Law Of Segregation Research Articles

An egg-sabotaging mechanism drives non-Mendelian transmission in mice

Selfish genetic elements drive in meiosis to distort their transmission ratio and increase their representation in gametes, violating Mendel's law of segregation. The two established paradigms for meiotic drive, gamete killing and biased segregation, are fundamentally different. In gamete killing, typically observed with male meiosis, selfish elements sabotage gametes that do not contain them. By contrast, killing is predetermined in female meiosis, and selfish elements bias their segregation to the single surviving gamete (i.e., the egg in animal meiosis). Here, we show that a selfish element on mouse chromosome 2, Responder to drive 2 (R2d2), drives using a hybrid mechanism in female meiosis, incorporating elements of both killing and biased segregation. We propose that if R2d2 is destined for the polar body, it manipulates segregation to sabotage the egg by causing aneuploidy, which is subsequently lethal in the embryo, ensuring that surviving progeny preferentially contain R2d2. In heterozygous females, R2d2 orients randomly on the metaphase spindle but lags during anaphase and preferentially remains in the egg, regardless of its initial orientation. Thus, the egg genotype is either euploid with R2d2 or aneuploid with both homologs of chromosome 2, with only the former generating viable embryos. Consistent with this model, R2d2 heterozygous females produce eggs with increased aneuploidy for chromosome 2, increased embryonic lethality, and increased transmission of R2d2. In contrast to typical gamete killing of sisters produced as daughter cells in a single meiosis, R2d2 prevents production of any viable gametes from meiotic divisions in which it should have been excluded from theegg.

Co-inheritance of recombined chromatids maintains heterozygosity in a parthenogenetic ant

According to Mendel’s second law, chromosomes segregate randomly in meiosis. Non-random segregation is primarily known for cases of selfish meiotic drive in females, in which particular alleles bias their own transmission into the oocyte. Here we report a rare example of unselfish meiotic drive for crossover inheritance in the clonal raider ant, Ooceraea biroi, in which both alleles are co-inherited at all loci across the entire genome. This species produces diploid offspring parthenogenetically via fusion of two haploid nuclei from the same meiosis. This process should cause rapid genotypic degeneration due to loss of heterozygosity, which results if crossover recombination is followed by random (Mendelian) segregation of chromosomes. However, by comparing whole genomes of mothers and daughters, we show that loss of heterozygosity is exceedingly rare, raising the possibility that crossovers are infrequent or absent in O. biroi meiosis. Using a combination of cytology and whole-genome sequencing, we show that crossover recombination is, in fact, common but that loss of heterozygosity is avoided because crossover products are faithfully co-inherited. This results from a programmed violation of Mendel’s law of segregation, such that crossover products segregate together rather than randomly. This discovery highlights an extreme example of cellular ‘memory’ of crossovers, which could be a common yet cryptic feature of chromosomal segregation.

An egg sabotaging mechanism drives non-Mendelian transmission in mice.

During meiosis, homologous chromosomes segregate so that alleles are transmitted equally to haploid gametes, following Mendel's Law of Segregation. However, some selfish genetic elements drive in meiosis to distort the transmission ratio and increase their representation in gametes. The established paradigms for drive are fundamentally different for female vs male meiosis. In male meiosis, selfish elements typically kill gametes that do not contain them. In female meiosis, killing is predetermined, and selfish elements bias their segregation to the single surviving gamete (i.e., the egg in animal meiosis). Here we show that a selfish element on mouse chromosome 2, R2d2, drives using a hybrid mechanism in female meiosis, incorporating elements of both male and female drivers. If R2d2 is destined for the polar body, it manipulates segregation to sabotage the egg by causing aneuploidy that is subsequently lethal in the embryo, so that surviving progeny preferentially contain R2d2. In heterozygous females, R2d2 orients randomly on the metaphase spindle but lags during anaphase and preferentially remains in the egg, regardless of its initial orientation. Thus, the egg genotype is either euploid with R2d2 or aneuploid with both homologs of chromosome 2, with only the former generating viable embryos. Consistent with this model, R2d2 heterozygous females produce eggs with increased aneuploidy for chromosome 2, increased embryonic lethality, and increased transmission of R2d2. In contrast to a male meiotic driver, which kills its sister gametes produced as daughter cells in the same meiosis, R2d2 eliminates "cousins" produced from meioses in which it should have been excluded from the egg.

