Pedigree Analysis: Introduction PDF

Text Introduction: Pedigree is a chart showing a record of inheritance of certain traits for two or more ancestral generation of human beings or domesticated animals in the form of a diagram or family tree. Pedigree analysis is a system of analysis by following the movement and distribution of cert...

Text Introduction: Pedigree is a chart showing a record of inheritance of certain traits for two or more ancestral generation of human beings or domesticated animals in the form of a diagram or family tree. Pedigree analysis is a system of analysis by following the movement and distribution of certain genetic traits. In pedigree, symbols represent people and lines represent genetic relationships. The pedigree is a visual tool for documenting the biological relationships in families and determining the mode of inheritance (dominant, recessive etc.) of genetic diseases. Pedigrees are most often constructed by medical geneticists or genetic counselors. Pedigree is a chart showing a record of inheritance of certain traits for two or more ancestral generation of human beings or domesticated animals in the form of a diagram or family tree. Pedigree analysis is a system of analysis by following the movement and distribution of certain genetic traits, this system has following conventions: However in real life situations various dilations distinguish from basic Mendelian pattern, some of them are discussed here: 1.1) Nonpenetrance: Nonpenetrance is the failure of a dominant condition to manifest itself. With dominant condition, nonpenetrance is a frequent complication. The penetrance of a character, for a given genotype, is defined as the probability that a person who has the genotype will manifest the character. By definition the, a dominant character is manifest in a heterozygous person and so should show 100% penetrance. However many characters fail to express themselves in a generation which would otherwise be expressed as dominant character. Very often the presence or absence of a character depends, in the main and in normal circumstances, on the genotype at one locus, but an unusual genetic background, particular lifestyle or may be just chance means that the occasional person may fail to manifest the character. Nonpanetrance is a major pitfall in genetic counseling. Nonpenetrance and variable expression are typically problems with dominant, rather than recessive, characters. Figure: autosomal dominant inheritance with nonpenetrance at II2 1.2) Variable expressivity: Expressivity quantifies variation in a non-binary phenotype across individuals carrying a particular genotype. It is equal to the proportion of individual’s carriers of a genotype for a trait who show the trait to a specifiable extent. This differs from penetrance, the term that measures how often a gene generates its associated phenotype to any extent that makes an individual carrier different from the wild type. Expressivity therefore characterizes non-binary qualitatively or quantitatively the extent of phenotypic variation within a particular genotype. With binary phenotypes (e.g., albino vs. wild type) expressivity and penetrance are the same. The term is analogous to the severity of a condition in clinical medicine. For example, the volume of blood ejected from the pumping heart with each contraction, relative to the total amount of blood contained in the heart's chamber can be quantified by echocardiography and is called the ejection fraction. If a specific genotype is associated with the development of congestive heart failure, the expressivity would be represented by the range of ejection fractions seen in patients that have that genotype. Expressivity is measured only when there is 100% penetrance. Variable expressivity occurs when a phenotype is expressed to a different degree among individuals with the same genotype. For example, individuals with the same allele for a gene involved in a quantitative trait like body height might have large variance (some are taller than others), making prediction of the phenotype from a particular genotype alone difficult. The expression of a phenotype may be modified by the effects of aging, other genetic loci, or environmental factors. Another example is neurofibromatosis, where patients with the same genetic mutation show different signs and symptoms of the disease. 1.3) Pleiotropy: The term pleiotropy comes from the Greek pleion, meaning "more", and tropos, meaning "way". Thus, pleiotropy means "more change"; a pleiotropic gene causes more than one phenotype to change. The term "pleiotropie" was coined in a 1910 Festschrift written by the German geneticist Ludwig Plate, a former student of Ernst Haeckel. Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously. Even though a structural gene may have many end effects, it usually has only one primary function, that of producing one polypeptide, this polypeptide may give rise to different expressions at the phenotypic level. Thus, pleiotropy describes the genetic effect of a single gene on multiple phenotypic traits. It is not essential that all the traits are equally influenced. Sometimes the effect of a pleiotropic gene is more evident in case of one trait (major effect) and less evident in case of others (secondary effect). Occasionally a number of related changes are caused by a gene. They are together called syndrome. The underlying mechanism is that the gene codes for a product that is, for example, used by various cells, or has a signaling function on various targets. Examples: Phenylketonuria: A classic example of pleiotropy is the human disease phenylketonuria (PKU). This disease can cause mental retardation and reduced hair and skin pigmentation, and can be caused by any of a large number of mutations in a single gene that codes for the enzyme phenylalanine hydroxylase, which converts the amino acid phenylalanine to tyrosine, another amino acid. Depending on the mutation involved, conversion of phenylalanine to tyrosine is reduced or ceases entirely. Unconverted phenylalanine concentrates in the bloodstream and can rise to levels that are toxic to the developing nervous system of newborn and infant children and which can cause effects such as mental retardation and abnormal gait posture. Because tyrosine is used by the body to make melanin (an important component of the pigment found in hair and skin) the failure to convert normal levels of phenylalanine to tyrosine results in less pigmentation being produced causing the fair hair and skin typically associated with phenylketonuria. Sickle-cell anemia: Sickling of erythrocytes occurs due to mutation in the beta- chain of hemoglobin. It not only causes hemolytic anemia but also has a following other phenotypic effects: i. The sickled RBCs are destroyed by the liver, causing hemolytic anemia. The phenotypic effects of this anemia include physical weakness, below average development and hypertrophy of bone marrow. ii. The sickled cells cause the interference of capillary blood flow by clumping of the odd-shaped cells, resulting in damage to every major organ. The individual can suffer pain, heart failure, rheumatism and other effects. iii. In heterozygous condition, it provides resistance to malaria caused by Plasmodium falciparum. Cystic fibrosis: It is a hereditary metabolic disorder in children that is controlled by a single autosomal recessive gene. The gene specifies an enzyme that produces a glycoprotein, which results in the production of mucus with abnormally high viscosity. This mucus interferes with the normal functioning of several exocrine glands including those in skin, lungs, liver and pancreas. Secreting cells in the liver and pancreas are damaged, decreasing the production of fat-emulsifying agents and digestive enzymes and thus interfering with digestion and absorption of food. 1.4) Late onset: Genetic conditions are not necessarily congenital. The genotype is fixed at conception, but the phenotype may not persist until adult life. In such cases the penetrance is age- related. Huntington disease is a well known example. Delayed onset might be caused by slow accumulation of a noxious substance, by slow tissue death or by inability to repair some form of environmental damage. Hereditary cancers are caused by a second mutation affecting a cell of a person who already carries one mutation in a tumor suppressor gene. Depending on the disease, the penetrance may become 100% if the person lives long enough, or there may be people who carry the gene but who will never develop symptoms no matter how long they live. Age- of-onset curves are important tools in genetics, because they enable the geneticists to estimate the chance that an at- risk but asymptomatic person will subsequently develop the disease. Figure: Age of onset curve for Huntington disease. Curve A: probability that an individual carrying the disease gene will have developed symptoms by a given age. Curve B: risk that a healthy child of an affected parent carries the disease gene at a give age. (Harper, 2001) 1.5) Dominance problems: According to Mendel’s law of dominance, “when a pair of alleles or allelomorphs are brought together in F1 hybrid, then only one of them expresses itself, masking the expression of other completely. Example in Tt F1 hybrid, ‘T’ is expressed ------ so dominant ‘t’ is masked ----------- so recessive Exceptions to principle of dominance: A) Incomplete dominance (or partial or mosaic dominance) When a dominant allele does not mask completely the phenotypic expression of the recessive allele in a heterozygote then a blending of both dominant and recessive traits takes place in the F1 and F2 heterozygotes, this phenomenon is known as incomplete dominance. In such cases the blending occurs only in the phenotype of the F1 heterozygotes and the alleles maintain their individual identities and segregate from each other during gametogenesis. The F1 gametes produce F2 progeny having the phenotypic and genotypic ratios of 1:2:1. Example, in Mirabilis jalapa there are two types of flower color in pure state: red and white. When a homozygous red flowered plant is crossed with a homozygous white flowered plant, the F1 heterozygotes are found to have pink flowers. B) Co-dominance It is the phenomenon of two alleles lacking dominant-recessive relationship and both expressing themselves in the organism. Example, the alleles governing the M-N blood group system in humans are co-dominants and may be represented by the symbols LM and LN, here three blood groups are possible- M, N and MN and these