What are silent carrier traits?

What are silent carrier traits?

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I couldn't find a definition but I have come to know the following from google search:

People who are carriers of a thalassemia gene show no thalassemia symptoms and might not know they're carriers.

So is thalassemia a "silent carrier trait"?

Thalassemia is a recessive trait. That means individuals with only one copy of the allele (Heterozygotes) have the "normal" phenotype (they don't have Thalassemia).

If an individual is homozygous for the recessive allele they will have Thalassemia.

Heterozygotes are often referred to as carriers as they can transmit the allele to their offspring, but do not themselves express the trait.

"Silent carrier trait" is quite unusual terminology, just call it a recessive trait.

Alpha Thalassemia Trait

Alpha thalassemia is common in people of African, Southern Chinese, Southeast Asian, Middle Eastern and Mediterranean descent.

  • Alpha thalassemia affects the amount of hemoglobin in the red blood cells.
  • Adult hemoglobin (hemoglobin A) is made of alpha and beta globins.
  • Normally, people have four genes for alpha globin with two genes on each chromosome (αα/αα).
  • People with alpha thalassemia trait only have two genes for alpha globin, so they make slightly lower amounts of hemoglobin.
  • All red blood cells contain hemoglobin, which carries oxygen from the lungs to all parts of the body.

1. First, it’s important to keep in mind that “asymptomatic” is different from “presymptomatic.”

Being presymptomatic means you’ve been infected and don’t feel any symptoms at the time you get tested, but will develop them later on. In contrast, asymptomatic people never have any symptoms during the course of their infections at all.

That difference matters for the scientists who are racing to identify and count cases to study the spread of the virus. If you’re truly asymptomatic, you’re probably not going to get tested and would therefore never be counted by the health care system. But you may still be contributing to the virus’s spread.

Alternatively, suppose you are symptom-free when you test positive, only to later develop a fever and cough that you don’t report to your doctor. You might be mistakenly counted as asymptomatic rather than presymptomatic.

“Previously we had commonly used asymptomatic to include both groups so it's tough to break out of that thinking and lexicon,” Tara Smith, an epidemiologist at Kent State University, said by email. “But I think this pandemic has shown that there may be nuance between those who are not YET symptomatic and those who might NEVER show symptoms, and that seems to be important here.”

To Jeffrey Shaman, an infectious disease expert at Columbia University, the simpler and more important distinction is between “documented” versus “undocumented” cases — the latter being all infected people who aren’t diagnosed. Those could include a person who is very sick but “hates to go to the hospital or see a doctor and toughs it out at home,” he told BuzzFeed News.

It’s more likely that a lot of these undocumented COVID-19 cases have symptoms mild enough that they don’t feel the need to see a doctor, but are walking around in the world anyway, he said.

The slight differences in these terms matter. “They’re not all the same thing but are getting conflated,” Smith said.

Corn genetics research exposes mechanism behind traits becoming silent

Diverse maize cobs showing the varying genetic traits of kernel and cob colors. Credit: Surinder Chopra, Penn State

For more than a century, plant geneticists have been studying maize as a model system to understand the rules governing the inheritance of traits, and a team of researchers recently unveiled a previously unknown mechanism that triggers gene silencing in corn. Gene silencing turns off genetic traits, an important consideration for plant breeders who depend on the faithful inheritance of traits from one generation to the next.

Historically, the maize p1 gene has been used as a model by maize geneticists. Previous researchers did not know that two types of overlapping DNA methylation marks could modify, silence or activate this gene. The discovery adds to geneticists' knowledge of different mechanisms of non-Mendelian inheritance, according to lead researcher Surinder Chopra, professor of maize genetics, College of Agricultural Sciences, Penn State.

In findings reported in PLOS One, Chopra's team showed that silencing the corn pericarp color 1 gene—regulator of the kernels'' outer layer color and the cob color—can have two "overlapping" epigenetic components—RNA dependent DNA methylation (RdDM) and non-RNA dependent DNA methylation (non-RdDM).

"DNA methylation, which is the addition of methyl groups to the DNA molecule, can change the activity of a DNA segment without changing the sequence," he said. "DNA methylation typically acts to repress gene transcription, which is the first step of gene expression."

