Phenotype In Heterozygotes Research Articles

Expression of a new mutation (Axd) causing axial defects in mice correlates with maternal phenotype and age

A new autosomal mutation, Axd (axial defects), is described. Axd segregates in a simple Mendelian fashion, and it is dominant with incomplete penetrance and variable expressivity. The phenotype of Axd heterozygotes ranges from a variety of tail anomalies to visibly normal tails. Approximately 12% of neonates from curly-tail (CT) F1 (Axd/+) x F1 (Axd/+) matings exhibit open neural tube defects (NTD) in the lumbosacral region and 16% have curly tails. Mean litter sizes and resorption rates comparable to wild type indicate that homozygosity for Axd is not obligately lethal. Genetic background plays a major role in Axd expression. Strains such as BALB/cByJ allow the highest penetrance of the mutation in single dose (46%), whereas, in CF-1 mice Axd is recessive. The tail phenotype of heterozygous Axd/+ dams, in part reflective of their genetic background, correlates with the incidence of NTD in F2 offspring: CT mothers produce significantly more neonates with frank NTD than normal tail mothers. At the one embryonic period examined for this study (D13/D14 post-coitus), an 85% higher incidence of total axial defects is observed than among the F2 at birth. Unchanging litter size and the relative increase in phenotypically normal offspring by birth suggest that Axd acts by delaying posterior neural tube closure. One of the most significant findings in this study is that maternal age influences the survival of Axd embryos in utero. Axd/+ dams older than 8 months yield fewer mean implants, higher resorption rates, and fewer viable embryos with axial defects than do Axd/+ dams younger than 8 months. Axd is not allelic to nor linked to the Sp (splotch) gene which also affects neurulation.

ENU‐induced allele of brachyury (Tkt1) exhibits a developmental lethal phenotype similar to the original brachyury (T) mutation

New alleles of brachyury (Tkt1, Tkt4) were induced in the mouse complete tw5 haplotype by ethylnitrosourea (ENU). Like the original brachyury (T) mutation, the new alleles cause a short-tailed phenotype in heterozygotes, and interact with the t complex tail interaction factor (tct) in trans to cause phenotypically tailless mice. Because ENU is mainly a point mutagen, it is important to determine that the new alleles are homozygous embryonic lethal mutations like the original T allele, and to characterize their embryonic lethal phenotype. Moreover, the Tkt1 mutation maps to an inverted position relative to quaking (qk) in t haplotypes as compared with its position on normal chromosome 17. The Tkt1 allele was separated from the resident tw5 lethal gene, tclw5, by recombination, allowing embryology studies to be performed. Embryological analyses show that the Tkt1 allele is nearly identical to the classic T allele. At 9 and 10 days of development, homozygous Tkt1/Tkt1 embryos are grossly abnormal with properties including 1) irregular, disorganized somite pairs, 2) a shortened posterior end of the embryo, 3) an irregular neural tube, and 4) an abnormal notochord. In addition, 10 day-old abnormal embryos have anterior limb buds that point dorsally rather than ventrally, and are smaller than normal littermates. We conclude that the Tkt1 mutation is a valuable allele for both mapping and molecular characterization of the brachyury locus.

Effects of the halothane-sensitivity gene on sarcoplasmic reticulum function.

Pigs heterozygous for the halothane-sensitivity gene exhibit a distinct phenotype with regard to both in vivo and in vitro muscle responses to halothane (E. M. Gallant, J. R. Mickelson, B. D. Roggow, S. K. Donaldson, C. F. Louis, and W. E. Rempel. Am. J. Physiol. 257 (Cell Physiol. 26): C781-C786, 1989). In this paper heavy sarcoplasmic reticulum (SR) preparations were isolated from the muscles of pigs of all three genotypes. The rate of calcium release from SR of pigs homozygous for the halothane-sensitivity gene was approximately twice that of SR from pigs homozygous for the normal allele. Furthermore, in the presence of 6 microM Ca2+, the binding of [3H]ryanodine to SR isolated from the homozygous halothane-sensitive pigs was of a higher affinity than was the binding to SR isolated from the homozygous normal pigs (Kd = 70-90 vs. 265 nM, respectively). The SR from pigs heterozygous for the halothane-sensitivity gene, however, demonstrated intermediate values for the rate of calcium release and the affinity for [3H]ryanodine (Kd = 192 nM). Thus the alterations in heavy SR calcium release and [3H]ryanodine binding in the pigs containing one copy of the halothane-sensitivity gene demonstrate a distinct heterozygote phenotype. These data also suggest that the protein product of this gene is closely associated with, and perhaps identical to, the SR calcium release channel-ryanodine receptor protein.