are determined by the genotypes LM LM, LN LN, LM LN, respectively (given by land Steiner and Levine in 1927). Blood groups actually represent the presence of an immunological antigen on the surface of RBCs. People of LM LN genotype have both antigens. C) Multiple alleles They are multiple forms of a Mendelian factor or genes which occur on the same gene locus, distributed in different organisms in the gene pool with an organism  carrying only two alleles and gamete only one allele. Example, ABO blood group system in human beings shows multiple alleles. Human beings have six genotypes and four blood groups or blood group phenotypes- A, B, AB and O. The blood groups are determined by two types of antigens (glycoprotein) present in the surface coating of red blood cells-A and B. Blood group A persons have antigen A, group B have antigen B, AB have both antigens while blood group O persons do not carry any antigen in the coating of their erythrocytes. In human population, three different alleles for ABO blood group system are found IA, IB and IO or i. IA and IB are mutant alleles and are dominant over IO or i which is a wild allele. IA and IB are responsible for A and B antigens while IO or i do not produce any of these A or B antigens. A person is having only two of these three alleles and blood group type can be determined by their antigen types. The genotypic combinations possible are given as: Blood Type Genotype Antigen Antibodies (Phenotype) A IA IA or IA Io A b B IB IB or IB IO B a AB IA IB both A and B neither a nor b O IO IO neither A nor B both a and b D) Heterodominance When the heterozygotes have a more extreme phenotype than either of the corresponding homozygotes (homozygous parents) then it is usually referred to as overdominance, superdominance or heterodominance. For example, there is heterodominance when the heterozygote Aa between a pair of factors which control size s bigger than the homozygotes AA or aa. This type of allelic relationship which implies interaction between the alleles, or of these with other factors of the genotype, may be found in qualitative characters and especially those such as size, production, vigour etc which are of importance in the breeding of animals and plants. 1.6) Anticipation: The signs and symptoms of some genetic conditions tend to become more severe and appear at an earlier age as the disorder is passed from one generation to the next. This phenomenon is called anticipation. Anticipation is a phenomenon whereby as a genetic disorder is passed on to the next generation, the symptoms of the genetic disorder become apparent at an earlier age with each generation. Anticipation describes the tendency of some variable dominant conditions to become more severe in successive generations. In most cases, an increase of severity of symptoms is also noted. Anticipation typically occurs with disorders that are caused by an unusual type of mutation called a trinucleotide repeat expansion. A trinucleotide repeat is a sequence of three DNA building blocks (nucleotides) that is repeated a number of times in a row. DNA segments with an abnormal number of these repeats are unstable and prone to errors during cell division. The number of repeats can change as the gene is passed from parent to child. If the number of repeats increases, it is known as a trinucleotide repeat expansion. In some cases, the trinucleotide repeat may expand until the gene stops functioning normally. This expansion causes the features of some disorders to become more severe with each successive generation. Most genetic disorders have signs and symptoms that differ among affected individuals, including affected people in the same family. Not all of these differences can be explained by anticipation. A combination of genetic, environmental, and lifestyle factors is probably responsible for the variability, although many of these factors have not been identified. Researchers study multiple generations of affected family members and consider the genetic cause of a disorder before determining that it shows anticipation. Anticipation is common in trinucleotide repeat disorders, such as Huntington's disease and myotonic dystrophy, where a dynamic mutation in DNA occurs. All of these diseases have neurological symptoms. Trinucleotide repeats and expansion: Trinucleotide repeats are apparent in a number of loci in the human genome. They have been found in introns, exons and 5' or 3' UTR's. They consist of a pattern of three nucleotides (e.g. CGG) which is repeated a number of times. During meiosis, unstable repeats can undergo triplet expansion; in this case, the germ cells produced have a greater number of repeats than are found in the somatic tissues. The mechanism behind the expansion of the triplet repeats is not well understood. One hypothesis is that the increasing number of repeats influence the overall shape of the DNA, which can have an effect on its interaction with DNA polymerase and thus the expression of the gene. Disease mechanisms: For many of the loci, trinucleotide expansion is harmless, but in some areas expansion has detrimental effects that cause symptoms. When the trinucleotide repeat is present within the protein-coding region, the repeat expansion leads to production of a mutant protein with gain of