Testing a candidate meiotic drive locus identified by pool sequencing.

Meiotic drive biases the transmission of alleles in heterozygous individuals, such that Mendel's law of equal segregation is violated. Most examples of meiotic drive have been discovered over the past century based on causing sex ratio distortion or the biased transmission of easily scoreable genetic markers that were linked to drive alleles. More recently, several approaches have been developed that attempt to identify distortions of Mendelian segregation genome wide. Here, we test a candidate female meiotic drive locus in Drosophila melanogaster, identified previously as causing a ∼54:46 distortion ratio using sequencing of large pools of backcross progeny. We inserted fluorescent visible markers near the candidate locus and scored transmission in thousands of individual progeny. We observed a small but significant deviation from the Mendelian expectation; however, it was in the opposite direction to that predicted based on the original experiments. We discuss several possible causes of the discrepancy between the 2 approaches, noting that subtle viability effects are particularly challenging to disentangle from potential small-effect meiotic drive loci. We conclude that pool sequencing approaches remain a powerful method to identify candidate meiotic drive loci but that genotyping of individual progeny at early developmental stages may be required for robust confirmation.

Quantitative Model of Hereditary Behavior for Carbon Segregation in Continuous Casting Billets Based on Grayscale Analysis

To obtain a reliable hereditary law of carbon segregation, the carbon element content in the entire area of the longitudinal sections of billet, wire rod and bolt samples is quantitatively calculated through the method of metal macrostructure grayscale analysis (MGA). Based on the calculation results, the hereditary behavior of carbon distribution characteristics at the “geometric similar” positions of the selected samples is quantitatively analyzed from three aspects: carbon element content, longitudinal carbon fluctuation and transverse carbon uniformity. The results show that the distribution characteristics of the carbon element content and carbon variation coefficient on the longitudinal sections of the selected samples are approximately the same, which can quantitatively reflect the hereditary behavior of the carbon segregation, and the corresponding hereditary equations during the rolling process are fitted. Then, the central segregation index, which can represent the transverse carbon uniformity, on the longitudinal sections of selected samples is calculated, and the corresponding hereditary equation is also obtained. According to the hereditary equation of central segregation index during the rolling process, to produce rolled products with more uniform quality and mechanical properties, the central segregation index of the corresponding billet should be controlled below 1.05.

A method for low-coverage single-gamete sequence analysis demonstrates adherence to Mendel's first law across a large sample of human sperm.

Recently published single-cell sequencing data from individual human sperm (n=41,189; 969-3377 cells from each of 25 donors) offer an opportunity to investigate questions of inheritance with improved statistical power, but require new methods tailored to these extremely low-coverage data (∼0.01× per cell). To this end, we developed a method, named rhapsodi, that leverages sparse gamete genotype data to phase the diploid genomes of the donor individuals, impute missing gamete genotypes, and discover meiotic recombination breakpoints, benchmarking its performance across a wide range of study designs. We then applied rhapsodi to the sperm sequencing data to investigate adherence to Mendel's Law of Segregation, which states that the offspring of a diploid, heterozygous parent will inherit either allele with equal probability. While the vast majority of loci adhere to this rule, research in model and non-model organisms has uncovered numerous exceptions whereby 'selfish' alleles are disproportionately transmitted to the next generation. Evidence of such 'transmission distortion' (TD) in humans remains equivocal in part because scans of human pedigrees have been under-powered to detect small effects. After applying rhapsodi to the sperm data and scanning for evidence of TD, our results exhibited close concordance with binomial expectations under balanced transmission. Together, our work demonstrates that rhapsodi can facilitate novel uses of inferred genotype data and meiotic recombination events, while offering a powerful quantitative framework for testing for TD in other cohorts and study systems.