In plant cells, when and at what level a gene is expressed is under tight control between transcription activation and suppression, Chopra explained. Small RNAs—molecules essential in regulation and expression of genes—can mediate methylation of DNA strands and shut down transcription activity, therefore playing a role in silencing inherited genes or transgenes introduced to produce desirable crop traits.

In corn, the pericarp color 1 gene regulates the accumulation of brick-red flavonoid pigments called phlobahpenes. The pattern of pigmentation on the corn kernel pericarp and "glumes"—membrane covering the cob—depends upon the expression of the pericarp color 1 gene. Some examples of these patterns are: white kernels, red cob red kernels, red cob variegated kernels, variegated cob red kernels, white cob and white kernels, white cob.

"Our study on maize pericarp color 1 gene has demonstrated the involvement of both small RNA-dependent and small RNA-independent mechanisms for gene suppression," Chopra said. "This study reveals the additional layer of gene regulation by small RNAs, and improves our understanding of how gene expression is regulated specifically in one tissue but not in the other."

Typically, when plant breeders are creating new types of cultivars, several traits they are breeding for may disappear or their expression gets reduced in the progeny, he said. "And that, we now know, is because of gene silencing."

A better understanding of how gene-silencing mechanisms cause the disappearance of desired traits has long been needed, Chopra believes. It can be disastrous for a farmer to buy seeds that do not behave in the grow-out the way they were promised by the producer.

If one or more genes that are controlling a trait become silent due to overlapping DNA methylation, then that trait basically disappears from the population.

"That is a big setback for anyone trying to breed for traits such as high yield, which is regulated by several genes," said Chopra. "If one or two of those genes that are essential for high yield become silent, then a reduction in the overall yield may result."

Inherited Bone Marrow Failure Syndromes

Heterozygote Phenotype

Heterozygote carriers of FANC gene mutations do not develop peripheral blood cytopenias or aplastic anemia, and cell lines from heterozygote carriers do not show excessive chromosome fragility in culture when exposed to DEB or MMC. The mean chromosomal breakage level of lymphocytes from FA carriers tested in cultures with a clastogenic agent may be higher than controls, but individual carrier testing may show overlap with normal values and severely limits its diagnostic utility. Literature from the early 1980s describes congenital anomalies of the hand and the genitourinary system in relatives of patients with FA, and parents of children with FA may have short stature. FA carriers may have increased levels of HbF, decreased natural killer (NK) cell counts, and diminished reactivity to mitogen stimulation.

Monoallelic carriers for FANCD1, FANCN, FANCJ, FANCS, FANCP, and FANCO are at increased risk of developing cancer. Female carriers of FANCD1/BRCA2 and FANCS/BRCA1 have an increased risk of breast cancer ranging from 40% at age 80 years to a lifetime risk of about 80%, and of ovarian cancer with a risk of up to 20% at age 70 years. Male carriers have a 7% risk of breast cancer and a 20% risk of prostate cancer before age 80 years. Heterozygous mutations in FANCP and FANCO are also associated with breast and ovarian cancers. Mutant FANCN and FANCJ are low-penetrance breast cancer susceptibility alleles with about a twofold increased risk in carriers compared with the general population.

Alpha Thalassemia

Alpha thalassemia is a blood disorder that reduces the production of normal hemoglobin. Hemoglobin is the protein in red blood cells that carries oxygen to cells throughout the body.

What are the symptoms of alpha thalassemia?

In people with the characteristic features of alpha thalassemia, a reduction in the amount of normal hemoglobin prevents enough oxygen from reaching the body's tissues. Affected individuals also have a shortage of red blood cells (anemia), which can cause pale skin, weakness, fatigue, and more serious complications.

Alpha Thalassemia Testing

Is there a test for thalassemia?

Blood tests and family genetic studies can show whether an individual has thalassemia or is a carrier. If both parents are carriers, they may want to consult with a genetic counselor for help in deciding whether to conceive or whether to have a fetus tested for thalassemia.

Prenatal testing can be done around the 11th week of pregnancy using chorionic villi sampling (CVS). This involves removing a tiny piece of the placenta. Or, the fetus can be tested with amniocentesis around the 16th week of pregnancy. In this procedure, a needle is used to take a sample of the fluid surrounding the baby for testing.

