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Medical Genetics 101: Introduction to Medical Genetics


Foreword


This 101 course will explore various aspects of medical genetics, including an introduction to the field, different types of genetic disorders, and specific diseases affecting different body systems. The understanding of genetics plays a crucial role in diagnosing, managing, and treating genetic disorders, ultimately improving the lives of individuals and families affected by these conditions. In the course each article will focus on a specific category of disorders, providing an overview of the genetic basis, clinical manifestations, diagnostic approaches, and available treatment for different diseases and disorders.


This series will be divided into eight articles:

  1. Medical Genetics 101: Introduction to Medical Genetics

  2. Medical Genetics 101: Chromosomal Aberrations

  3. Medical Genetics 101: Neuromuscular Diseases

  4. Medical Genetics 101: Mental and Behavioral Diseases

  5. Medical Genetics 101: Central Nervous System Diseases

  6. Medical Genetics 101: Hematological Diseases

  7. Medical Genetics 101: Metabolic Diseases

  8. Medical Genetics 101: Skeletal and Connective Tissue Diseases


Introduction

Medical genetics is a fascinating and rapidly evolving branch that plays a pivotal role in understanding, diagnosing, managing, and preventing genetic disorders. The intricate relationship between genetics and human health has long intrigued scientists and physicians, leading to significant advancements in the field over the past century. Medical genetics centres around the study of genes, which are the fundamental physical and functional units of heredity. Genes are made up of DNA and contain instructions for making molecules called proteins. In the human genome, which is all the DNA of one organism, genes vary in size from a few hundred DNA bases to more than 2 million bases. An international research effort called the Human Genome Project, which aimed to determine the sequence of the human genome and identify the genes that it contains, estimated that humans have between 20,000 and 25,000 genes (International Human Genome Sequencing Consortium, 2004). This article will explore the basic concepts of medical genetics, including inheritance patterns, genetic counselling, and genetic testing, to be able to understand the mechanism of action of different diseases related to different genes and chromosomes.



Understanding Inheritance Patterns

One of the key aspects of medical genetics is comprehending the inheritance patterns of genetic disorders. Understanding the fundamental laws of inheritance is crucial for comprehending the transmission of conditions within a family. An accurate family health history serves as a valuable tool in illustrating the inheritance patterns of conditions across generations. The inheritance of traits and disorders can follow different patterns, each with distinct implications for the risk of passing on the condition to offspring.


In the human organism genes are organised into chromosomes, with each human cell containing 23 pairs of chromosomes, and a total of 46 (Figure 1) (Trask, 2002). These chromosomes are further divided into thousands of genes, and any alterations or mutations in these genes or in the regulation of those genes can lead to various genetic disorders, diseases, and conditions. Every cell has two copies of all genes, one copy in each chromosome, one copy from the mother, and one copy from the father. Some changes in the genes are very minor and do not affect the way a gene works. These changes are often called single nucleotide polymorphisms (SNPs) or gene variants. Other changes, called mutations, affect how a gene works and can lead to disease. In some conditions, family members with the same mutation may not have the same symptoms. In other conditions, individuals with different mutations can have similar characteristics. This is because in many instances a disease is not caused only by a single mutation in a single gene. These diseases are normally multi-factorial diseases that can be produced by different mutations or by a combination of them.


Figure 1: Karyotype (a visual representation of the chromosomes) shows a normal male chromosomal constitution (Ferguson-Smith, 2013).

Diseases caused by mutations in a single gene are usually inherited in a simple pattern, depending on the location of the gene and whether one or two normal copies of the gene are needed for a normal function of a particular protein. These patterns, known as Mendelian inheritance, were first observed by Gregor Mendel in garden pea plants (Clarke, 2022). While most single gene disorders are rare to find, their impact affects millions of people worldwide. There are several primary modes of inheritance for single-gene disorders: autosomal dominant, autosomal recessive, X-linked dominant, and X-linked recessive. However, it is essential to note that not all genetic conditions adhere to these patterns, and rare forms of inheritance, such as mitochondrial inheritance, also exist.

Autosomal dominant pattern mutations occur when a single copy of the mutated gene from one parent is sufficient to trigger the disorder in the offspring. The affected gene is located on one of the autosomal chromosomes (non-sex chromosomes, numbers 1-22), resulting in a 50% chance of passing on the disorder to each child. Dominantly inherited genetic diseases typically manifest in every generation of a family (Figure 2) (Zschocke et al., 2023).