A mouse model for neural tube defects: the curtailed (Tc) mutation produces spina bifida occulta in Tc/+ animals and spina bifida with meningomyelocele in Tc/t.

Curtailed (Tc), a dominant mutation on mouse chromosome 17, causes a tailless phenotype and occasional hindlimb paralysis in heterozygotes. Histologically, Tc/+ embryos show a variety of abnormalities including budding and ventral duplication of the developing spinal cord, duplication and intermittent absence of the notochord, and partial or complete absence of bony vertebrae, all posterior to midliver level. When Tc is heterozygous with t-haplotypes that contain the "tail interaction factor," tct, the phenotype is more severe, and a dorsal blood blister exists in the lumbosacral area. Our microscopic observations reveal that Tc/tw5 mice have a lumbosacral spina bifida with meningomyelocele. This results from the absence of bony vertebrae, extensive thinning of the dermis dorsally, and the rupturing of the previously closed neural tube, probably by increased cerebrospinal fluid (CSF) pressure on the necrotic, attenuated roof plate. Thinning of the roof plate, which facilitates the rupturing of the spinal cord, is not observed in Tc/+, which suggests that this phenomenon is associated with the interaction of Tc with the t-allele. Later in the development of Tc/tw5 embryos, adjacent blood vessels are ruptured, resulting in hemorrhage into the CSF space to give the external appearance of a blood blister. Tc/+ mice also show an absence of bony vertebrae dorsally in the lumbosacral region, but they lack the dorsal blood blister, and the dermal layer overlying the bony defect retains its normal thickness; these observations describe a spina bifida occulta.

A frequent A?-persistence of fetal hemoglobin in northern Sardinia: its molecular basis and hematologic phenotype in heterozygotes and compound heterozygotes with ?-thalassemia

A survey of hemoglobinopathies in northern Sardinia revealed a high frequency (0.3%) of carriers of a hematologic condition characterized by increased expression of fetal hemoglobin during adult life (hereditary persistence of fetal hemoglobin or HPFH). In spite of a normal hematologic phenotype, the heterozygous carriers for this condition display about 12% HbF, almost exclusively of the A gamma type; compound heterozygotes with beta-thalassemia have 20%-26% HbF and run a very mild clinical course. The sequence analysis of the cloned A gamma gene linked to the HPFH determinant revealed the presence of a G----A substitution at position -117 of the A gamma-globin gene promoter; the same mutation occurs also in Greek HPFH, although associated with different restriction polymorphisms. Another hereditary condition characterized by increased HbF (alpha 2 A gamma 2) level and a mild thalassemia phenotype in Sardinia is associated with the -196C----T substitution in the A gamma-globin gene promoter (Sardinian delta beta-thalassemia). Population studies using oligonucleotides complementary both to the -117 G----A and -196C----T mutations and the corresponding normal sequences confirm the presence of these mutations only in HPFH and delta beta-thalassemia chromosomes and exclude these changes being common DNA polymorphisms.

Genetic analysis of "transvection" effects involving contrabithorax mutations in Drosophila melanogaster.

