Science II

Human Heritage

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Chromosomes and Karyotypes Chromosomes A chromosome is a structure within cells that contains the genes. In eukaryotes, it consists of a single linear DNA molecule associated with proteins. Human cells each have 46 chromosomes, consisting of 23 nearly identical pairs. Each of these 46 chromosomes contains hundreds or thousands of genes that play important roles in determining how a person’s body develops and functions. The DNA is replicated during the S phase, and the replicas are separated during the M phase. Chromosomes are composed of chromatin, a complex of DNA and protein; most are about 40% DNA and 60% protein. A significant amount of RNA is also associated with chromosomes because chromosomes are the sites of RNA synthesis. The DNA of a chromosome is one very long, double-stranded fiber that extends unbroken through the entire length of the chromosome. A typical human chromosome contains about 140 million (1.4 × 108) nucleotides in its DNA.

Karyotypes Chromosomes may differ widely in appearance. They vary in size, staining properties, the location of the centromere (a constriction found on all chromosomes), the relative length of the two arms on either side of the centromere, and the positions of constricted regions along the arms (National Human Genome Research Institute). An individual's particular array of chromosomes is called its karyotype (National Human Genome Research Institute). Karyotypes show marked differences among species and sometimes even among individuals of the same species (National Human Genome Research Institute).

A karyotype is an individual's collection of chromosomes. The karyotype is used to look for abnormal numbers or structures of chromosomes (National Human Genome Research Institute). The term also refers to a laboratory technique that produces an image of an individual's chromosomes (National Human Genome Research Institute).

 

Sex Inheritance A sex chromosome is a type of chromosome that participates in sex determination. Humans and most other mammals have two sex chromosomes, the X and the Y. Females have two X chromosomes in their cells, while males have both X and a Y chromosome in their cells. Egg cells all contain an X chromosome, while sperm cells contain an X or Y chromosome. This arrangement means that the male determines the sex of the offspring when fertilization occurs chromosomes (National Human Genome Research Institute).

 

Female, Male, and Hermaphroditism Female Typically, biologically female individuals have two X chromosomes (XX), inherit an X chromosome from their father, and the other X chromosome from their mother. Male Typically, biologically male individuals have one X and one Y chromosome (XY). The sex chromosomes determine the sex of offspring. The father can contribute an X or a Y chromosome, while the mother always contributes an X.

 

Hermaphroditism (Intersex) Hermaphroditism is the condition of having both male and female reproductive organs (Encyclopedia Britannica, 2021).  In humans, conditions that involve discrepancies between external genitalia and internal reproductive organs are described by the term intersex (Encyclopedia Britannica, 2021). Intersex conditions are sometimes also referred to as disorders of sexual development (DSDs). Such conditions are extremely rare in humans. In ovotesticular disorder (sometimes also called true hermaphroditism), an individual has both ovarian and testicular tissue (Encyclopedia Britannica, 2021). The ovarian and testicular tissue may be separate, or the two may be combined in what is called an ovotestis (Encyclopedia Britannica, 2021). Affected individuals have sex chromosomes showing male-female mosaicism (where one individual possesses both the male XY and female XX chromosome pairs) (Encyclopedia Britannica, 2021). Most often, but not always, the chromosome complement is 46, XX. In every such individual, there also exists evidence of Y chromosomal material on one of the autosomes (any of the 22 pairs of chromosomes other than the sex chromosomes) (Encyclopedia Britannica, 2021). Individuals with a 46, XX chromosome complement usually have ambiguous external genitalia with a sizable phallus and are often reared as males (Encyclopedia Britannica, 2021). However, they develop breasts during puberty and menstruate and, in only rare cases, actually produce sperm (Encyclopedia Britannica, 2021). In 46, XX intersex (female pseudohermaphroditism) individuals have male external genitalia but a female's chromosomal constitution and reproductive organs (Encyclopedia Britannica, 2021). In 46, XY (male pseudohermaphroditism), individuals have ambiguous or female external genitalia but a male's chromosomal constitution and reproductive organs. However, the testes may be malformed or absent (Encyclopedia Britannica, 2021).