On the other hand, autosomal recessive pattern mutations require both copies of the gene, one from each parent, to be mutated for the disorder to manifest. If both parents carry one copy of the mutated gene without displaying symptoms, their children face a 25% chance of inheriting the disorder, a 50% chance of becoming carriers like their parents, and a 25% chance of not inheriting the mutated gene at all. Recessive genetic diseases are typically not observed in every generation of an affected family. The parents of an affected individual are usually carriers —unaffected individuals carrying one copy of the mutated gene. If both parents are carriers of the same mutated gene and pass it on to their child, the child will be affected (Figure 2) (Zschocke et al., 2023).


Figure 2: The inheritance pattern of autosomal dominant and autosomal recessive inheritance (Genetic Education, 2023).

The inheritance patterns of genes located on sex chromosomes (X and Y) differ from those located on autosomes (non-sex chromosomes). This distinction arises from the fact that females typically carry two X chromosomes (XX), while males have one X and one Y chromosome (XY). As a result, females possess two copies of each X-linked gene, whereas males carry only one copy of X-linked genes and one copy of Y-linked genes.


When considering diseases caused by mutated genes on the X chromosome, they can be inherited in either a dominant or recessive manner. Since males have only one X chromosome, any mutated gene on their X chromosome, whether dominant or recessive, will lead to the disease. In contrast, females, with two X chromosomes, will not be affected by inheriting a single recessive mutation on an X-linked gene. For X-linked recessive diseases to affect females, both copies of the gene must be mutated. As a result, families with X-linked recessive disorders often have affected males in each generation, but rarely affected females (Figure 3) (Migeon, 2020).

Y-linked disorders pertain to genes situated on the Y chromosome and predominantly impact males. These disorders are infrequent due to the Y chromosome’s relatively smaller size and fewer gene content compared to the X chromosome. An example of such a Y-linked disorder is infertility caused by mutations in Y chromosomal genes, such as the SRY gene. Infertility leads to the inability to conceive and produce offspring, making Y-linked disorders exceptionally rare, even among rare genetic conditions (Jedidi et al., 2019).


Figure 3: The inheritance pattern of X-linked dominant and recessive inheritance (Genetic Education, 2023).

To finish with the inheritance patterns, we have to talk about mitochondrial inheritance, also known as maternal inheritance. It is a unique pattern of genetic transmission that involves genes located in the mitochondria, the energy-producing organelles within cells. Unlike nuclear DNA, which is inherited from both parents, mitochondrial DNA (mtDNA) is exclusively inherited from the mother. This is because the mitochondria present in the sperm are typically destroyed upon fertilization, leaving only the mother’s mitochondria to be passed on to the offspring (Figure 4).


Mitochondrial DNA is circular (Figure 4) and contains a small number of genes, which primarily encode proteins essential for the mitochondrial energy production process called oxidative phosphorylation. Any mutations in these mitochondrial genes can lead to dysfunctional mitochondria and impaired energy production, affecting various organs and systems in the body. Mitochondrial diseases are a group of rare and complex disorders resulting from mutations in mitochondrial DNA. The severity and manifestation of these diseases can vary widely, as their impact depends on the proportion of defective mitochondria in different tissues and cells.


Figure 4: The inheritance pattern of mitochondrial DNA (Centrum voor Medische Genetica, 2023).

Genetic Testing

Genetic testing is a powerful and rapidly advancing tool used in medical genetics to analyse an individual’s DNA and identify genetic variations or mutations that may be associated with specific genetic disorders or conditions. It plays a crucial role in diagnosing, predicting, and managing various genetic conditions, enabling personalised healthcare approaches and informed decision-making. There are many different kinds of genetic tests that can be classified through different aspects, such as the timing of the test, the type of test or the purpose of it. No individual genetic test can identify all genetic conditions. The approach to genetic testing is individualised, based on medical and family history and what condition is the subject of the test


Single-gene tests look for changes in only one gene (Fiorentino et al., 2003). Single gene testing is done when it is believed that a patient has symptoms of a specific condition or syndrome. Single-gene testing is also done when there is a known genetic mutation in a family (Figure 5). A panel genetic test examines numerous genes in a single test and is typically categorized based on various medical concerns. For instance, panel genetic tests may focus on conditions such as low muscle tone, short stature, or epilepsy. Additionally, they can be organized according to genes linked to an increased risk of specific types of cancer, such as breast or colorectal (colon) cancer (Figure 5) (Winship & Southey, 2016).


