Single Mendelian Gene Research Articles

Improvement of the submergence stress tolerance of local South Sumatran rice through the introgression of the Sub1 gene by using marker-assisted selection

Submergence stress due to unpredictable soil flooding is one of the main constraints encountered in rainfed growing areas, especially in Southern Sumatran riparian swamps. The development of submergence-stress-tolerant cultivars through the introgression of Sub1 via marker-assisted backcrossing (MABC) is an ideal solution. This study was carried out during 2020 at Sriwijaya University, Palembang, Indonesia, with the aim to select Sub1-introgressed lines in BC3F1 generations on the basis of MABC and to screen out the SSR markers that were unlinked to the target gene for application in subsequent background selection studies. Results revealed that almost all the backcrossed progenies segregated from the rice parental cultivars ‘FR13A’ and ‘Pegagan’. The backcrossed lines showed significantly improved submergence stress tolerance and recovery rates compared with their parents. Sub1 introgression into the BC3F1 generation was confirmed by the tightly linked Sub1 marker SUB1C173, and marker RM23915 was used for recombinant selection. These markers followed the expected marker segregation ratio in accordance with the Mendelian single gene model. In the parental polymorphism survey, 84 out of 237 SSR markers that were unlinked to the target loci were found to be available for background study. Twenty-seven backcrossed lines were selected on the basis of foreground selection. Seven plants were selected on the basis of the recombinant marker RM23915. Five backcrossed lines were further selected on the basis of submergence stress tolerance and agronomic performance.

Co-located quantitative trait loci mediate resistance to Agrobacterium tumefaciens, Phytophthora cinnamomi, and P. pini in Juglans microcarpa \xd7 J. regia hybrids

Soil-borne plant pathogens represent a serious threat that undermines commercial walnut (Juglans regia) production worldwide. Crown gall, caused by Agrobacterium tumefaciens, and Phytophthora root and crown rots, caused by various Phytophthora spp., are among the most devastating walnut soil-borne diseases. A recognized strategy to combat soil-borne diseases is adoption of resistant rootstocks. Here, resistance to A. tumefaciens, P. cinnamomi, and P. pini is mapped in the genome of Juglans microcarpa, a North American wild relative of cultivated walnut. Half-sib J. microcarpa mother trees DJUG 31.01 and DJUG 31.09 were crossed with J. regia cv. Serr, producing 353 and 400 hybrids, respectively. Clonally propagated hybrids were genotyped by sequencing to construct genetic maps for the two populations and challenged with the three pathogens. Resistance to each of the three pathogens was mapped as a major QTL on the long arm of J. microcarpa chromosome 4D and was associated with the same haplotype, designated as haplotype b, raising the possibility that the two mother trees were heterozygous for a single Mendelian gene conferring resistance to all three pathogens. The deployment of this haplotype in rootstock breeding will facilitate breeding of a walnut rootstock resistant to both crown gall and Phytophthora root and crown rots.

Biochemical and genetic implications of the slow ripening phenotype in peach fruit

The peach [Prunus persica L. (Batsch)] slow ripening (SR) trait is a mutation preventing the normal fruit ripening process. It is determined by a single Mendelian gene (Sr/sr) located on linkage group 4, where only homozygous individuals for a recessive allele (sr) show the SR phenotype and are generally discarded from breeding programs. Ripening-related traits such as fruit weight, firmness loss, ethylene production, ACO activity, sugars and organic acids composition, malondialdehyde, antioxidant capacity and total phenolic content were evaluated in a segregating population for the SR trait during two consecutive harvest seasons and at different maturity stages. Although there is no commercial value for the slow ripening (srsr) individuals, our results demonstrate that a heterozygous combination involving sr and another allele at this locus (Sr2) showed interesting traits including a longer harvest window and improved postharvest behaviour if harvested at the appropriate maturity (IAD ≥ 2). All these traits seem to be linked to a delayed ripening behaviour mediated, in turn, by a lower ethylene production capacity and an altered sugar (mainly sucrose) and organic acid accumulation/utilisation on-tree. The selection of this allelic combination could be an easy and efficient strategy to obtain new peach cultivars with potentially improved shelf life.