Assisted reproductive therapy is also an option for carriers who don't want to risk giving birth to a child with thalassemia. A new technique, pre-implantation genetic diagnosis (PGD), used in conjunction with in vitro fertilization, may enable parents who have thalassemia or carry the trait to give birth to healthy babies. Embryos created in-vitro are tested for the thalassemia gene before being implanted into the mother, allowing only healthy embryos to be selected.

SOURCE: National Heart Lung and Blood Institute

What are the types of alpha thalassemia?

Two types of alpha thalassemia can cause health problems. The more severe type is known as hemoglobin Bart hydrops fetalis syndrome or Hb Bart syndrome. The milder form is called HbH disease.

Hb Bart syndrome is characterized by hydrops fetalis, a condition in which excess fluid builds up in the body before birth. Additional signs and symptoms can include severe anemia, an enlarged liver and spleen (hepatosplenomegaly), heart defects, and abnormalities of the urinary system or genitalia. As a result of these serious health problems, most babies with this condition are stillborn or die soon after birth. Hb Bart syndrome can also cause serious complications for women during pregnancy, including dangerously high blood pressure with swelling (preeclampsia), premature delivery, and abnormal bleeding.

HbH disease causes mild to moderate anemia, hepatosplenomegaly, and yellowing of the eyes and skin (jaundice). Some affected individuals also have bone changes such as overgrowth of the upper jaw and an unusually prominent forehead. The features of HbH disease usually appear in early childhood, but affected individuals typically live into adulthood.

How Common Is Alpha Thalassemia?

Alpha thalassemia is a fairly common blood disorder worldwide. Thousands of infants with Hb Bart syndrome and HbH disease are born each year, particularly in Southeast Asia. Alpha thalassemia also occurs frequently in people from Mediterranean countries, North Africa, the Middle East, India, and Central Asia.


What Genes Are Related to Alpha Thalassemia

Alpha thalassemia typically results from deletions involving the HBA1 and HBA2 genes. Both of these genes provide instructions for making a protein called alpha-globin, which is a component (subunit) of hemoglobin.

People have two copies of the HBA1 gene and two copies of the HBA2 gene in each cell. Each copy is called an allele. For each gene, one allele is inherited from the father, and the other is inherited from the mother. As a result, there are four alleles that produce alpha-globin. The different types of alpha thalassemia result from the loss of some or all of these alleles.

Hb Bart syndrome, the most severe form of alpha thalassemia, results from the loss of all four alpha-globin alleles. HbH disease is caused by a loss of three of the four alpha-globin alleles. In these two conditions, a shortage of alpha-globin prevents cells from making normal hemoglobin. Instead, cells produce abnormal forms of hemoglobin called hemoglobin Bart (Hb Bart) or hemoglobin H (HbH). These abnormal hemoglobin molecules cannot effectively carry oxygen to the body's tissues. The substitution of Hb Bart or HbH for normal hemoglobin causes anemia and the other serious health problems associated with alpha thalassemia.

Two additional variants of alpha thalassemia are related to a reduced amount of alpha-globin. However, cells still produce some normal hemoglobin, these variants tend to cause few or no health problems, and the loss of two of the four alpha-globin alleles results in alpha thalassemia trait. People with alpha thalassemia trait may have unusually small, pale red blood cells and mild anemia. A loss of one alpha-globin allele is found in alpha thalassemia silent carriers. These individuals typically have no thalassemia-related signs or symptoms.

How Do People Inherit Alpha Thalassemia?

The inheritance of alpha thalassemia is complex. Each person inherits two alpha-globin alleles from each parent. If both parents are missing at least one alpha-globin allele, their children are at risk of having Hb Bart syndrome, HbH disease, or alpha thalassemia trait. The precise risk depends on how many alleles are missing and which combination of the HBA1 and HBA2 genes is affected.

6.9 Recessive traits are expressed when two copies are present

Figure 6.11 Recessive Mutation Chart

For many genes in your body, it doesn’t really matter if one copy, on one of the homologues, has a mutation that reduces the function of the gene product or protein. Typically, the healthy copy can prompt the production of enough functioning protein to ensure normal development. In these cases, there won’t be noticeable changes to an individual’s phenotype unless the individual inherits two copies of the mutation. In other words, both alleles at a single locus code for proteins with reduced, altered, or no function. These conditions, which are only expressed when two alleles are inherited, are recessive.