 

Chromosomal Abnormalities Many types of chromosomal abnormalities exist, but they can be categorized as either numerical or structural (Genetic Alliance, 2009). Numerical abnormalities are whole chromosomes either missing from or extra to the normal pair (Genetic Alliance, 2009). Structural abnormalities are when part of an individual chromosome is missing, extra, switched to another chromosome or turned upside down (Genetic Alliance, 2009).  Chromosomal abnormalities can occur as an accident when the egg or the sperm is formed or during the early developmental stages of the fetus (Genetic Alliance, 2009). The mother's age and certain environmental factors may play a role in the occurrence of genetic errors (Genetic Alliance, 2009). Prenatal screening and testing can be performed to examine the fetus's chromosomes and detect some, but not all, types of chromosomal abnormalities (Genetic Alliance, 2009). Chromosomal abnormalities can have many different effects, depending on the specific abnormality (Genetic Alliance, 2009). For example, an extra copy of chromosome 21 causes Down syndrome (trisomy 21) (Genetic Alliance, 2009). Chromosomal abnormalities can also cause miscarriage, disease, or problems in growth or development (Genetic Alliance, 2009). The most common type of chromosomal abnormality is known as aneuploidy, an abnormal chromosome number due to an extra or missing chromosome (Genetic Alliance, 2009). Most people with aneuploidy have trisomy (three copies of a chromosome) instead of monosomy (single copy of a chromosome) (Genetic Alliance, 2009). Down syndrome is probably the most well-known example of chromosomal aneuploidy. Besides trisomy 21, the major chromosomal aneuploidies seen in live-born babies are trisomy 18; trisomy 13; 45, X (Turner syndrome); 47, XXY (Klinefelter syndrome); 47, XYY; and 47, XXX (Genetic Alliance, 2009). Structural chromosomal abnormalities result from breakage and incorrect rejoining of chromosomal segments (Genetic Alliance, 2009). A range of structural chromosomal abnormalities results in disease. Structural rearrangements are defined as balanced if the complete chromosomal set is still present, though rearranged, and unbalanced if the information is added or missing (Genetic Alliance, 2009). Unbalanced rearrangements include deletions, duplications, or insertions of a chromosomal segment (Genetic Alliance, 2009). Ring chromosomes can result when a chromosome undergoes two breaks, and the broken ends fuse into a circular chromosome (Genetic Alliance, 2009). An isochromosome can form when an arm of the chromosome is missing and the remaining arm duplicates (Genetic Alliance, 2009). Balanced rearrangements include inverted or translocated chromosomal regions (Genetic Alliance, 2009). Since the full complement of DNA material is still present, balanced chromosomal rearrangements may go undetected because they may not result in disease (Genetic Alliance, 2009). The disease can arise as a result of a balanced rearrangement if the breaks in the chromosomes occur in a gene, resulting in an absent or nonfunctional protein, or if the fusion of chromosomal segments results in a hybrid of two genes, producing a new protein product whose function is damaging to the cell (Genetic Alliance, 2009).

 

Polyploidy Polyploidy is the condition whereby a biological cell or organism has more than two homologous sets of chromosomes, with each set essentially coding for all the biological traits of the organism (New world encyclopedia). Polyploid types are termed according to the number of chromosome sets in the nucleus: triploid (three sets; 3n), tetraploid (four sets; 4n), pentaploid (five sets; 5n), hexaploid (six sets; 6n), and so on (New world encyclopedia). A haploid (n) only has one set of chromosomes. A diploid cell (2n) has two sets of chromosomes. Polyploidy involves three or more times the haploid number of chromosomes. Polyploidy occurs in humans in the form of triploidy (69, XXX) and tetraploidy (92, XXXX) (New world encyclopedia). Triploidy occurs in about two to three percent of all human pregnancies and around 15 percent of miscarriages. (New world encyclopedia) Most triploid conceptions end as miscarriage, and those who survive to term typically die shortly after birth (New world encyclopedia). In some cases, survival past birth may occur longer if there is mixoploidy, with both a diploid and a triploid cell population present (New world encyclopedia). Triploidy may be the result of either diandry (the extra haploid set is from the father) or digyny (the extra haploid set is from the mother) (New world encyclopedia). Diandry is almost always caused by fertilizing an egg with two sperm (dispermy). Digyny is most commonly caused by either failure of one meiotic division during oogenesis leading to a diploid oocyte, or failure to extrude one polar body from the oocyte (New world encyclopedia).