Contrabithorax (Cbx) alleles are dominant mutations in the Ultrabithorax (Ubx) gene of Drosophila that cause its ectopic expression in the mesothorax. We have studied the role of the homologous chromosome in the mesothoracic phenotype in several Cbx heterozygotes. None of the Cbx alleles studied shows variations in phenotype with extra doses of the Ubx gene. Only Cbx2 and CbxIRM (a revertant of Cbx1) show synapsis-dependent gene expression ("transvection"). The mesothoracic phenotypes of CbxIRM and Cbx2 heterozygotes are strongly modified when the homologous chromosome carries breakpoints proximal to or at the Ubx locus or null alleles of this gene. These lesions in the homologous chromosome enhance the Cbx2 mutant phenotype and reduce that of CbxIRM one. The genetic analysis of these transvection effects suggests that the transcription of the CbxIRM and Cbx2 alleles depends on RNAs of short radius of action from the homologous Ubx gene.

Genetic Variation in a Wintering Flock of the American Crow (Corvus brachyrhynchos)

Genetic Variation in a Wintering Flock of the American Crow (Corvus brachyrhynchos)

Hereditary thrombophilia: identification of nonsense and missense mutations in the protein C gene.

The structure of the gene for protein C, an anticoagulant serine protease, was analyzed in 29 unrelated patients with hereditary thrombophilia and protein C deficiency. Gene deletion(s) or gross rearrangement(s) was not demonstrable by Southern blot hybridization to cDNA probes. However, two unrelated patients showed a variant restriction pattern after Pvu II or BamHI digestion, due to mutations in the last exon: analysis of their pedigrees, including three or seven heterozygotes, respectively, with approximately 50% reduction of both enzymatic and antigen level, showed the abnormal restriction pattern in all heterozygous individuals, but not in normal relatives. Cloning of protein C gene and sequencing of the last exon allowed us to identify a nonsense and a missense mutation, respectively. In the first case, codon 306 (CGA, arginine) is mutated to an inframe stop codon, thus generating a new Pvu II recognition site. In the second case, a missense mutation in the BamHI palindrome (GGATCC----GCATCC) leads to substitution of a key amino acid (a tryptophan to cysteine substitution at position 402), invariantly conserved in eukaryotic serine proteases. These point mutations may explain the protein C-deficiency phenotype of heterozygotes in the two pedigrees.

Dominance is not Inevitable

In a diploid organism, a mutant gene that results in elimination of an enzyme activity in the homozygote is almost universally found to be recessive, so that the heterozygote phenotype is virtually indistinguishable from the wild type. It has been argued (H. Kacser & J. A. Burns, Genetics 97, 639-666 (1981)) that there is no need to look to evolution for an explanation of this phenomenon, as it is an inevitable consequence of the low control coefficients for metabolic flux possessed by nearly all enzymes. However, it is possible to envisage pathways in which every enzyme is more than half-saturated, so that moderate changes in the concentration of any enzyme result in substantial changes in metabolic flux. Such behaviour can occur, for example, if the limiting rates of the enzymes decrease as one proceeds along the pathway and the precursor concentration is large compared with the Michaelis constants of all the enzymes. Consequently one does require an explanation in terms of natural selection of why such pathways are apparently not observed in nature.

Evaluation of the nature of mildly prolonged prothrombin times.

Twenty-one asymptomatic individuals with a mildly prolonged prothrombin time (greater than 2 SD from the prothrombin time of the reference plasma) were found to have a mild isolated factor VII (F VII) defect (mean 38.8 U/dl; SD 13.2). Factor VII antigen levels were also found to be reduced (mean 45.5 U/dl; SD 7.8) in 13 of them. These figures were compared with those of 50 normals and 28 obligatory heterozygotes for F VII deficiency. The phenotypical behaviors in the propositi were found to be equal to those of the F VII congenital deficiency heterozygotes: the discrepant one (VII+) and the nondiscrepant one (VII-/R). Fifteen families of the propositi could also be studied, totalling 55 additional individuals; in 25 of them (ten pedigrees) a mild F VII deficiency was found showing the same phenotypical features of the corresponding propositi. We therefore believe that these individuals with mild F VII deficiency can be considered as heterozygotes for the defect, since the other vitamin K-dependent clotting factors were normal; the defect is transmitted throughout the kindred with the same mode of inheritance as F VII congenital deficiency; and F VII:C and F VII:Ag levels are highly comparable with those of known obligatory heterozygotes for F VII deficiency. On the grounds of a careful statistical analysis we propose a formula which allows a discrimination between the two phenotypes of the heterozygotes for F VII congenital deficiency. In addition it is suggested that sensitive tissue thromboplastins should be used to pick up these mild defects.