A carrier is an individual who has a single copy of a recessive allele. In the case of a recessive trait, having only one mutated copy will not cause the individual to display the phenotype. These recessive inheritance patterns allow for an individual to carry potentially problematic mutations without suffering any of the consequences. Although carriers will not display the trait, they can pass the mutation on to children. If two healthy carriers both pass on a recessive trait, the phenotype will present in the individual offspring. One such example is Tay-Sachs disease , a disorder that is caused by recessive mutations in the HEXA gene, located on chromosome 15. Children with Tay-Sachs disease live for a very short period of time. However, this mutated HEXA gene still is able to persist in populations since carriers can be completely healthy, and grow up to reproduce.

What are silent carrier traits? - Biology

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  • Alpha thalassemia is a group of inherited blood disorders characterized by reduced or absent production of &alpha-globin subunits, resulting in low levels of hemoglobin, decreased mean corpuscular volume (MCV) and decreased mean corpuscular hemoglobin (MCH)
  • Group of inherited autosomal recessive diseases caused by an &alpha-globin chain synthesis defect
  • There are four clinical pictures of &alpha-thalassemia, according to the number of genes affected by loss of function with hemoglobin Bart's hydrops fetalis (Hb Bart's) syndrome and HbH disease being clinically significant
  • Also classified as &alpha-thalassemia minima (heterozygous &alpha + -thalassemia, -&alpha/&alpha&alpha) and &alpha-thalassemia minor (heterozygous &alpha 0 -thalassemia, –/&alpha&alpha or homozygous &alpha + -thalassemia, -α/-α) (Dtsch Arztebl Int 2011108:532)
  • 15% of American blacks are silent carriers for &alpha-thalassemia and about 3% have &alpha-thalassemia trait (Medscape: Alpha Thalassemia [Accessed 23 April 2019])
  • In Southeast Asian and Mediterranean populations, HbH disease and Hb Bart’s syndrome (&gamma4) are common
  • The adequacy of the oxygen transport system depends on the affinity of hemoglobin for oxygen
  • In adults, HbA is the major hemoglobin (97%), composed of 2 &alpha subunits and 2 &beta subunits (&alpha₂&beta₂) with minor amount of HbA2 (approximately 1.5 - 3.5% &alpha2&delta2) and HbF (approximately 0 (no output of &alpha-globin from the chromosome)
    • An individual with the genotype --/&alpha&alpha is referred to as an &alpha 0 carrier (GeneReviews 2005: NBK1435)
    • This is common in Southeast Asia and Mediterranean but rare in African Americans
    • An individual with the genotype -&alpha/&alpha&alpha is referred to as an &alpha+ carrier (GeneReviews 2005: NBK1435)
    • This is common in African Americans