 

XXX (Super female) Triple X syndrome, also called trisomy X or 47, XXX, is characterized by the presence of an additional X chromosome in each of a female's cells (MedlinePlus). Although females with this condition may be taller than average, this chromosomal change typically causes no unusual physical features (MedlinePlus). Most females with triple X syndrome have normal sexual development and can conceive children (MedlinePlus). Triple X syndrome is associated with an increased risk of learning disabilities and delayed development of speech and language skills (MedlinePlus). Delayed development of motor skills (such as sitting and walking), weak muscle tone (hypotonia), and behavioral and emotional difficulties are also possible. Still, these characteristics vary widely among affected girls and women (MedlinePlus). Seizures or kidney abnormalities occur in about 10 percent of affected females (MedlinePlus). Trisomy X (also known as triple X syndrome) occurs in approximately 1 in 1000 female births; however, it is estimated that only 10% of females with trisomy X are diagnosed in their lifetime (Hutaff-Lee, 2013). 

 

XYY (Super male) 47, XYY syndrome is characterized by an extra copy of the Y chromosome in each of a male's cells (MedlinePlus). Although many males with this condition are taller than average, the chromosomal change sometimes causes no unusual physical features (MedlinePlus). Most males with 47, XYY syndrome have normal production of the male sex hormone testosterone and normal sexual development and are usually able to father children (MedlinePlus).  

47, XYY syndrome is associated with an increased risk of learning disabilities and delayed development of speech and language skills (MedlinePlus). Affected boys can have delayed development of motor skills (such as sitting and walking) or weak muscle tone (hypotonia) (MedlinePlus). Other signs and symptoms of this condition include hand tremors or other involuntary movements (motor tics), seizures, and asthma. Males with 47, XYY syndrome have an increased risk of behavioral, social, and emotional difficulties compared to unaffected peers (MedlinePlus). These problems include attention-deficit/hyperactivity disorder (ADHD); depression; anxiety; and autism spectrum disorder, which is a group of developmental conditions that affect communication and social interaction (MedlinePlus).

 

Trisomy 21 (Down Syndrome) Down syndrome is a condition in which a person is born with an extra copy of chromosome 21(MedlinePlus). People with Down syndrome can have physical problems as well as intellectual disabilities (MedlinePlus). Every person born with Down syndrome is different (MedlinePlus). People with the syndrome may also have other health problems (MedlinePlus). They may be born with heart disease. They may have dementia (MedlinePlus). They may have hearing problems and problems with the intestines, eyes, thyroid, and skeleton (MedlinePlus). The chance of having a baby with Down syndrome increases as a woman ages (MedlinePlus). Down syndrome cannot be cured. Early treatment programs can help improve skills. They may include speech, physical, occupational, and/or educational therapy (MedlinePlus). With support and treatment, many people with Down syndrome live happy, productive lives (MedlinePlus).

 

Aneuploidy Having missing or extra chromosomes is a condition called aneuploidy—the risk of having a child with an aneuploidy increases as a woman ages.

 

Turner X0 Turner syndrome is a chromosomal condition that affects development in females (MedlinePlus). The most common feature of Turner syndrome is short stature, which becomes evident by about age 5 (MedlinePlus). An early loss of ovarian function (ovarian hypofunction or premature ovarian failure) is also very common (MedlinePlus). The ovaries develop normally at first, but egg cells (oocytes) usually die prematurely, and most ovarian tissue degenerates before birth (MedlinePlus). Many affected girls do not undergo puberty unless they receive hormone therapy, and most are unable to conceive (infertile) (MedlinePlus). A small percentage of females with Turner syndrome retain normal ovarian function through young adulthood (MedicinePlus).

 

Deletions and Mutations Deletion is a type of mutation involving the loss of genetic material. It can be small, involving a single missing DNA base pair, or large, involving a piece of a chromosome (National Human Genome Research Institute).