function. This is the case for Huntington's disease, where the trinucleotide repeat encodes a long stretch of glutamine residues. When the repeat is present in an untranslated region, it could affect the expression of the gene in which the repeat is found (ex. fragile X) or many genes through a dominant negative effect (ex. myotonic dystrophy). In order to have a deleterious effect, the number of repeats must cross a certain threshold. For example, normal individuals have between 5 and 30 CTG repeats within the 3' UTR of DMPK, the gene that is altered in myotonic dystrophy. If the number of repeats becomes greater than 50, the person is only mildly affected – perhaps having only cataracts. However, meiotic instability could result in a dynamic mutation that increases the number of repeats in offspring inheriting the mutant allele. Once the number of copies reaches over 100, the disease will manifest earlier in life (although the individual will still reach adulthood before the symptoms are evident) and the symptoms will be more severe – including electrical myotonia. As the number progresses upwards past 400, the symptoms show themselves during childhood or infancy. Examples of diseases showing anticipation: Autosomal dominant Several spinocerebellar ataxias Huntington's disease – CAG  Myotonic dystrophy – CTG Dyskeratosis congenita – TTAGGG (telomere repeat sequence) Autosomal recessive Friedreich ataxia – GAA X-linked Fragile X syndrome – CGG 1.7) Genetic heterogeneity: Most Mendelian diseases were originally classified from their phenotype, in other words their clinical manifestations. As molecular causes have been identified for these diseases, it has turned out that in many cases the same or a similar phenotype can result from a different underlying genotype. This phenomenon is known as genetic heterogeneity. Thus, Genetic heterogeneity is a phenomenon in which a single phenotype or genetic disorder may be caused by any one of a multiple number of alleles or non-allele (locus) mutations. This is in contrast to pleiotropy, where a single gene may cause multiple phenotypic expressions or disorders. Genetic heterogeneity describes genetic variation from the normal population. Clinically, genetic heterogeneity refers to diseases that result from multiple gene abnormalities. An example is autosomal dominant polycystic kidney disease, which in most affected families is caused by mutation of a gene on chromosome 16 but can also arise from mutation of a different gene, on chromosome 4. When different mutations at the same locus cause the same disease, this is known as allelic heterogeneity. Multiple gene abnormalities are seen in disorders such as autism, cystic fibrosis, and retinitis pigmentosa. Genetic Heterogeneity is responsible for the presence of many medical disorders in humans. Heritable diseases are a result of a genotype that varies from the population standard. In relation to diseases, one gene mutation (varying from population) can cause a phenotypic disorder. The mutation can be expressed differently in different individuals. Additionally, mutations in multiple genes can result in phenotype of one disorder. An inherited predisposition for the development of breast cancer has been investigated. Multiple alleles are involved in this disease. 1.8) Uniparental disomy Uniparental disomy (UPD) occurs when a person receives two copies of a chromosome, or part of a chromosome, from one parent and no copies from the other parent. UPD can occur as a random event during the formation of egg or sperm cells or may happen early fetal development. UPD can be the result of heterodisomy, in which a pair of non- identical chromosomes are inherited from one parent (an earlier stage meiosis I error) or isodisomy, in which a single chromosome from one parent is duplicated (a later stage meiosis II error). Uniparental disomy may have clinical relevance for several reasons. For example, either isodisomy or heterodisomy can disrupt parent-specific genomic imprinting, resulting in imprinting disorders. Additionally, isodisomy leads to large blocks of homozygosity, which may lead to the uncovering of recessive genes, a similar phenomenon seen in children of consanguineous partners. In many cases, UPD likely has no effect on health or development. Because most genes are not imprinted, it doesn’t matter if a person inherits both copies from one parent instead of one copy from each parent. In some cases, however, it does make a difference whether a gene is inherited from a person’s mother or father. A person with UPD may lack any active copies of essential genes that undergo genomic imprinting. This loss of gene function can lead to delayed development, intellectual disability, or other health problems. Several genetic disorders can result from UPD or a disruption of normal genomic imprinting. The most well-known conditions include Prader-Willi syndrome, which is characterized by uncontrolled eating and obesity, and Angelman syndrome, which causes intellectual disability and impaired speech. Both of these disorders can be caused by UPD or other errors in imprinting involving genes on the long arm of chromosome 15. Other conditions, such as Beckwith-Wiedemann syndrome (a disorder characterized by accelerated growth and an increased risk of cancerous tumors), are associated with abnormalities of imprinted genes on the short arm of chromosome 11. 