Association of fetal hemoglobin level with frequency of acute pain episodes in sickle cell disease (HbS-only phenotype) patients

BackgroundSickle cell disease (SCD) is a Mendelian single gene disorder with highly variable phenotypic expression. In the present study, we analyzed the influence of HbF, alpha thalassemia and other hematological indices to determine their association with acute pain episodes. MethodThis case control study consisted of SCD subjects with HbS phenotype experiencing three or more acute pain episodes in last twelve months (cases) and without any episode of acute pain during last twelve months (controls). Hematological parameters, HbF, and presence of alpha thalassemia were assessed in all subjects. ResultsA statistically significant difference between HbF levels (P < 0.025, χ2 test) and alpha thalassemia (P < 0.008, χ2 test) was observed between controls and cases group. Univariate analysis indicated that increased HbF levels > 25% (OR: 0.37, 95% CI: 0.18‐0.77, P < 0.008) and presence of alpha thalassemia (OR: 0.53, 95% CI: 0.33–0.85, P < 0.009) provided protection, while multivariate analysis revealed significant protection was attributable only by higher HbF levels (OR: 0.39, 95% CI: 0.17–0.88, P < 0.025). Significantly higher HbF levels were observed only in the 11–20 age group of cases in comparison to controls (Student's t-test, P < 0.001). ConclusionHigher concentrations of HbF are associated with protection against frequent episodes of acute pain crisis in SCD patients.

Cellular and molecular mechanisms of cleft palate development.

Cleft lip and palate are common craniofacial deformities. The etiology underlying these deformities is complex and multifactorial and they can occur as part of one of many chromosomal syndromes, Mendelian single gene disorders, teratogenic effects, and as yet uncharacterized syndromes. Our paper will provide an overview of the multiple genes and molecular pathways that have been implicated in palatal fusion. We believe that understanding the molecular mechanisms of cleft formation can help clinicians anticipate which patients may have difficulties healing and in the future allow them to make surgical and medical treatment decisions based on genetic information.

The Genetic Basis of Albinism in the Hermaphroditic Freshwater SnailPlanorbella trivolvis

In many species, the difference between pigmented and non-pigmented individuals is due to a single recessive Mendelian gene. We have demonstrated that this is also the case in the freshwater hermaphroditic snail, Planorbella trivolvis (Say, 1817), and established an albino population with comparable genetic background to a wild population so that pigmentation can be used as a visible genetic marker to better understand the reproductive biology of these mollusks. We carried out Mendelian crosses between albino (A) individuals from an inbred laboratory strain and pigmented (P) individuals one generation removed from a natural population and assessed pigmentation of progeny from eggs collected immediately after mating and 11 weeks later. Results of parental, F1, and F2 generations and backcrosses to pure-breeding albinos were consistent with a Mendelian single gene inheritance pattern. Because we never observed albino progeny from any albino snails mated to pigmented sperm donors, we also confirmed that selfing is rare in P. trivolvis. A simple non-invasive paternity marker in this unusual hermaphrodite snail that displays almost no self-fertilization will facilitate experiments to understand its reproductive biology and provide a more complete picture of hermaphrodite mating strategies.

Arabidopsis glutamate:glyoxylate aminotransferase 1 (Ler) mutants generated by CRISPR/Cas9 and their characteristics.

Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated 9 (CRISPR/Cas9) technology provides an efficient tool for editing the genomes of plants, animals and microorganisms. Glutamate:glyoxylate aminotransferase 1 (GGAT1) is a key enzyme in the photorespiration pathway; however, its regulation mechanism is largely unknown. Given that EMS-mutagenized ggat1 (Col-0 background) M2 pools have been generated, ggat1 (Ler background) should be very useful in the positional cloning of suppressor and/or enhancer genes of GGAT1. Unfortunately, such ggat1 (Ler) mutants are not currently available. In this study, CRISPR/Cas9 was used to generate ggat1 (Ler) mutants. Two GGAT1 target single-guide RNAs (sgRNAs) were constructed into pYLCRISPR/Cas9P35S-N, and flowering Arabidopsis (Ler) plants were transformed using an Agrobacterium tumefaciens-mediated floral dip protocol. Eleven chimeric and two heterozygous GGAT1-edited T1 lines of target 1 were separately screened from positive transgenic lines. Two ggat1 homozygous mutants, CTC-deletion and T-deletion at target 1, were generated from T2 generations of the 13 T1 lines. The edited mutation sites were found to be stable through generations regardless of whether the T-DNA was present. In addition, the genetic segregation of the mutation sites obeyed the Mendelian single gene segregation rule, and no mutations were detected at the possible off-target site. Also, the two independent ggat1 mutants had similar photorespiration phenotypes and down-regulated GGAT enzyme activity. Together, these results indicate that genetically stable ggat1 (Ler) mutants were generated by CRISPR/Cas9 genome editing, and these mutants will be used to promote the positional cloning of suppressor and/or enhancer genes of GGAT1 in our subsequent study.