Biochemical polymorphism in Parascaris equorum, Toxocara canis and Toxocara cati

Vertical starch gel electrophoresis was used to resolve proteins encoded by 18 gene loci in ascaridoid nematodes. Estimates of genetic variability were made from population samples of the dog ascarid ( Toxocara canis), cat ascarid ( Toxocara cati), and the horse ascarid ( Parascaris equorum). Levels of polymorphism and mean heterozygosity were high, which is not consistent with the hypothesis that the intestinal environment selects for monomorphism among endoparasites. Most observed allele frequencies conformed to Hardy-Weinberg equilibrium expectations as tested by χ 2 goodness-of-fit, suggesting that the proteins evaluated are inherited in a Mendelian fashion and that these nematodes are mating at random. Subunit structures of the following enzymes, deduced from electrophoretic phenotypes of heterozygotes, corresponded to those of vertebrates: lactate dehydrogenase; malate dehydrogenase; 6-phosphogluconate dehydrogenase; phosphoglucomutase; esterase D; peptidase B; peptidase D; and mannose-6-phosphate isomerase. This observation substantiates the conservative nature of polypeptide subunit number across phylogenetically diverse groups of organisms.

Autosomal dominant inheritance of hereditary canine spinal muscular atrophy.

Hereditary canine spinal muscular atrophy ( HCSMA ) is a motor neuron disease in Brittany spaniels. Three phenotypes are recognized (accelerated, intermediate, and chronic) and are distinguished on the basis of rate of progression and age at onset. Breeding studies within a kindred of more than 125 dogs (Brittany spaniel and beagle-Brittany outcrosses ) have established an autosomal dominant inheritance for HCSMA . Pups homozygous for the trait have accelerated disease whereas heterozygous dogs have intermediate or chronic disease. The reason for the two phenotypes in heterozygotes is under study. HCSMA provides a unique opportunity to study the genetic and pathophysiological mechanisms of a motor neuron disease, and findings may have broad relevance to investigations of autosomal dominant degenerative disorders of the central nervous system.

A genetic analysis of the pteridine biosynthetic enzyme, guanosine triphosphate cyclohydrolase, in Drosophila melanogaster.

Strains with mutant eye color were surveyed for levels of GTP cyclohydrolase (GTP CH), the first enzyme acting in the biosynthesis of pteridines, the pigments causing red eye color in Drosophila. Six strains were found to have reduced GTP CH activity. In five of the six strains, the reduction of activity is apparent only in the adult head of homozygous mutants. We show that mutations in Punch (2-97, Pu) have severe effects on GTP CH activity. In most cases, the reduction of activity is apparent in all tissues and stages that express the enzyme. The activity of GTP CH is shown to be closely correlated with the number of Pu+ genes in the genome. One ethyl methanesulfonate (EMS)-induced Pu mutant has a GTP CH enzyme that is unstable when compared with the wild-type enzyme. Mutations in Pu fall into three general classes. The largest class has a recessive lethal and eye color phenotype, 50% or higher GTP CH activity in heterozygotes, and equivalent defects in all tissues. A second class is dominant in eye color phenotype and recessive lethal, with less than 50% GTP CH activity in heterozygotes. The third class is homozygous viable and has severe reduction of activity in the adult head, but no or less severe loss in other tissues.