1.9) Spontaneous mutation A sudden and heritable change in the sequence of an organism’s genome that gives rise to alternate forms of any gene is called mutation. It can be simply be put as an abrupt change in the genotype of an organism that is not the result of recombination. Spontaneous mutations are those mutations that occur without a known cause. They may be truly spontaneous, resulting from an inherent low level of metabolic errors, that is, mistakes during DNA replication or they may actually be caused by mutagenic agents present in the environment. Spontaneous mutations arise from a variety of sources, including errors in DNA replication, spontaneous lesions, and transposable genetic elements. A): Error in DNA replication a. Tautomeric shift Each of the bases in DNA can appear in one of several forms, called tautomers, which are isomers that differ in the positions of their atoms and in the bonds between the atoms. The forms are in equilibrium. Formation of tautomer of any base alters its base pairing properties. The more stable keto forms of thymine and guanine and amino forms of adenine and cytosine may infrequently undergo tautomeric shifts to less stable enol and imino forms, respectively. The basis would be expected to exist in their less stable tautomeric form for only very short period of time. However, if a base existed in the rear tautomeric form at the movement that it was being replicated or being incorporated into a nascent DNA chain, a mutation might result. In tautomeric form the basis can form adenine-cytosine and guanine thymine base pairs. b. Substitution mutation Substitution mutation is also known as point mutation, simple mutation or single site mutation. It is a gene mutation that results from the substitution of one base pair from another (or one base from another in the case of single-strand DNA genomes) is known as Substitution mutation. It may be:  I. Transition: the most common class, comprising the substitution of one pyrimidine by the other, or of one purine by the other. This replaces a G-C pair with an A-T or vice versa. II. Transversion: In transversion purine is replaced by pyrimidine, or vice versa c. Frameshift mutation Aberrant replication can also result in small numbers of extra nucleotides being inserted into the polynucleotide being synthesized, or some nucleotides in the template not being copied. Addition or deletion of base pair that occurs within the protein-coding portion of a gene has the effect of shifting the translational reading frame. In majority of the cases, these results in a failure to synthesize a functional protein, thus, allowing the mutation to be identified by its phenotypic consequences. Because these mutations cause a shift in the translational reading frame, they are termed frameshift mutations. For example, the human HD gene contains the sequence 5′-CAG-3′ repeated between 6 and 35 times in tandem, coding for a series of glutamines in the protein product. In the case of Huntington′s disease this repeat expands to a copy no. of 36-121 resulting in dysfunctional protein. B): Spontaneous lesion: In addition to replication errors, spontaneous lesions, naturally occurring damage to the DNA, can generate mutations. Two of the most frequent spontaneous lesions result from depurination and deamination. a. Depurination and depyrimidination Depurination, the more common of the two, consists of the interruption of the glycosidic bond between the base and deoxyribose and the subsequent loss of a guanine or an adenine residue from the DNA, resulting in significant genetic damage because, in replication, the resulting apurinic sites cannot specify a base complementary to the original purine. Similarly, depyrimidination is the loss of pyrimidine base. b. Deamination It is the removal of an amino group from a molecule. Three of the fore nitrogenous basis normally present in DNA (cytosine, adenine and guanine) contain exocyclic amino group. The deamination of cytosine yields uracil. Unrepaired uracil residues will pair with adenine in replication, resulting in the conversion of a G–C pair into an A–T pair (a GC → AT transition). Deamination of adenine and guanine results in the formation of hypoxanthine and xanthine, respectively. Deamination changes the standard base pairing patterns. For example, xanthine pairs with thymine instead of cytosine and hypoxanthine selectively pairs with cytosine instead of thymine. C) Transposable genetic elements: Some segments in a genome cane move quite readily from one place to another. The movement of those elements has effects on the phenotype of the organism, primarily at the transcriptional level. Transposable elements (also called as mobile genes, jumping genes, or transposon) are segments of DNA that can move from one position in a genome to another. They were first discovered by Barbara Mc Clintock in 1940s. This DNA segment translocates to other sites in the genome essentially independent of sequence homology. Usually such elements are flanked by short, inverted repeats of 20 to 40 base pairs at each end. Insertion repeats into a structural gene can produce a mutant phenotype, often inactivating the gene into which they become inserted. Insertion and excision of transposable elements depend on two enzymes, transposase and resolvase. 