What can we learn from selfish loci that break Mendel's law?

Exceptions to Mendel's law of segregation were important for demonstrating that chromosomes carry genetic material. Scrutiny of additional exceptions to Mendel's law caused by selfish genes has the potential to unravel other unsolved mysteries of genetics.

Adaptive meiotic drive in selfing populations with heterozygote advantage

The egalitarian allotment of gametes to each allele at a locus (Mendel’s law of segregation) is a near-universal phenomenon characterizing inheritance in sexual populations. As exceptions to Mendel’s law are known to occur, one can investigate why non-Mendelian segregation is not more common using modifier theory. Earlier work assuming sex-independent modifier effects in a random mating population with heterozygote advantage concluded that equal segregation is stable over long-term evolution. Subsequent investigation, however, demonstrated that the stability of the Mendelian scheme disappears when sex-specific modifier effects are allowed. Here I derive invasion conditions favoring the repeal of Mendelian law in mixed and obligate selfing populations. Oppositely-directed segregation distortion in the production of male and female gametes is selected for in the presence of overdominant fitness. The conditions are less restrictive than under panmixia in that strong selection can occur even without differential viability of reciprocal heterozygotes (i.e. in the absence of parent-of-origin effects at the overdominant fitness locus). Generalized equilibria are derived for full selfing.

Meiotic drive in house mice: mechanisms, consequences, and insights for human biology.

Meiotic drive occurs when one allele at a heterozygous site cheats its way into a disproportionate share of functional gametes, violating Mendel's law of equal segregation. This genetic conflict typically imposes a fitness cost to individuals, often by disrupting the process of gametogenesis. The evolutionary impact of meiotic drive is substantial, and the phenomenon has been associated with infertility and reproductive isolation in a wide range of organisms. However, cases of meiotic drive in humans remain elusive, a finding that likely reflects the inherent challenges of detecting drive in our species rather than unique features of human genome biology. Here, we make the case that house mice (Mus musculus) present a powerful model system to investigate the mechanisms and consequences of meiotic drive and facilitate translational inferences about the scope and potential mechanisms of drive in humans. We first detail how different house mouse resources have been harnessed to identify cases of meiotic drive and the underlying mechanisms utilized to override Mendel's rules of inheritance. We then summarize the current state of knowledge of meiotic drive in the mouse genome. We profile known mechanisms leading to transmission bias at several established drive elements. We discuss how a detailed understanding of meiotic drive in mice can steer the search for drive elements in our own species. Lastly, we conclude with a prospective look into how new technologies and molecular tools can help resolve lingering mysteries about the prevalence and mechanisms of selfish DNA transmission in mammals.

Str Locus Mutations In Paternity Case

DNA analysis is widely applied in solving forensic cases, especially Short Tandem Repat (STR) because of its advantages. In identifying the individual, the National Police compared the individual's DNA with that of his parents. Each anal has a pair of DNA fragments of which half are inherited by the father and the remainder by the mother according to Mendel's Law of Segregation. In this study, we compared DNA typing between the child and the mother with the help of PCR extracted by the Chelex method to find the mother fragment and obtain the father fragment. A child is the biological child of the alleged father if he or she has less than 2 exclusion STR loci. The results of this study revealed that all paternal fragments from the child were identical to the DNA fragments of the alleged father, except for one locus, namely CSF1PO which had a mutation. Mutations in the STR locus lower the paternity index, although it can still be concluded that the child is the biological child of the alleged father.
 