The Litoria ewingi Complex (Anura : Hylidae) in South-Eastern Australia VI.* Geographic Variation in Transferrins of Four Taxa

Transfenins are highly polymorphic in taxa of the L, ewingi complex. Nine transferrins with different electrophoretic mobilities occur in L. ewingi, and four in each of the other taxa studied. Populations of L. paraewingi are relatively similar for transferrin, but in L, ewingi, L. v. verreauxi and L. v. alpina there is geographic differentiation of transferrins across the range. In L. ewingi, four areas can be recognized on the basis of transferrin composition of populations and by changes in transferrin gene frequencies. In L. v. verreauxi, two areas can be recognized: a western area in which populations are monomorphic for transferrin, and an eastern and northern area in which populations are polymorphic. Two areas are recognized in L. v. alpina. Individuals from two geographic isolates have a 'fixed heterozygote' transferrin phenotype; population samples from the remainder of the range are polymorphic, but the fixed heterozygote phenotype does not occur there. Geographic isolation of L. ewingi in Flinders I. and King I. and in Tasmania for more than 9000 y has not resulted in marked differences in the transferrin compositions of these populations compared with those from the southern mainland.

Rescue of a lethal T/t locus genotype by chimaerism with normal embryos.

THE mutation THp at the T/t locus in the mouse is unique among mammalian genes because it has a sharply defined maternal effect. Like other known T/t locus dominant mutations, THp yields a short-tailed phenotype in heterozygotes of both sexes. However, THp/+ females when mated with wild type fail to give birth to short–tailed progeny; examination of embryos shows that heterozygous embryos die late in gestation with seemingly minor defects such as spina bifida and polydactyly1–3. The precise reason for death has not been determined, but must depend on some defect intrinsic to the egg, because THp/+ females successfully carry to term heterozygous embryos whose THp gene is paternally derived. I report here that lethal THp/+ maternal heterozygotes are readily rescued if they are aggregated with normal embryos, but that the germ cell defect persists, because the chimaeras fail, if female, to transmit THp to viable offspring.

Synthesis of haemoglobin Wayne in erythroid cells.

HETEROZYGOTES for haemoglobin (Hb) Wayne possess two minor haemoglobin components that migrate more rapidly than HbA on electrophoresis at pH8.6. Each of the minor haemoglobin components contains an abnormal α chain in which the carboxyl-terminal tripeptide sequence, Lys-Tyr-Arg, has been replaced by an octapeptide1. The slower of the two components, henceforth designated Wayne-Asn, has the following octapeptide sequence: Asn-Thr-Val-Lys-Leu-Glu-Pro-Arg, whereas the faster one (Wayne-Asp) has exactly the same sequence except that asparagine at position 139 is replaced by aspartic acid. (These components, formerly designated1 Wayne-1 and Wayne-2, respectively, have been renamed for clarity.) This is the first variant described in which deamidation of the gene product is believed to occur. The purpose of this study was to demonstrate the proposed deamidation and to explain another interesting feature of the Hb Wayne phenotype in heterozygotes; namely, the presence of the variant haemoglobins in markedly reduced quantities relative to HbA (3% and 4%, respectively, for Hb Wayne-Asn and Hb Wayne-Asp).

Are macronuclear subunits in Paramecium functionally diploid?

SUMMARYThe organization of the genetic material in the macronucleus of ciliates has been the subject of considerable controversy. Two of the four models of macronuclear structure predict assortment of the alternative phenotypes of heterozygotes during vegetative growth. Early studies of the phenotypic behaviour of heterozygotes after macronuclear regeneration (Sonneborn, 1947) had supported a diploid subunit model. The availability of quantitative predictions of the rate of assortment for the haploid and chromosomal models (Preer, this volume) and the existence of two alleles controlling a quantitative trait, copper tolerance, inParamecium tetraurelia, has provided an opportunity to test these models. The median tolerance limits to copper of unselected sublines were measured as a function of age. There was no increase in the variance among sublines, as the haploid and chromosomal models predict. Quantitative evaluation shows that subdiploid models with a kinetic complexity of 860 or less are not compatible with the results. This experiment was not sensitive enough, however, to exclude subdiploid models if the kinetic complexity is 2000 or greater. Selection on heterozygotes also failed to provide evidence in favour of assortment. All the results are consistent with and support the diploid subunit model of theParameciummacronucleus.