Apart from single gene and panel genetic tests, there are two different kinds of large-scale genetic tests. Exome sequencing looks at all the genes in the DNA (whole exome) or just the genes that are related to medical conditions (clinical exome) (Jelin & Vora, 2018). Genome sequencing is the largest genetic test and looks at all of a person’s DNA, not just the genes (Balloux et al., 2018). Exome and genome sequencing are only ordered by doctors for people with complex medical histories as they are much more expensive and time-consuming than the previously mentioned ones (Figure 5). Large-scale genomic testing is also used in research to learn more about the genetic basis of conditions. Moreover, large-scale genetic tests can have findings unrelated to the reason for the test ordering in the first place (secondary findings). Examples of secondary findings are genes associated with a predisposition to cancer or rare heart conditions when what was looking for was y a genetic diagnosis to explain a child’s developmental disabilities.


Figure 5: Genetic sequencing: pros and cons (TESS, 2023).

Depending on the timing of the genetic testing the tests can also be classified in different variants (Figure 6). Diagnostic, predictive, and carrier testing are performed in adulthood (Figure 6). The diagnostic one is performed when a healthcare professional suspects a specific genetic disorder based on the individual’s symptoms and medical history. Diagnostic testing aims to confirm or rule out the presence of the suspected genetic mutation (Toft, 2014). Predictive testing is utilised when an individual has a family history of a genetic disorder, but no symptoms are present. It helps assess the risk of developing the disorder later in life (Marzullo et al., 2014). The carrier testing is performed to determine if an individual carries one copy of a mutated gene for a recessive disorder. Carriers do not usually display symptoms but can pass on the mutation to their children (Vears et al., 2023).


Newborn screening is a set of laboratory tests performed on newborn babies (Figure 6) with the aim to detect a set of known genetic diseases. Typically, this testing is performed on a blood sample obtained from a heel prick when the baby is two or three days old (Rajabi, 2018). Prenatal genetic testing is conducted during pregnancy (Figure 6) in order to detect genetic disorders or chromosomal abnormalities in the fetus. It allows parents to make informed decisions about the pregnancy and potential treatment options (Jelin & Vora, 2018). The Preimplantation Genetic Diagnosis (PGD) is conducted during in vitro fertilization (IVF) to test embryos for genetic disorders before implantation (Figure 6). This process helps parents to avoid passing on certain genetic conditions to their offspring (Sullivan-Pyke & Dokras, 2018).


Figure 6: Timing of genetic testing (Learn.Genetics, 2023).

Genetic testing offers several benefits. It can identify genetic conditions before symptoms manifest, allowing for early interventions and better disease management. Moreover, test results enable individuals and families to make informed decisions about family planning, lifestyle choices, and medical treatments. Lastly, modern genetic tests also open up the future of precision medicine, facilitating personalised treatment plans based on an individual’s genetic makeup, improving treatment effectiveness, and minimising adverse reactions. However, genetic testing also has certain limitations. Some genetic tests may provide uncertain or inconclusive results, making it challenging to predict the exact onset or severity of a genetic condition. In addition, information obtained from genetic testing can raise ethical dilemmas and concerns about privacy, confidentiality, and potential misuse of data, causing emotional distress to individuals and their families.


Genetic Counselling

Genetic counselling is a specialised and vital service provided by certified genetic counsellors to individuals and families who may be at risk of or affected by genetic disorders. The primary aim of genetic counselling is to help individuals understand their genetic risks, make informed decisions regarding genetic testing, family planning, and medical management, and provide support throughout the genetic testing process and beyond.


Genetic counsellors are skilled healthcare experts who possess advanced training in both medical genetics and counselling. They play a critical role in the genetic counselling process and work in collaboration with other healthcare providers, such as geneticists, physicians, and nurses. Genetic counsellors collect detailed medical and family histories to identify any pattern or indication of genetic conditions within the family. Based on the gathered information, they assess the individual’s or family’s risk of having a genetic disorder or passing it on to future generations. In addition, genetic counsellors educate individuals and families about specific genetic conditions, their inheritance patterns, the health implications, and available testing and management options. They discuss different types of genetic testing available and explain the benefits, limitations, and potential outcomes of testing. In addition to addressing the medical aspects, they also provide emotional support to individuals and families dealing with the possibility of having a genetic condition (Figure 7).