 Keywords: Paternity Test, DNA, STR, Mutation

Sulfur segregation and inclusion modification in steel using magnesium addition

In order to investigate the effect of magnesium treatment on the distribution and segregation of sulfides in carbon high-sulfur free-cutting steel in industrial tests, scanning electron microscope, energy dispersive spectrometer, electrolysis device, and other characterization methods are used to examine the sulfide morphology in the steel continuous casting billet. The sulfide type, spatial distribution and three-dimensional morphology of the slab in the casting slab are analyzed, and the law of sulfur segregation is calculated and summarized using the diffusion growth model. The results of the study showed that the area of cluster-like sulfides before and after magnesium modification decreased by 6.4%, the area of chain-like sulfides reduced by 6.6%, and the area of single-type sulfides increased by 12.7%. In the magnesium modified steel, a fine composite sulfide with MgO·Al2O3 as the core is formed, which provides a heterogeneous nucleation site for the later precipitated MnS, enhancing the shape and size of the sulfide. The sulfide segregation index was reduced from 1.30 to 1.04 before and after magnesium modification, and the sulfur concentration was 1.56 times the original precipitation when the middle position of the slab was entirely cemented. The macroscopic shrinkage cavity on the surface of the cast slab is minimized after magnesium treatment, which enhances sulfide dispersion and segregation dramatically.

Diverse mating phenotypes impact the spread of wtf meiotic drivers in Schizosaccharomyces pombe.

Meiotic drivers are genetic elements that break Mendel's law of segregation to be transmitted into more than half of the offspring produced by a heterozygote. The success of a driver relies on outcrossing (mating between individuals from distinct lineages) because drivers gain their advantage in heterozygotes. It is, therefore, curious that Schizosaccharomyces pombe, a species reported to rarely outcross, harbors many meiotic drivers. To address this paradox, we measured mating phenotypes in S. pombe natural isolates. We found that the propensity for cells from distinct clonal lineages to mate varies between natural isolates and can be affected both by cell density and by the available sexual partners. Additionally, we found that the observed levels of preferential mating between cells from the same clonal lineage can slow, but not prevent, the spread of a wtf meiotic driver in the absence of additional fitness costs linked to the driver. These analyses reveal parameters critical to understanding the evolution of S. pombe and help explain the success of meiotic drivers in this species.

Comparative study on the growth, survival, gonad development and trait segregation of F2 hybrids and their grandparent species (Crassostrea ariakensis and C. hongkongensis)

In previous studies, we showed that F1 hybrids of Crassostrea ariakensis and C. hongkongensis had faster growth and higher survival rates than the parent species. Further intraspecific crossing of F1 hybrids allowed us to further understand the law of trait segregation and indicated a potential direction for breeding of a new strain with high salinity tolerance and fast growth. In this study, we systematically analyzed the growth, survival, fertility and trait segregation of F2 hybrids and their grandparent species (C. ariakensis and C. hongkongensis) at two sites. Our results confirmed that there were no significant differences in fertilization, cleavage, D larval and survival rates among the three groups of larvae, but the shell heights of F2 hybrids and C. ariakensis were significantly higher than those of C. hongkongensis on the 12th and 16th days. At the high salinity site (Zhulin Pond), the shell heights and survival rates of the F2 hybrids were significantly higher than those of C. hongkongensis from the 90th to the 360th day. However, at the low salinity site (Dafeng River), the F2 hybrids consistently had higher shell heights than C. ariakensis, but there was no significant difference in the survival rates among the three groups. F2 hybrids were classified into three types based on size and number of holes on the gill water tubes, and only F2-A (like C. ariakensis) and F2-H/A (intermediate form) were confirmed to survive at the high salinity site. Moreover, three number of ITS-2 gene fragment were identified in the F2 hybrids, and only A (like C. ariakensis) and HA (like F1 hybrids) existed at the high salinity site. Paraffin sections of gonad tissue confirmed that all three types of F2 hybrids were fertile and could produce mature gametes. In summary, through this comprehensive comparative analysis, we confirmed that F2-A and F2-H/A hybrids had clear advantages in terms of high salinity tolerance and fast growth over their grandparent species, and had potential to be used as the foundations of a new strain.