The simplest, but perhaps most damaging, structural change is deletion, the complete loss of a part of one chromosome (Encyclopedia Britannica, 2019). In a haploid cell, this is lethal because part of the essential genome is lost (Encyclopedia Britannica, 2019). However, deletions are generally lethal even in diploid cells or have other serious consequences (Encyclopedia Britannica, 2019). A diploid heterozygous deletion results in a cell with one normal chromosome set and another set containing a truncated chromosome (Encyclopedia Britannica, 2019). Such cells show genomic imbalance, which increases in severity with the size of the deletion. Another potential source of damage is that any recessive, deleterious, or lethal alleles that are in the normal counterpart of the deleted region will be expressed in the phenotype (Encyclopedia Britannica, 2019). In humans, the cri-du-chat syndrome is caused by a heterozygous deletion at the tip of the short arm of chromosome 5 (Encyclopedia Britannica, 2019). Infants are born with this condition due to a deletion arising in parental germinal tissues or even in sex cells (Encyclopedia Britannica, 2019). In addition to the “cat cry” that gives the syndrome its name, the manifestations of this deletion include severe intellectual disability and an abnormally small head (Encyclopedia Britannica, 2019).

 

Cri-du-chat Cri du Chat or "Cat Cry syndrome" is found in approximately 1 in 20,000 to 50,000 live births in the U.S. Cri du Chat is caused by a deletion of chromosome 5p, which is written, "5p-." Babies with Cri du Chat have a high-pitched cry, poor muscle tone, a small head size, and low birth weight. They also have problems with language and may express themselves by using a small number of words or sign language. Other health problems can be present. These include delays in walking, problems with feeding, hyperactivity, scoliosis, and severe intellectual disability. Most people with Cri du Chat may have a normal lifespan unless they are born with other serious organ defects. In addition to physical and language therapy, educational intervention at an early age is important for children with Cri du Chat to reach their full potential (Stanford Children’s).

 

Heritable Diseases and Conditions Diabetes Type 1 and type 2 diabetes have different causes, but two factors are important in both. You inherit a predisposition to the disease, then something in your environment triggers it (American Diabetes Association). That’s right: genes alone are not enough. One proof of this is identical twins. Identical twins have identical genes. Yet when one twin has type 1 diabetes, the other gets the disease, at most, only half the time. When one twin has type 2 diabetes, the other's risk is three in four at most. Type 1 diabetes is an autoimmune disorder in which the body attacks its pancreatic beta cells (Dean & McEntyre, 2004). The onset of type 1 diabetes is attributed to both an inherited risk and external triggers, such as diet or an infection. The hunt for these genetic and environmental risk factors is ongoing (Dean & McEntyre, 2004). About 18 regions of the genome have been linked with influencing type 1 diabetes risk (Dean & McEntyre, 2004). These regions may contain several genes which have been labeled IDDM1 to IDDM18 (Dean & McEntyre, 2004). The most well-studied is IDDM1, which contains the HLA genes that encode immune response proteins (Dean & McEntyre, 2004). Variations in HLA genes are an important genetic risk factor, but they alone do not account for the disease, and other genes are involved (Dean & McEntyre, 2004). Two other non-HLA genes have been identified thus far (Dean & McEntyre, 2004). One of these non-HLA genes, IDDM2, is the insulin gene, and the other non-HLA gene maps close to CTLA4, which has a regulatory role in the immune response (Dean & McEntyre, 2004). Type 2 diabetes has been loosely defined as "adult-onset" diabetes, although as diabetes becomes more common throughout the world, cases of type 2 diabetes are being observed in younger people. It is increasingly common in children (Dean & McEntyre, 2004). In determining the risk of developing diabetes, environmental factors such as food intake and exercise play an important role (Dean & McEntyre, 2004). The majority of individuals with type 2 diabetes are either overweight or obese. Inherited factors are also important, but the genes involved remain poorly defined (Dean & McEntyre, 2004). In rare forms of diabetes, mutations of one gene can result in disease (Dean & McEntyre, 2004). However, in type 2 diabetes, many genes are thought to be involved. "Diabetes genes" may show only a subtle variation in the gene sequence, and these variations may be extremely common (Dean & McEntyre, 2004). The difficulty lies in linking such common gene variations, known as single nucleotide polymorphisms (SNPs), with an increased risk of developing diabetes (Dean & McEntyre, 2004). One method of finding the diabetes susceptibility genes is by whole-genome linkage studies (Dean & McEntyre, 2004). The entire genome of affected family members is scanned, and the families are followed over several generations and/or large numbers of affected sibling-pairs are studied (Dean & McEntyre, 2004). Associations between parts of the genome and the risk of developing diabetes are looked for. To date only two genes, calpain 10 (CAPN10) and hepatocyte nuclear factor 4 alpha (HNF4A), have been identified by this method (Dean & McEntyre, 2004).