Figure 7: Aspects covered during genetic counselling (Genosalut, 2023).

Genetic counselling is recommended in various situations. If there is a known family history of a genetic disorder or if multiple family members are affected, genetic counselling can help assess the risk and discuss preventive measures. Individuals concerned about the risk of passing on a genetic condition to their children can benefit from genetic counselling and carrier testing. Pregnant individuals with abnormal results from prenatal screening tests may seek genetic counselling for further evaluation and guidance. Couples experiencing recurrent pregnancy loss or infertility may benefit from genetic counselling to explore potential genetic factors that may interfere with their fertility. Individuals with a personal history of genetic conditions can receive support through genetic counselling and guidance for managing their health and making informed decisions.


Genetic counselling plays a pivotal role in the field of medical genetics, providing individuals and families with essential information, support, and guidance in understanding their genetic risks and making informed decisions about their health and family planning. With the continued advancement of genetic technologies, genetic counselling remains a crucial component of personalised healthcare, ensuring that individuals are empowered to navigate the complexities of their genetic health with knowledge and compassion.


Determinations

Medical genetics is an ever-evolving field, and its advancements continue to reshape healthcare. As we move forward, the knowledge and insights gained from medical genetics will lead to further breakthroughs in personalised medicine and targeted therapies, and improve healthcare outcomes for individuals and families affected by genetic disorders.


In conclusion, medical genetics stands at the forefront of scientific innovation and compassion, providing a basis for a future where genetic disorders are better understood, diagnosed early, and managed effectively. With continued research, education, and genetic counselling, we can build a healthier world where individuals and families can face genetic challenges with resilience and optimism. Together, we can embrace the power of genetics that transforms lives and shapes a brighter future for generations to come. The next chapters of the 101 series will cover in-depth different diseases and disorders, deciphering their genetic basis, clinical features, diagnostic approaches, and available treatments.


Bibliographical References

Abacan, M., Alsubaie, L., Barlow-Stewart, K., Caanen, B., Cordier, C., Courtney, E., Davoine, E., Edwards, J., Elackatt, N.J., Gardiner, K., et al. (2019). The Global State of the Genetic Counseling Profession. Eur. J. Hum. Genet. 27, 183–197. https://doi.org/10.1038/s41431-018-0252-x Balloux, F., Brønstad Brynildsrud, O., van Dorp, L., Shaw, L.P., Chen, H., Harris, K.A., Wang, H., & Eldholm, V. (2018). From Theory to Practice: Translating Whole-Genome Sequencing (WGS) into the Clinic. Trends Microbiol. 26, 1035–1048. https://doi.org/10.1016/j.tim.2018.08.004 Biesecker, B. (2020). Genetic Counseling and the Central Tenets of Practice. Cold Spring Harb. Perspect. Med. 10. https://doi.org/10.1101/cshperspect.a038968

Clarke, J. (2022). Mendel’s legacy in modern genetics. PLoS Biol. 20, e3001760. https://doi.org/10.1371/journal.pbio.3001760

Fiorentino, F., Magli, M.C., Podini, D., Ferraretti, A.P., Nuccitelli, A., Vitale, N., Baldi, M., & Gianaroli, L. (2003). The minisequencing method: an alternative strategy for preimplantation genetic diagnosis of single gene disorders. Mol. Hum. Reprod. 9, 399–410. https://doi.org/10.1093/molehr/gag046 International Human Genome Sequencing Consortium (2004). Finishing the euchromatic sequence of the human genome. Nature 431, 931–945. https://doi.org/10.1038/nature03001 Jedidi, I., Ouchari, M., & Yin, Q. (2019). Sex chromosomes-linked single-gene disorders involved in human infertility. Eur. J. Med. Genet. 62, 103560. https://doi.org/10.1016/j.ejmg.2018.10.012