1.10) X inactivation X-inactivation (also called lyonization) is a process by which one of the copies of the X chromosome present in female mammals is inactivated. The inactive X chromosome is silenced by its being packaged in such a way that it has a transcriptionally inactive structure called heterochromatin. As nearly all female mammals have two X chromosomes, X-inactivation prevents them from having twice as many X chromosome gene products as males, who only possess a single copy of the X chromosome. The choice of which X chromosome will be inactivated is random in placental mammals such as humans, but once an X chromosome is inactivated it will remain inactive throughout the lifetime of the cell and its descendants in the organism. Unlike the random X- inactivation in placental mammals, inactivation in marsupials applies exclusively to the paternally derived X chromosome. In 1959 Susumu Ohno showed that the two X-chromosomes of mammals were different: one appeared similar to the autosomes; the other was condensed and heterochromatic. This finding suggested, independently to two groups of investigators, that one of the X-chromosomes underwent inactivation. In 1961, Mary Lyon proposed the random inactivation of one female X chromosome to explain the mottled phenotype of female mice heterozygous for coat color genes. The Lyon hypothesis also accounted for the findings that one copy of the X chromosome in female cells was highly condensed, and that mice with only one copy of the X chromosome developed as infertile females. This suggested to Ernest Beutler, studying heterozygous females for Glucose-6-phosphate dehydrogenase (G6PD) deficiency, that there were two red cell populations of erythrocytes in such heterozygotes: deficient cells and normal cells, depending on whether the inactivated X chromosome contains the normal or defective G6PD allele. It has been demonstrated that in homogametic XX female individuals, one X chromosome gets characteristically condensed and inactivated. Such chromatin material is called facultative heterochromatin, since it becomes inactive in certain part of the life cycle and resumes activity before entering the germ line. The phenomenon of inactivation of X chromosome was confirmed by the observation of the barr body. XX females have one Barr body per cell, XXX females have 2 Barr bodies per cell, and XXY Klinefelter males have one Barr body per cell. Inactivation of X-chromosome is initiated by a non coding RNA molecule (rather than a protein) that is transcribed from one of the genes (called XIST in humans) on X-chromosome that becomes inactivated. 1.11) Dosage compensation Dosage compensation is the process by which the expression levels of sex-linked genes are altered in one sex to offset a difference in sex chromosomes number between females and males of a heterogametic species. It was first discovered by Muller in Drosophila. Across species, different sexes are often characterized by different types and numbers of sex chromosomes. In order to account for varying numbers of sex chromosomes, different organisms have acquired unique methods to equalize gene expression amongst the sexes. Dosage Compensation is a term that describes the processes by which organisms equalize the expression of genes between members of different biological sexes. Because sex chromosomes contain different numbers of genes, different species of organisms have developed different mechanisms to cope with this inequality. Replicating the actual gene is impossible; thus organisms instead equalize the expression from each gene. For example, in humans, females (XX) silence the transcription of one X chromosome of each pair, and transcribe all information from the other, expressed X chromosome. Thus, human females have the same number of expressed X-linked genes as do human males (XY), both genders having essentially one X chromosome per cell, from which to transcribe and express genes. There are three main mechanisms of achieving dosage compensation which are widely documented in the literature and which are common to most species. These include random inactivation of one female X chromosome (as observed in Mus musculus), a two-fold increase in the transcription of a single male X chromosome (as observed in Drosophila melanogaster), and decreased transcription by half in both of the X chromosomes of a hermaphroditic organism (as observed in Caenorhabditis elegans). However, there are also other less common forms of dosage compensation, which are not as widely researched and are sometimes specific to only one species (as observed in certain bird and monotreme species). In Drosophila melanogaster, X chromosome is not inactivated, but both X chromosomes are transcribed in females. Compensation occurs by making the single X chromosome in male hyperactive. As a result, the level of transcription of the genes carried by the X chromosome is the same in both sexes. It is mediated by the products of male-specific lethal (msl) genes. In C. elegance (males, XO and hermaphrodites, XX), dosage compensation is achieved by down-regulating transcription of genes on both hermaphrodite X chromosomes.