 

Schizophrenia As with most other mental disorders, schizophrenia is not directly passed from one generation to another genetically, and there is no single specific cause for this illness (MedicineNet). Rather, it results from a complex group of genetic and other biological vulnerabilities, as well as psychological and environmental risk factors (MedicineNet). Biologically, it is thought that people who have abnormalities in the brain neurochemical dopamine and lower brain matter in some areas of the brain are at higher risk for developing the condition (MedicineNet). Other brain issues that are thought to predispose people to develop schizophrenia include abnormalities in the connections between different areas of the brain, called default mode network connectivity(MedicineNet). Recent research is emerging that implicates potential abnormalities in the transmission of the brain neurochemical glutamate as a risk factor for schizophrenia (MedicineNet). Schizophrenia is thought to have a significant but not solely genetic component (MedicineNet). Genetically, schizophrenia and bipolar disorder have much in common in that the two disorders share a number of the same risk genes (MedicineNet). However, the fact is that both illnesses also have some unique genetic factors. There are some genetic commonalities between schizophrenia and epilepsy (MedicineNet).

 

Epilepsy Over the last decade, advances in science and medicine have led to a better understanding of how genetic factors contribute to epilepsy (Epilepsy Foundation). Some types of epilepsy run in families and are passed down from one generation to the next (Epilepsy Foundation). These epilepsies are both inherited and genetic (Epilepsy Foundation). Other types of epilepsy may be due to genetic changes that were inherited or happened for the first time in an individual (Epilepsy Foundation). In such instances, there may not have been any family history of epilepsy (Epilepsy Foundation). Thus, not all epilepsies that are due to genetic causes are inherited (Epilepsy Foundation). Some forms of epilepsy are due to acquired (happen for another reason) causes, like in the case of a head injury, and are neither genetic nor inherited (Epilepsy Foundation). In general, if a person has a first-degree relative (mother, father, sibling) with epilepsy, the risk of developing epilepsy by the age of 40 is less than 1 in 20 (Epilepsy Foundation). The risk differs somewhat between focal and generalized epilepsy (Epilepsy Foundation). There is an increased risk of developing epilepsy if the first-degree relative has generalized epilepsy rather than focal epilepsy (Epilepsy Foundation). These estimates come from population-based studies, meaning they are based on the average across a large group and may not apply to all individuals (Epilepsy Foundation). The likelihood of inheriting epilepsy may differ significantly if a person has a relative with a known genetic epilepsy diagnosis (Epilepsy Foundation). In this case, the chance of developing epilepsy depends on the specific gene and inheritance pattern involved (Epilepsy Foundation).

 