A Systemic Perspective

A Systemic Perspective

CRISPR/Cas9 induces exon skipping that facilitates development of fragrant rice.

Splicing of precursor-mRNAs (pre-mRNAs) is a critical biological process of gene expression. Normally, pre-mRNAs are processed by spliceosomes to produce mature mRNAs by removing introns at 5'-(donor) and 3'-(acceptor) splice sites based on the canonical GU-AG rule (Reddy et al., 2013). Mutation at either the intron donor or acceptor sites should cause mRNA mis-splicing. Recently, an exon skipping method has been developed to generate loss of gene function in mammalian cells using base editors to mutate nucleotides at the acceptor sites (Gapinske et al., 2018).

Small-Molecule Control of Super-Mendelian Inheritance in Gene Drives

SUMMARYSynthetic CRISPR-based gene-drive systems have tremendous potential in public health and agriculture, such as for fighting vector-borne diseases or suppressing crop pest populations. These elements can rapidly spread in a population by breaching the inheritance limit of 50% dictated by Mendel’s law of gene segregation, making them a promising tool for population engineering. However, current technologies lack control over their propagation capacity, and there are important concerns about potential unchecked spreading. Here, we describe a gene-drive system in Drosophila that generates an analog inheritance output that can be tightly and conditionally controlled to between 50% and 100%. This technology uses a modified SpCas9 that responds to a synthetic, orally available small molecule, fine-tuning the inheritance probability. This system opens a new avenue to feasibility studies for spatial and temporal control of gene drives using small molecules.

Phenylketonuria: Genes in phenylketonuria, diagnosis, and treatments

Introduction: Phenylketonuria (PKU) is a rare autosomal-recessive disorder inherited in accordance with the law of segregation. Detection tools for people with PKU can include Sanger Sequencing (SS) and Next Generation Sequencing (NGS). Diet therapy, Large Neutral Amino Acids (LNAA), and Specific Nutrient Combination (SNC) can help alleviate people with PKU. In the future, genetic manipulation techniques can help to eliminate PKU. Objectives: In this review, the author describes the progress in a study that focused on detection tools such as SS and NGS, the phenylalanine hydroxylase (PAH) gene and mutations in the PAH gene, use of drugs for PKU, and genetic manipulation techniques such as Adeno-associated virus (AAV) vectors and clustered regularly interspaced short palindromic repeats (CRISPR) RNA-guided FokI nuclease system (FokI-dCas9 system). AAV is abbreviation of AAV. CRISPR system is abbreviation of clustered regularly interspaced short palindromic repeats. Methods: The author searched the PubMed Database at National Center for Biotechnology Information (NCBI) for articles on PKU disorder. These articles were published between 2014 and 2019. Articles were open access and in English. Searches were also done at Google and ScienceDirect. Results: PKU derives from mutations in the PAH gene. Features of PKU may include ataxia, intellectual ability, and seizures. MassARRAY method, minisequencing method, SS and NGS can detect PKU on humans. Diet therapy, BH4, LNAA, SNC, enzyme therapy can help patients with PKU. However, these drugs cannot treat PKU permanently. In the future, genetic manipulation techniques can be used. AAV vectors and FokI-dCas9 system can be useful to eliminate PKU disorder. Conclusion: Guthrie method, MassARRAY, minisequencing, SS and NGS are tools for detecting PKU. Treatments with such as diet therapy, LNAA, SNC, and enzyme therapy are useful for PKU disorder. AAV vectors and FokI-dCas9 system are methods that can be useful for eliminating PKU in the future.

Nondisjunction and unequal spindle organization accompany the drive of Aegilops speltoides B chromosomes.