Fine-mapping and identifying candidate genes conferring resistance to Soybean mosaic virus strain SC20 in soybean.

The Mendelian gene conferring resistance to Soybean mosaic virus Strain SC20 in soybean was fine-mapped onto a 79-kb segment on Chr.13 where two closely linked candidate genes were identified and qRT-PCR verified. Soybean mosaic virus (SMV) threatens the world soybean production, particularly in China. A country-wide SMV strain system composed of 22 strains was established in China, among which SC20 is a dominant strain in five provinces in Southern China. Resistance to SC20 was evaluated in parents, F1, F2 and the F2:7 RIL (recombinant inbred line) population derived from a cross between Qihuang-1 (resistant) and NN1138-2 (susceptible). The segregation ratio of resistant to susceptible in the populations suggested a single dominant gene involved in the resistance to SC20 in Qihuang-1. A "partial genome mapping strategy" was used to map the resistance gene on Chromosome 13. Linkage analysis between 178 RILs and genetic markers showed that the SC20-resistance gene located at 3.9 and 3.8cM to the flanking markers BARCSOYSSR_13_1099 and BARCSOYSSR_13_1185 on Chromosome 13. Subsequently, a residual heterozygote segregating population with 346 individuals was developed by selfing four plants heterozygous at markers adjacent to the tentative SC20-resistance gene; then, the candidate region was delimited to a genomic interval of approximately 79kb flanked by the new markers gm-ssr_13-14 and gm-indel_13-3. Among the seven annotated candidate genes in this region, two genes, Glyma.13G194700 and Glyma.13G195100, encoding Toll Interleukin Receptor-nucleotide-binding-leucine-rich repeat resistance proteins were identified as candidate resistance genes by quantitative real-time polymerase chain reaction and sequence analysis. The two closely linked genes work together to cause the phenotypic segregation as a single Mendelian gene. These results will facilitate marker-assisted selection, gene cloning and breeding for the resistance to SC20.

Matching Genotype and Phenotype: A Pragmatist Semiotic Analysis of Clinical Exome Sequencing

Based on three years of ethnographic research, the author examines how laboratory geneticists using next-generation genomic sequencing technologies causally link a patient’s genotype and phenotype. At stake is the extent to which this technology breaks with Mendelian single gene–single trait genetic determinism. In spite of an abundance of genetic information that could be used to establish complex multicausal interactions at the molecular level, geneticists actively work to reconcile uncertainties, unknowns, and anomalies to broker monogenic causality. The author develops pragmatist semiotics as a novel analytical strategy to capture collective processes of signification.

Analysis of Simple Sequence Repeat Markers Linked to Submergence Tolerance on Newly Developed Rice Lines Derived from MR263 × Swarna-Sub1

Nowadays, in extreme changing environments, development of submergence tolerance variety is necessary for ensuring crop production stability where, it is known that Malaysian commercial rice varieties such as MR219, MR220 and MR263 were severely susceptible to submergence. First step towards the development of submergence tolerance variety starts with the breeding program by crossing MR263 and Swarna-Sub1. Marker-assisted selection (MAS) was carried out through the utilization of simple sequence repeats (SSR) markers, considering its reliability as pre-selection tools to conduct this research. F1 generations plants were confirmed by tightly linked markers. In case of background study, out of 180 SSR markers, 38 were found polymorphic between two parents. Association of molecular markers and submergence tolerance were determined using Chi-square test. MR263 × Swarna-Sub1 F2 lines were tested for Sub1 gene conformation using the markers RM8300 and RM219. These markers showed a good fit to the expected marker segregation ratio (1:2:1) in a Mendelian single gene model (DF=1.0, p≤0.05). Eleven homozygous lines with Sub1 gene out of 256 were selected for future development of submergence tolerant varieties. Eleven lines were selected based on phenotypic study and agronomic performance.

Genetic Basis and Genetic Modifiers of β-Thalassemia and Sickle Cell Disease.