    HbH disease HPLC and capillary electrophoresis

    Hb Bart's HPLC and capillary electrophoresis

    • There are four &alpha-thalassemia syndromes, according to the number of genes affected, correlating with different clinical pictures
      • Hb Bart's hydrops fetalis syndrome: complete absence of all 4 &alpha chains
        • Because of the absence of &alpha chains, no HbA or HbF is present (GeneReviews 2005: NBK1435)
        • Large amount of Hb Bart’s, a variable amount of Hb Portland and traces of HbH present
        • Red cells with Hb Bart's have an extremely high oxygen affinity and are incapable of effective oxygen delivery
        • Incompatible with life, fetuses are still born with severe anemia, marked edema and hepatosplenomegaly
        • There is chronic hemolytic anemia, mild jaundice and hepatosplenomegaly
        • Most individuals are clinically well and survive without treatment (GeneReviews 2005: NBK1435), transfusion is rarely needed
        • Benign condition with most patients diagnosed on routine screening
        • Does not require treatment
        • No clinical abnormalities
        • Electrophoresis or high performance liquid chromatography (HPLC):
          • Hb Bart's hydrops fetalis syndrome: Hb Bart’s migrates faster than HbA on alkaline electrophoresis
            • Hb Bart’s > 80%, a variable amount of Hb Portland and HbH present (See figure)
            • No HbA
            • HbH > 15% at birth (GeneReviews 2005: NBK1435)
            • Normal or decreased HbA2
            • Hb Bart’s in newborns (up to 20%) (See figure)
            • Normal electrophoresis in adults and the diagnosis is made by excluding iron deficiency, anemia of chronic disease and beta thalassemia
            • Normal HbA2 and HbF (GeneReviews 2005: NBK1435)
            • Hb Bart’s in newborns (up to 2%)
            • Normal electrophoresis in adults and diagnosis is made by molecular or globin chain synthesis studies
            • Hb Bart's hydrops fetalis syndrome:
              • CBC: severe microcytic hypochromic anemia and reticulocytosis
              • Hb Bart’s > 80%, HbH and Hb Portland
              • CBC: decreased MCV and MCH, reticulocytosis (4 - 5%), increased RBCs
              • Hb Bart’s:
                • 20 - 40% at birth
                • 5 - 30% in adults
                • CBC: may show mild hypochromic (low MCH), microcytic (low MCV) anemia (GeneReviews 2005: NBK1435)
                • Hb Bart’s:
                  • 2 - 10% in neonate
                  • None in adults
                  • CBC: either normal or mild reduction of MCV and MCH (GeneReviews 2005: NBK1435)
                  • Hb Bart’s:
                    • 1 - 2% in neonates
                    • None in adults
                    • Hb Bart's hydrops fetalis syndrome is suspected in fetuses with increased nuchal thickness, thickened placenta, increased cerebral media artery velocity and increased cardiothoracic ratio on ultrasonography examination at 13 to 14 weeks' gestation (GeneReviews 2005: NBK1435)

                    Hb Bart’s, midpregnancy sonographic features

                    • Neonate with &alpha-thalassemia major (Pediatr Dev Pathol 201922:166)
                    • 16 year old boy with co-inheritance of heterozygous &alpha + -thalassemia and sickle cell trait (BMC Ophthalmol 201717:6)
                    • 22 year old woman with HbH disease (Biomed Rep 20165:23)
                    • 28 year old Chinese woman with &alpha-thalassemia trait (J Med Case Rep 20159:58)
                    • 36 year old Chinese woman with HbH disease (Case Rep Med 20182018:8057045)
                    • Hb Bart's syndrome is a universally fatal condition and death usually occurs in the neonatal period (GeneReviews 2005: NBK1435)
                    • Most individuals with HbH disease, thalassemia trait and carriers are clinically well and survive without any treatment (GeneReviews 2005: NBK1435)
                    • Hb Bart's hydrops fetalis syndrome:
                      • Large, hypochromic red cells and severe anisopoikilocytosis (GeneReviews 2005: NBK1435) (See figure)
                      • Hypochromia, basophilic stippling, target cells, anispoikilocytosis
                        • Red blood cell supravital stain show HbH inclusions (&beta4 tetramers) (GeneReviews 2005: NBK1435)
                        • Hypochromia and microcytosis
                          • HbH inclusions found in &alpha 0 -thalassemia but rarely in &alpha + -thalassemia

                          &alpha 0 -thalassemia carrier inclusion bodies

                          Hb Bart's hydrops fetalis syndrome

                          Red cell inclusion bodies

                          • Electrophoresis report:
                            • Features suggestive of &alpha-thalassemia, if other causes of microcytosis are excluded
                            • &alpha-globin (HBA1 and HBA2) deletion / duplication
                            • Deletion:
                              • Result: two pathogenic deletions detected in the &alpha-globin gene cluster
                              • DNA variant(s):
                                • Pathogenic deletion: -&alpha 3.7
                                • Heterozygous pathogenic deletion: -&alpha 4.2
                                • Heterozygous predicted genotype: -&alpha/-&alpha

                                  A 30 year old man presents to his physician for partner screening. Routine laboratory studies are shown:


                                The Silent Generation grew up in hard times that was character forming for them. On the one hand, they became traditionalists that worked hard and loyally to improve their living conditions. They saved their money and became the wealthiest generation.

                                On the other hand, this generation produced enduring entertainers, top sporting achievements, activists that brought about lasting social change.

                                This generation might have been silent to start out, but they are not going quietly.