Alzheimer's When diseases like Alzheimer's and other dementias tend to run in families, genetics (hereditary factors), environmental factors, or both may play a role (Alzheimer Association). A family history is not necessary for an individual to develop Alzheimer’s. However, research shows that those who have a parent or sibling with Alzheimer's are more likely to develop the disease than those who do not have a first-degree relative with Alzheimer’s (Alzheimer Association). Those with more than one first-degree relative with Alzheimer’s are at an even higher risk. Two categories of genes influence whether a person develops a disease: (1) risk genes and (2) deterministic genes (Alzheimer Association). Researchers have identified hereditary Alzheimer's genes in both categories (Alzheimer Association).  Risk genes increase the likelihood of developing a disease but do not guarantee it will happen (Alzheimer Association). Researchers have found several genes that increase the risk of Alzheimer's. APOE-e4 is the first risk gene identified and remains the gene with strongest impact on risk. Researchers estimate that between 40-65% of people diagnosed with Alzheimer's have the APOE-e4 gene (Alzheimer Association). APOE-e4 is one of three common forms of the APOE gene; the others are APOE-e2 and APOE-e3 (Alzheimer Association). We all inherit a copy of some form of APOE from each parent (Alzheimer Association). Those who inherit one copy of APOE-e4 from their mother or father have an increased risk of developing Alzheimer's (Alzheimer Association). Those who inherit two copies from their mother and father have an even higher risk, but not a certainty (Alzheimer Association). In addition to raising risk, APOE-e4 may tend to make symptoms appear at a younger age than usual (Alzheimer Association).  An estimated 20-30% of individuals in the United States have one or two copies of APOE-e4; approximately 2% of the U.S. population has two copies of APOE-e4 (Alzheimer Association). Deterministic genes directly cause a disease, guaranteeing that anyone who inherits one will develop a disorder (Alzheimer Association). Scientists have found rare genes that cause Alzheimer's in only a few hundred extended families worldwide (Alzheimer Association). These genes, which are estimated to account for 1% or less of Alzheimer's cases, cause familial early-onset forms in which symptoms usually develop between a person's early 40s and mid-50s (Alzheimer Association). Most individuals with Alzheimer's have the late-onset disease at age 65 or later (Alzheimer Association). Although the hereditary genes that cause "familial Alzheimer's" are rare, their discovery has provided important clues that help our understanding of Alzheimer's (Alzheimer Association). All of these genes affect the processing or production of beta-amyloid, the protein fragment that is the main component of plaques. Beta-amyloid is a prime suspect in the decline and death of brain cells (Alzheimer Association). Aducanumab (Aduhelm™), a drug granted accelerated approval by the FDA, is the first therapy to demonstrate that removing amyloid from the brain is reasonably likely to reduce the cognitive and functional decline in people living with early Alzheimer’s. Several other amyloid-targeting therapies are also in development (Alzheimer Association).

 

Hypercholesterolemia Familial hypercholesterolemia  (FH) is an autosomal codominant inherited disorder of lipoprotein metabolism characterized by very high plasma concentrations of low-density lipopropotein cholesterol (LDLc), tendon xanthomas (TX), and increased risk of premature coronary heart disease (CHD) (De Castro-Orós, Pocoví & Civeira, 2010). The penetrance of FH is almost 100%, which means that half of the offspring of an affected parent have a severely increased plasma cholesterol level from birth onwards, being both males and females equally affected (De Castro-Orós, Pocoví & Civeira, 2010). Although mutations cause the vast majority of FH cases in the LDL receptor gene (LDLR) gene, other causative genes, such as apolipoprotein B (APOB), codify for the natural ligand of the LDLr protein7. Furthermore, a third gene, proprotein convertase subtilisin/kexin type 9 (PCSK9) has been more recently identified as a cause of FH. However, mutations in this latter gene seem to be rare in the populations studied so far (De Castro-Orós, Pocoví & Civeira, 2010).

 

Colon Cancer One in 18 individuals (5.5 percent) will develop colon cancer in their lifetime (UTSouthwestern Medical Center). Of all colon cancer cases, only about 5 to 10 percent are hereditary, linked to gene mutations inherited from one’s mother or father (UTSouthwestern Medical Center). Multiple genes are associated with hereditary colon cancer, but mutations in genes associated with Lynch syndrome (MLH1, MSH2, MSH6, PMS2, EPCAM) are the most common cause of the hereditary form of the disease. Prediction models can estimate an individual’s risk for a Lynch syndrome mutation (UTSouthwestern Medical Center). Approximately 25 to 35 percent of colon cancer is familial, meaning the disease occurs more often in family members than can be expected in the general population, even though a particular gene mutation has not been identified in the family (UTSouthwestern Medical Center). With familial colon cancer, the specific cause of colon cancer is unknown but likely due to combinations of risk factors, including genetics, lifestyle, and environment, that increase risk in the family (UTSouthwestern Medical Center).

 