β-thalassemia and sickle cell disease (SCD) are prototypical Mendelian single gene disorders, both caused by mutations affecting the adult β-globin gene. Despite the apparent genetic simplicity, both disorders display a remarkable spectrum of phenotypic severity and share two major genetic modifiers-α-globin genotype and innate ability to produce fetal hemoglobin (HbF, α2γ2).This article provides an overview of the genetic basis for SCD and β-thalassemia, and genetic modifiers identified through phenotype correlation studies. Identification of the genetic variants modifying HbF production in combination with α-globin genotype provide some prediction of disease severity for β-thalassemia and SCD but generation of a personalized genetic risk score to inform prognosis and guide management requires a larger panel of genetic modifiers yet to be discovered.Nonetheless, genetic studies have been successful in characterizing some of the key variants and pathways involved in HbF regulation, providing new therapeutic targets for HbF reactivation.

Mapping of the clubroot disease resistance in spring Brassica napus canola introgressed from European winter canola cv. ‘Mendel’

Clubroot disease caused by the soil-borne pathogen Plasmodiophora brassicae Woronin is a major threat to the production of Brassica crops worldwide. The European winter canola cv. ‘Mendel’ shows resistance to many P. brassicae isolates including pathotypes 3, 5, 6 and 8 that are prevalent in Canada. To introgress clubroot resistance (CR) into Canadian spring Brassica napus canola, crosses between Canadian spring and European winter B. napus canola cv. ‘Mendel’ were made and several resistant lines were developed through pedigree breeding. Two of the resistant lines were further crossed with the clubroot susceptible spring canola line A07-26NR to produce two doubled haploid (DH) populations from nine F1 plants. Segregation for resistance followed a 1:1 ratio for resistant and susceptible phenotypes suggesting that a single Mendelian gene is involved in the control of resistance to P. brassicae single spore isolate SACAN-ss1 (pathotype 3) in the DH population where the ‘favourable allele’ for resistance is derived from the cv. ‘Mendel’. Genetic and physical mapping study positioned five previously described CR loci (CRk, Crr3, CRb, CRa and CRb Kato ) on the B. rapa chromosome A3, and identified twelve markers (1.5–2.0 % recombination) from the genomic region that houses the CRa and CRb Kato loci to be associated with the resistance derived from ‘Mendel’. The identified markers can be used in breeding as well as pyramiding of multiple clubroot resistance genes.

Conference summary: Navigating the Sea of Genomic Data, October 28-29, 2015

BackgroundThe rapid pace of biomedical discoveries in the past few years has resulted in substantial advances in our ability to diagnose, treat, and prevent a wide variety of diseases. The sequencing of the human genome offered the possibility of understanding the etiology, pathogenesis, and risk of developing disease from a genetic perspective and has resulted, for example, in the development of genomic-based diagnostic or risk-assessment tests for a number of medical and dental conditions. To assess the scientific evidence underlying such tests and determine whether they may be useful in clinical practice, practitioners need to have a basic understanding of the state-of-the-science of genomics and genetic testing. ObjectiveTo assist practitioners in understanding the science of genomics, the American Dental Association and the Task Force on Design and Analysis in Oral Health Research co-sponsored a landmark conference, Navigating the Sea of Genomic Data, held October 28-29, 2015, at the American Dental Association headquarters building in Chicago, IL. The purpose of this conference was to review the basics of genomic science, promote sound design and analysis of genomic studies of oral diseases, and provide a basis or “framework” to guide practitioners in assessing new development in genomics and genetic tests for oral diseases. OverviewPresentations at this conference were made by 9 world-renowned scientists who discussed a wide range of topics involving genomic science, genetic testing for rare mendelian single gene disorders, and genetic testing for assessing the risk of experiencing common complex diseases. This article summarizes the key points and concepts presented by the speakers. Practical ImplicationsIt is essential for oral health care professionals to have a fundamental understanding of genomic science so that they can evaluate new advances in this field and the use of genetic testing for the benefit of their patients.