B chromosomes (Bs) are supernumerary chromosomes, which are often preferentially inherited. When transmission rates of chromosomes are higher than 0.5, not obeying the Mendelian law of equal segregation, the resulting transmission advantage is collectively referred to as 'chromosome drive'. Here we analysed the drive mechanism of Aegilops speltoides Bs. The repeat AesTR-183 of A.speltoides Bs, which also can be detected on the Bs of Aegilops mutica and rye, was used to track Bs during pollen development. Nondisjunction of CENH3-positive, tubulin interacting B sister chromatids and an asymmetric spindle during first pollen grain mitosis are key for the accumulation process. A quantitative flow cytometric approach revealed that, independent of the number of Bs present in the mother plant, Bs accumulate in the generative nuclei to >93%. Nine out of 11 tested (peri)centromeric repeats were shared by A and B chromosomes. Our findings provide new insights into the process of chromosome drive. Quantitative flow cytometry is a useful and reliable method to study the drive frequency of Bs. Nondisjunction and unequal spindle organization accompany during first pollen mitosis the drive of A.speltoides Bs. The prerequisites for the drive process seems to be common in Poaceae.

Statistics of Mendelian segregation-A mixture model.

Mendel's law of segregation explains why genetic variation can be maintained over time. In diploid organisms, an offspring receives one allele from each parent, not just half of the blended genetic material of the parents. Which of the two alleles is received is purely random. This stochastic process generates genetic variation among members of the same family, called Mendelian segregation variance or within-family variance. In statistics, the genetic value of a quantitative trait for an offspring follows a mixture distribution consisting of the four alleles of the two parents, guided by a Mendelian variable from each parent. The mixture model allows us to partition the total genetic variance into between-family and within-family variances. In the absence of inbreeding, the genetic variance splits half to the between-family variance and half to the within-family variance. With inbreeding, however, the between-family variance is increased at the cost of the within-family variance, leading to a net increase of the total genetic variance. This study defines multiple Mendelian variables and develops a mixture model of quantitative genetics. The phenomenon that allelic variance is maintained over time is guided by "the law of conservation of allelic variance" in biology, which is comparable to "the law of conservation of mass" in physics.

Phenylketonuria: Genes in Phenylketonuria, Diagnosis, and Treatments

Introduction: Phenylketonuria (PKU) is a rare autosomal-recessive disorder inherited in accordance with the law of segregation. Detection tools for people with PKU can include Sanger Sequencing (SS) and Next Generation Sequencing (NGS). Diet therapy, Large Neutral Amino Acids (LNAA), and Specific Nutrient Combination (SNC) can help alleviate people with PKU. In the future, genetic manipulation techniques can help to eliminate PKU. Objectives: In this review, the author describes the progress in a study that focused on detection tools such as SS and NGS, the phenylalanine hydroxylase (PAH) gene and mutations in the PAH gene, use of drugs for PKU, and genetic manipulation techniques such as Adeno-associated virus (AAV) vectors and clustered regularly interspaced short palindromic repeats (CRISPR) RNA-guided FokI nuclease system (FokI-dCas9 system). AAV is abbreviation of AAV. CRISPR system is abbreviation of clustered regularly interspaced short palindromic repeats. Methods: The author searched the PubMed Database at National Center for Biotechnology Information (NCBI) for articles on PKU disorder. These articles were published between 2014 and 2019. Articles were open access and in English. Searches were also done at Google and ScienceDirect. Results: PKU derives from mutations in the PAH gene. Features of PKU may include ataxia, intellectual ability, and seizures. MassARRAY method, minisequencing method, SS and NGS can detect PKU on humans. Diet therapy, BH4, LNAA, SNC, enzyme therapy can help patients with PKU. However, these drugs cannot treat PKU permanently. In the future, genetic manipulation techniques can be used. AAV vectors and FokI-dCas9 system can be useful to eliminate PKU disorder. Conclusion: Guthrie method, MassARRAY, minisequencing, SS and NGS are tools for detecting PKU. Treatments with such as diet therapy, LNAA, SNC, and enzyme therapy are useful for PKU disorder. AAV vectors and FokI-dCas9 system are methods that can be useful for eliminating PKU in the future.