and is distinct from isolated cleft palate. Cleft lip embryos recovered in 2.4% of 1485 first backcross (BC₁) segregants from a cross of A/WySnJ (24% cleft lip) and C57BL/6J (no cleft lip) in A/WySnJ mothers, and in testcrosses of 10 recombinant inbred (RI) strains (AXB/Pgn or BXA/Pgn), were used for gene mapping and for inference of genetic architecture. The A/WySnJ maternal genotype increased cleft lip risk in reciprocal crosses; the relevant genetic difference between AXB-6/Pgn (8%) and A/WySnJ (24%) is entirely maternal. A combination of new mapping panels (325 meioses), new markers, and a recombinant cleft lip embryo redefined the location of a recessive factor essential to cleft lip risk, clf1, and candidate genes Itgb3 and Crhr, to between D11Mit146/360 and D11Mit166/147. A screen of 54 YACs for 46 genes and SSLP loci located Wnt15, Wnt3, Crhr, Mtapt, Itgb3, Dlx3, and Dlx7 within the clf1 candidate region. The clf2 locus was newly mapped to Chromosome (Chr) 13 by a genome screen of BC₁ segregants, and further defined to a 4-cM region between D13Mit13/54 and D13Mit231 by strain distribution patterns of cleft lip liability and markers in testcrossed RI strains. Specific combinations of marker genotypes associated with cleft lip risk indicated that high risk in A/WySnJ mice is caused by epistatic interaction between clf1 and clf2 in the context of a genetic maternal effect. Human homologs of clf1 and clf2 are expected to be on 17q and 5q/9q. Received: 17 May 2000 / Accepted: 30 November 2000