Transferring clubroot resistance from Chinese cabbage (Brassica rapa) to canola (B. napus)

Clubroot, caused by Plasmodiophora brassicae, is one of the most important diseases of Brassica species worldwide, including vegetable and oilseed crops. A dominant clubroot resistance gene from B. rapa (Chinese cabbage) was previously fine mapped and molecular markers were developed in Chinese cabbage that could be used for marker-assisted selection (MAS) in other Brassica crops. To transfer this clubroot resistance gene to B. napus (canola), an interspecific hybridization was made between B. napus (canola) and B. rapa (Chinese cabbage). Subsequently, the F1 was backcrossed to the canola recurrent parent for three generations to produce BC1, BC2 and BC3 progenies. Using these populations, simple sequence repeat (SSR) markers flanking the clubroot resistance gene were used to perform MAS in canola. These molecular markers were then evaluated in 13 different canola and rapeseed quality genotypes in B. napus and B. rapa. These markers exhibited high reliability in identifying clubroot resistance in this diverse set of Brassica genotypes. Clubroot resistance also co-segregated with the SSR markers flanking the clubroot resistance gene in the BC3 and BC3S1. The segregation ratio of resistant and susceptible individuals in the BC3 supported the expected 1:1 ratio for the segregation of a single Mendelian gene. BC3S1 families with homozygous clubroot resistance were developed during this process, and should be valuable sources of clubroot resistance in B. napus breeding activities.

Evaluation of nose shape as a Mendelian-inherited trait in the determination of parentage among Nigerians in Port Harcourt

Background: Nose shape might be environmentally influenced; however, there are evidences of it being inherited in simple Mendelian dominant‑recessive patterns. In such instance, a nose can be broad or narrow with respect to its wideness in comparison to the intercanthal bridge. Therefore, this study was aimed at determining the inheritance pattern of nose shape in the bid to ascertain its usability in parentage determination. Methods: Three hundred and thirty‑seven subjects from 101 families comprising 202 parents and 135 offspring were recruited for this study. The families were randomly selected from within Port Harcourt by a multistage sampling technique. Their nose shape were observed physically in the father, mother, and at least a child in each family and documented. The offspring traits were tabularized in patterns of parental combinations (when both parents’ nose are broad, both parents’ nose are narrow, and a combination of broad and narrow). SPSS IBM (r) version 20 was used to analyze the data. Descriptive statistics and test for association between sex and nose shape was carried out by Chi‑square analysis and the conformance to Mendelian inheritance pattern was analyzed using Mendelian Chi‑square gene distribution model. Results: Broad nose shape was more frequent with 298 (88.4%) when compared to narrow nose shape (11.6%). About 46.9% of males had broad nose against 41.5% for females. However, this distribution was not observed to follow any sexual preference ( χ 2 = 0.141, P > 0.932). The observed and expected outcome were tested for significance on the assumption that offspring outcome conforms to Mendelian simple dominant‑recessive monohybrid cross; conformance was observed. Conclusion: The distribution of nose shape was observed to be genetically determined and follows Mendelian single gene dominant‑recessive pattern with the allele for narrow nose dominant over the allele for broad nose. This result can be used for preliminary screening in parentage dispute. It can also be useful in forensic and genetic studies. Keywords: Mendelian pattern, Nigerians, nose shape, Port Harcourt, trait

Mendelization and fine mapping of a bread wheat spot blotch disease resistance QTL

The wheat (Triticum aestivum L.) quantitative trait locus (QTL) QSb.bhu-5B, which determines resistance to spot blotch (causative pathogen Bipolaris sorokiniana), was mapped to an interval of 0.62 cM on chromosome arm 5BL via the analysis of a recombinant inbred line population bred from a cross between ‘YS116’ and ‘Sonalika.’ ‘YS116’ is an F12 selection from the cross ‘Yangmai 6’ (source of resistance) × ‘Sonalika,’ which contains only one spot blotch resistance QTL. Lines tested from F3 to F5 were inoculated in the field with B. sorokiniana over three consecutive seasons. Measurements of both disease severity and the area under the disease progress curve were used to assess each line’s disease response. In the present population, QSb.bhu-5B, which is flanked by the SSR loci Xgwm639 (0.28 cM) and Xgwm1043, behaved as a single Mendelian gene and was given the gene symbol Sb2. The physical location of Sb2 is within the 5BL1-0.55-0.75 deletion bin. Both Xgwm1043 and Xgwm639 were effective as indirect selection tools in various genetic backgrounds.