Other Forms of Cancer Inherited genetic mutations play a major role in about 5 to 10 percent of all cancers (National Cancer Institute). Researchers have associated mutations in specific genes with more than 50 hereditary cancer syndromes, which are disorders that may predispose individuals to develop certain cancers (National Cancer Institute). Genetic tests for hereditary cancer syndromes can tell whether a person from a family that shows signs of such a syndrome has one of these mutations (National Cancer Institute). These tests can also show whether family members without the obvious disease have inherited the same mutation as a family member who carries a cancer-associated mutation (National Cancer Institute). Many experts recommend that genetic testing for cancer risk be considered when someone has a personal or family history that suggests an inherited cancer risk condition, as long as the test results can be adequately interpreted (that is, they can clearly tell whether a specific genetic change is present or absent) and when the results provide information that will help guide a person’s future medical care (National Cancer Institute). Cancers that are not caused by inherited genetic mutations can sometimes appear to “run in families.” For example, a shared environment or lifestyle, such as tobacco use, can cause similar cancers to develop among family members (National Cancer Institute). However, certain patterns in a family such as the types of cancer that develop, other non-cancer conditions that are seen, and the ages at which cancer develops—may suggest the presence of a hereditary cancer syndrome (National Cancer Institute). Even if a cancer-predisposing mutation is present in a family, not everyone who inherits the mutation will necessarily develop cancer (National Cancer Institute).  Here are examples of genes that can play a role in hereditary cancer syndromes:  --The most commonly mutated gene in all cancers is TP53, which produces a protein that suppresses the growth of tumors. In addition, germline mutations in this gene can cause Li-Fraumeni syndrome, a rare, inherited disorder that leads to a higher risk of developing certain cancers (National Cancer Institute). --Inherited mutations in the BRCA1 and BRCA2 genes are associated with hereditary breast and ovarian cancer syndrome, which is a disorder marked by an increased lifetime risk of breast and ovarian cancers in women (National Cancer Institute). Several other cancers have been associated with this syndrome, including pancreatic and prostate cancers and male breast cancer (National Cancer Institute). --Another gene that produces a protein that suppresses the growth of tumors is PTEN (National Cancer Institute). Mutations in this gene are associated with Cowden syndrome, an inherited disorder that increases the risk of breast, thyroid, endometrial, and other types of cancer (National Cancer Institute).

 

Obesity Obesity is a complex condition because it results from the interaction of multiple genes with the environment (Jiménez, 2011). Genes involved in the etiology of obesity include genes encoding peptides targeted to transmit hunger and satiety signals, genes involved in adipocyte growth and differentiation, and genes involved in energy expenditure control (Jiménez, 2011).  The obesity map thus suggests that all chromosomes, except for chromosome Y, have genes involved in obesity occurrence and development (Jiménez, 2011). There is now sufficiently solid scientific evidence, provided by 222 studies conducted on genes and obesity, for us to be able to say that there are 71 genes that are potential inducers of the occurrence of obesity (Jiménez, 2011). Of these, 15 genes are closely associated with body fat volume (Jiménez, 2011). It is, therefore, natural to think that there is not a single type of obesity but several types with similar phenotypes (Jiménez, 2011).

 

Genetic Manipulation Genetic engineering is the artificial manipulation, modification, and recombination of DNA or other nucleic acid molecules to modify an organism or population of organisms (Encyclopedia Britannica, 2021). Genetic engineering generally refers to methods of recombinant DNA technology, which emerged from basic research in microbial genetics (Encyclopedia Britannica, 2021). The techniques employed in genetic engineering have led to the production of medically important products, including human insulin, human growth hormone, and hepatitis B vaccine, and the development of genetically modified organisms such as disease-resistant plants (Encyclopedia Britannica, 2021). The term genetic engineering initially referred to various techniques used to modify or manipulate organisms through the processes of heredity and reproduction (Encyclopedia Britannica, 2021). As such, the term embraced both artificial selection and all the interventions of biomedical techniques, among them artificial insemination, in vitro fertilization (e.g., “test-tube” babies), cloning, and gene manipulation (Encyclopedia Britannica, 2021). In the latter part of the 20th century, however, the term came to refer more specifically to methods of recombinant DNA technology (or gene cloning), in which DNA molecules from two or more sources are combined either within cells or in vitro and are then inserted into host organisms in which they can propagate (Encyclopedia Britannica, 2021).

 

References

Hermaphroditism (Intersex). News Medical.  https://www.news-medical.net/health/Hermaphroditism-(Intersex).aspx Error! Filename not specified. Links to an external site. Deletion. National Human Genome Research Institute.  https://www.genome.gov/genetics-glossary/Deletion Error! Filename not specified. Links to an external site. WHO issues new recommendations on human genome editing for the advancement of public health. World Health Organization. WHO issues new recommendations on human genome editing for the advancement of public health Karyotype. National Human Genome Research Institute.  https://www.genome.gov/genetics-glossary/Karyotype Error! Filename not specified.