Abnormal hemoglobinopathies in a University student population

Abnormal hemoglobins were found in 1.26% of 500 University of Costa Rica freshmen (1.06% A-S phenotype and 0.20% A-C) . An important factor is the discovery of 0.24% of abnormal hemoglobins, primarily due the S gene, in Caucasian Costa Rican students, or 0.5 % if three cases, apparently corresponding to a heterozygote beta thalassemia with a high F hemoglobin ( delta-beta variant), are included. Two asymptoma tic heterozygote phenotypes of persistent fetal hemoglobin (one in a white student an the other in a mulatto) were found among the abnormal hemoglobins. The differences between Costa Rican and foreign students, based on nationality and racial groups, are briefly commented.

Alpha1-Antitrypsin: 1. Evidence for two genes causing low concentrations in serum; 2. Association of heterozygosity and Chronic obstructive lung disease

Alpha1-antitrypsin (alpha1-at) is one of several proteinase inhibitors in human serum. It is a glycoprotein with molecular weight of approximately 50,000. The normal concentration in serum is 213±32 mg/100 ml. The corresponding trypsin inhibiting capacity is between 1.0 and 1.3 mg of trypsin inhibited by one ml of serum. During recent years, different phenotypes of alpha1-at have been identified and a genetic model with several codominant alleles at one locus has been developed. Two phenotypes can be distinguished quantitatively and electrophoretically while others can be recognized by electrophoresis only. I consider here only the phenotypes with lower than normal alpha1-at concentrations in serum. The method most commonly used to determine the total trypsin inhibiting capacity of serum is an inhibition assay with H,α-benzoyl-arginine-p-nitroanilide as substrate for trypsin. A convenient method for a specific determination of alpha1-at is the radial immunodiffusion employing an antiserum against alpha1-at. For the electrophoretic distinction of the different phenotypes starch gel electrophoresis at an acidic pH (4.95) in combination with a second electrophoresis at an alkaline pH (8.65) in agarose gel that contains an antiserum at alpha1-at is a sensitive method. With this technique, the alpha1-at phenotypes of heterozygotes and homozygotes for the two genes, Pin and Pz (nomenclature of Fagerhol), that cause lower concentrations of serum alpha1-at, can be recognized as a characteristic pattern of bands. The protein that is determined by the gene Piz moves on electrophoresis more slowly towards the anode than that determined by Pi”, but both are moving slower than the normal protein controlled by the most common gene PiM. In heterozygotes, both patterns of alpha1-at can be recognized. The resulting phenotypes are called: M/M, S/S, Z/Z, M/S, M/Z, etc. The alpha1-at concentrations for these phenotypes are in mg/100 ml of serum: M/M; 213±32, S/S; 122±10, Z/Z; 25±6, M/S; 159±39, M/Z; 125±46. These concentrations are only found under normal physiologic conditions. In the presence of an inflammation, during pregnancy, and due to certain steroid hormones, the alpha1-at concentration of normal individuals can rise to levels 100 percent above the starting value, and heterozygotes can show values in the normal range of 200 mg percent. The electrophoretic characteristics are, however, independent of concentration and for that reason the electrophoretic determination of the alpha1-at phenotype is more reliable than the quantitative one, particularly in patients with inflammation.

Threshold phenomena versus cell heredity in the manifestation of sex-linked genes in mammals

ABSTRACT The mode of action of sex-linked genes in mammals has been the subject of much discussion in recent years. This has centred round the concept that in the mammalian female, either the maternal or the paternal X-chromosome is randomly and irreversibly inactivated during development, with the result that in the female, as in the male, one X-chromosome only is active in a given cell (Lyon, 1966, and earlier papers). The Lyon Hypothesis (L.H.) is based, in part, on the mottled or striped phenotypes of heterozygotes for certain sexlinked genes affecting the coat of the mouse and other mammals: these phenotypes are regarded as a mixture of clones descended from cells in which either one or the other X-chromosome has been inactivated. As Baker (1967) rightly insists, the interpretation of a pattern as clonal must be based on known cell lineages, as without such a basis one simply postulates what should be put to the test.