Two quantitative trait loci for grain yield and plant height on chromosome 3 are tightly linked in coupling phase in rice

Plant height is a very important trait affecting rice production. The introduction of the Green Revolution gene, sd-1, into rice cultivars to create semi-dwarfism has increased grain production. In the development of modern rice breeding, there is an urgent need for a somewhat higher plant for not only maintenance of strong lodging resistance, but also more biomass. In this study, the plant height quantitative trait locus (QTL) qPH3 with moderate effect and the yield per plant QTL qYD3 were identified on chromosome 3 in a population derived from two semi-dwarf cultivars. The 02428 alleles of both QTLs contributed to increasing phenotypic values. In the near-isogenic background, both QTLs exhibited the characteristics of a single Mendelian gene. qPH3 and qYD3 were tightly linked, no more than 130 kb apart, with additive effects of 5.2 cm and 4.4 g, respectively. They were further fine-mapped to a 19-kb region and a 65-kb region, respectively, where the recombination hotspot occurred. The gene encoding gibberellin 20-oxidase associated with plant height was regarded as the candidate of qPH3. Both linked QTLs are in coupling phase, which is convenient for simultaneously selecting both genes for breeding high-yield varieties maintaining strong lodging resistance. Isolation of qYD3 is very promising due to its location in a recombination hotspot region.

Genetic analysis and molecular mapping of genes controlling seed coat colour in adzuki bean (Vigna angularis)

Seed coat colour is an important agronomic trait in adzuki bean (Vigna angularis). Here, we identified four genes controlling seed coat colour in this species. We conducted a quantitative trait locus (QTL) study of 133 recombinant inbred lines (F6) derived from a cross between V. angularis Shumari (red) and landrace Acc2265 (olive buff), by using simple sequence repeat (SSR) markers. We identified a strong QTL, designated OLB1, which explained 54, 43, and 56 % of the total variance in the L* (lightness), a* (redness), and b* (yellowness) values, respectively. In addition, we identified a minor QTL, designated OLB2, which explained 6 % of the total variance in redness. OLB1 and OLB2 were located on linkage group (LG) 1, >80 cM apart. The F1 phenotype was olive buff, implying that the red colour trait was recessive. Next, we crossed two spontaneous mutants, ‘Shiro-shouzu’ (ivory yellow) and ‘TA230002’ (pale olive buff), to the red-seeded ‘Beni-shouzu’. Genetic analysis of the F1, F2, and F3 progenies derived from each parental cross revealed that the ivory yellow and pale olive buff traits were recessive to the red trait. Molecular mapping studies indicated that the ivory yellow trait was controlled by a single Mendelian gene, designated IVY, located on LG 8. The pale olive buff trait was also controlled by a single Mendelian gene, designated POB, located on LG 10. Our findings will facilitate the development of novel red seed cultivars and the introgression of ivory yellow genes into desirable cultivars, via marker-assisted selection.

Dissection of wheat spot blotch disease resistance QTLs in to single Mendelian genes

Two QTLs imparting spot blotch resistance and located on chromosomes 2BS and 5BL were dissected in to a single Mendelian gene using 341 single seed descent (SSD) derived F3 and F4 lines of YS102 × Sonalika and 335 F3, F4 and F5 lines of YS116 × Sonalika cross. The resistant parental lines YS102 and YS116 were selected on the basis of phenotypic and genotyping data of base mapping population (Yangmai 6 × Sonalika in F12 generation) used for QTL mapping. Both the populations were tested for spot blotch resistance under artificial epiphytotic conditions in the field. Disease severity (%) and AUDPC values of each line were calculated. The parent YS116 was known to possess a single major QTL on chromosome 5BL, while YS102 possess QTL on chromosome 2BS. The test for goodness of fit (1: 2: 1 in F3, 3: 2: 3 in F4 and 7: 2: 7 in F5 generations) confirmed segregation at one locus in both the crosses. These SSD derived lines from both the crosses may further be used for fine mapping and map based cloning of the QTLs of spot blotch resistance OTLs.

Genetics of common malformations

Advanced technology has recently allowed us to study rare Mendelian disorders in an unprecedented manner. The same technology should allow us also to study more common malformations. Many of these are not caused by a variant in a single Mendelian gene but by interplay between series of genetic variants and exogenous influences. Likely the site from which the DNA is derived is of great importance in studying malformations as mosaicism may be much more common than earlier anticipated. Factors other than simple variants in our genomic DNA should be considered in the studies as well. Not only is recognition of someone's liability to disease important, but also determining exogenous factors involved in malformations should receive more attention as it may allow us decrease the burden of malformations in humans.