Jelin, A. C., & Vora, N. (2018). Whole Exome Sequencing: Applications in Prenatal Genetics. Obstet. Gynecol. Clin. North Am. 45, 69–81. https://doi.org/10.1016/j.ogc.2017.10.003 Marzuillo, C., De Vito, C., D’Andrea, E., Rosso, A., & Villari, P. (2014). Predictive genetic testing for complex diseases: a public health perspective. QJM 107, 93–97. https://doi.org/10.1093/qjmed/hct190 Migeon, B. R. (2020). X-linked diseases: susceptible females. Genet. Med. Off. J. Am. Coll. Med. enet. 22, 1156–1174. https://doi.org/10.1038/s41436-020-0779-4 Rajabi, F. (2018). Updates in Newborn Screening. Pediatr. Ann. 47, e187–e190. https://doi.org/10.3928/19382359-20180426-01 Sullivan-Pyke, C., & Dokras, A. (2018). Preimplantation Genetic Screening and Preimplantation Genetic Diagnosis. Obstet. Gynecol. Clin. North Am. 45, 113–125. https://doi.org/10.1016/j.ogc.2017.10.009 Toft, M. (2014). Advances in genetic diagnosis of neurological disorders. Acta Neurol. Scand. Suppl. 20–25. https://doi.org/10.1111/ane.12232

Trask, B. J. (2002). Human cytogenetics: 46 chromosomes, 46 years and counting. Nat. Rev. Genet. 3, 769–778. https://doi.org/10.1038/nrg905 Vears, D. F., Boyle, J., Jacobs, C., McInerney-Leo, A., & Newson, A. J. (2023). Human Genetics Society of Australasia Position Statement: Genetic Carrier Testing for Recessive Conditions. Twin Res. Hum. Genet. Off. J. Int. Soc. Twin Stud. 26, 188–194. https://doi.org/10.1017/thg.2023.15 Wei, W., & Chinnery, P. F. (2020). Inheritance of mitochondrial DNA in humans: implications for rare and common diseases. J. Intern. Med. 287, 634–644. https://doi.org/10.1111/joim.13047 Winship, I., & Southey, M. C. (2016). Gene panel testing for hereditary breast cancer. Med. J. Aust. 204, 188–190.https://doi.org/10.5694/mja15.01335 Zschocke, J., Byers, P.H., & Wilkie, A. O. M. (2023). Mendelian inheritance revisited: dominance and recessiveness in medical genetics. Nat. Rev. Genet. 24, 442–463. https://doi.org/10.1038/s41576-023-00574-0


Visual Sources

Cover Image: The Basis of Clinical Genetics: What You Need to Know [Image]. Medanta. Retrieved July 27th, 2023, from https://www.medanta.org/patient-education-blog/the-basics-of-clinical-genetics-what-you-need-to-know/


Figure 1: Ferguson-Smith. (2013). Karyotype (a visual representation of the chromosomes) shows a normal male chromosomal constitution [Image]. Brenner’s Encyclopedia of Genetics (Second Edition). Retrieved July 27th, 2023, from https://www.sciencedirect.com/topics/medicine-and-dentistry/human-chromosome


Figure 2: The inheritance pattern of autosomal dominant and recessive inheritance [Image]. Genetic Education. Retrieved July 27th, 2023, from https://geneticeducation.co.in/different-types-of-genetic-inheritance-patterns/?utm_content=cmp-true


Figure 3: The inheritance pattern of X-linked dominant and recessive inheritance [Image]. Genetic Education. Retrieved July 27th, 2023, from https://geneticeducation.co.in/different-types-of-genetic-inheritance-patterns/?utm_content=cmp-true


Figure 4: The inheritance pattern of mitochondrial DNA [Image]. Centrum voor Medische Genetica. Retrieved July 27th, 2023, from https://www.uzbrussel.be/web/genetics/mitochondrial-inheritance


Figure 5: Genetic sequencing: pros and cons. [Image]. TESS. Retrieved July 27th, 2023, from https://www.tessresearch.org/genetic-sequencing/


Figure 6: Timing of genetic testing [Image]. Learn.Genetics. Retrieved July 27th, 2023, from https://learn.genetics.utah.edu/content/disorders/whatispgt


Figure 7: Aspects covered during genetic counselling [Image]. Genosalut. Retrieved July 27th, 2023, from https://www.genosalut.com/en/genetic-testing-and-counselling/genetic-counselling/


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