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Genetic Disorders

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Biology

Genetic variation can confer increased fitness in an organism or population because the variants may be more adaptive to an environment or certain circumstances. Genetic variations and mutations can lead to pleasing visual and aesthetic differences so that not all flowers, puppies, or humans look the same. However, genetic variation can also lead to unfortunate outcomes when these variants or mutations reduce fitness due to causing a disease or genetic disorders.

Genetic Disorder Definition

A genetic disorder is defined as any disease that has negative effects on an organism due (at least partially) to an alteration in the organism's DNA sequence, to the extent that its sequence is different from the normal (wild-type) DNA sequence.

Sometimes genetic disorders are syndromic - and you may see disorders referred to as syndromes, such as Down's Syndrome or Turner Syndrome - and sometimes genetic disorders are referred to as diseases - such as maple syrup urine disease or sickle cell disease.

A syndrome is essentially a set of symptoms that occur together, with some variation over time; while the term disease implies we know the exact pathophysiology (this means the scientific, medical cause) for the disorder.

Regardless, the important thing to know is that there are many of these genetic diseases, disorders, and syndromes.

Tools for Determining Genetic Disorders and Genetic Disorder Testing

Before we get into detail about the many types and examples of genetic disorders, let's first analyze how we could determine if an individual has one. This is through a process called genetic testing, and there are many possibilities at different stages of life and different levels of technology.

  • Pedigree analysis:
    • Pedigrees look like family trees, where the emphasis is not just on keeping a lovely reminder of your genealogy but on determining the health status (carrier, non-carrier, has a disorder, etc.) of members of an extended family.
    • This is best used for an existing family when everyone has already expressed their traits, and we can determine their status.
    • Pedigrees are also used to determine inheritance patterns of existing disorders. For example, we can tell in the image below that this disorder has recessive inheritance because two healthy individuals gave birth to a child who has the disorder (Fig. 1). This would be impossible for a dominantly inherited allele.

Genetic Disorders, Recessively Inherited Genetic Disease Pedigree, StudySmarterFigure 1: Recessively Inherited Genetic Disease Pedigree. Wikimedia.

  • Amniocentesis
    • Amniocentesis involves taking a small amount of amniotic fluid from the uterus of a pregnant woman and using it to analyze the genetic material of her baby.
    • Abnormal or defective cells may be swimming around in amniotic fluid
    • This method is used only to analyze fetuses. Down Syndrome and other trisomies are checked in this manner.
  • Chorionic Villus Sampling (CVS)
    • The chorionic villus is the placenta. CVS involves taking a sample of the placenta and testing it for genetic abnormalities
    • It is invasive, just like amniocentesis. Its benefit over amniocentesis is that it can be done a few weeks earlier in pregnancy.
  • Karyotyping
    • Karyotyping involves visualizing the full set of chromosomes under a microscope. The chromosomes are best visualized in the metaphase of mitosis (Fig. 2).
    • This is done to analyze the size, shape, and number of chromosomes and any abnormalities like breakage.
    • Karyotyping is done at any age and merely requires a DNA sample.

Genetic Disorders, Human Karyotypes, StudySmarterFigure 2: Human Karyotype. Proprofs.

Sometimes karyotypes add on FISH (fluorescent in-situ hybridization), allowing different chromosomes to fluoresce different colors. This can help scientists see if chromosomes have broken off and rejoined in the wrong place if there's a color where it should not be!

Genetic Disorders List

We can examine a subset of important genetic disorders and divide them into categories based on their inheritance pattern.

Autosomal recessive genetic disorders:

  1. Sickle cell anemia
  2. Cystic Fibrosis
  3. Tay Sachs Disease
  4. Alpha Thalassemia
  5. Beta Thalassemia
  6. Wilson's Disease
  7. Neimann-Pick Disease
  8. Hemochromatosis
  9. Hurler's Syndrome
  10. Phenylketonuria

Autosomal dominant genetic disorders:

  1. Achondroplasia (Dwarfism)
  2. Familial Hypercholesterolemia
  3. Marfan Syndrome
  4. Huntington's Disease
  5. Hypertrophic Cardiomyopathy
  6. Romano-Ward Syndrome
  7. Diamond-Blackfan Anemia
  8. Aniridia
  9. Ehler's Danlos Syndrome (some forms)
  10. Osteogenesis Imperfecta (some forms)

X-linked dominant genetic disorders:

  1. Aicardi Syndrome
  2. Goltz Syndrome
  3. Alport Syndrome
  4. Hypophosphatemic Rickets
  5. Rett Syndrome

X-linked recessive genetic disorders:

  1. Hemophilia A
  2. Hemophilia B
  3. Duchenne Muscular Dystrophy
  4. Becker's Muscular Dystrophy
  5. Ocular Albinism
  6. Fabry's Disease
  7. Lesch-Nyhan Syndrome
  8. G6PD Deficiency
  9. Sideroblastic Anemia
  10. Bruton Agammaglobulinemia

Chromosomal disorders

  1. Kleinfelter Syndrome
  2. Turner's Syndrome
  3. Cri du chat Syndrome
  4. Down's Syndrome (Trisomy 21)
  5. Edward's Syndrome (Trisomy 18)
  6. Patau Syndrome (Trisomy 13)

Mitochondrial inheritance disorders

  1. MELAS Syndrome
  2. Leber's Hereditary Optic Neuropathy (LHON)
  3. Diabetes and Deafness (DAD syndrome)

Select Examples of Genetic Disorders

Let's examine one disorder of each category, starting with sickle cell disease (also known as sickle cell anemia or SCD). Sickle cell is an autosomal recessive disorder, meaning you must have two copies of this allele to have the disorder. It occurs most frequently in people of West African descent.

Genetic Disorders: Sickle Cell Disease (SCD)

A point mutation causes SCD on the β-globin gene on chromosome 13. The normal gene codes for glutamate, but this point mutation codes for valine.

This single amino acid substitution leads to a defective hemoglobin protein, known as Hemoglobin S. Red blood cells with HbS are less elastic and have a sickled shape, instead of the normal biconcave-disc shape, under stressful or low oxygen conditions (Fig. 3). This shape also causes anemia because these sickled, abnormal red blood cells get destroyed by the body, and the rate of creating new ones cannot compensate.

Beyond anemia, SCD has a wide range of difficult symptoms: extreme pain, swelling in the limbs, predisposition to strokes, predisposition to infections, and recurrent lung disease.

Genetic Disorders, Sickle Cell and Normal RBC IDPH, StudySmarterFigure 3: Sickle cell vs. Normal RBC. IDPH.

Genetic Disorders: Marfan Syndrome

Marfan syndrome is an autosomal dominant disorder due to a mutation on chromosome 15. Normally, this chromosome should code for fibrillin, which helps strengthen the elastic fibers in our connective tissues. When this mutation occurs, connective tissues are abnormal. People with this syndrome are usually very lanky and thin. They're also very flexible but tend to have scoliosis and back problems. Even the elastic fibers in their blood vessels and heart are affected, so they tend to have heart problems and aneurysms.

Genetic Disorders: Rett Syndrome

Rett syndrome is an X-linked dominant disorder that causes severe developmental regression and premature death. A highly lethal mutation causes this disorder, and because of its lethality, boys (XY) who have this gene don't survive till birth.

Girls (XX) who have one normal X chromosome can survive until their 20s. The symptoms of this disorder usually start in girls around ages 2 - 5, with hand flapping, seizures, and loss of ability to walk and talk.

Genetic Disorders: Hemophilia A

Hemophilia is the classic X-linked recessive disorder. Famously, Alexander Romanov had this, and his symptoms of severe bleeding from minor wounds or painful hemarthroses (bleeding between the joints) are tell-tale of this disease. Because Hemophilia A is X-linked recessive, it occurs most often in boys, while girls tend only to be carriers. For a female to actually have Hemophilia A disease, the mutation would need to be on both of their X chromosomes.

Genetic Disorders: Down Syndrome

Down Syndrome is both a trisomy and a chromosomal disorder. A chromosomal disorder means that the genetic error is at the level of the chromosome, not a single gene on a chromosome. Trisomy tells us that the specific chromosomal error, in this case, is three copies (instead of the normal 2) of a particular chromosome. That is why Down Syndrome is also called Trisomy 21, three copies of chromosome 21.

Individuals with Down Syndrome have certain characteristics, like up-slanted eyes, a flat facial profile, and one crease (instead of several) in their palms (Fig. 4). Other effects like intellectual disability, and increased risk of Alzheimer's, are also seen.

Genetic Disorders, Down syndrome features, StudySmarterFigure 4: Down Syndrome features. Arizona Department of Health Services.

Genetic Disorders: MELAS Syndrome

MELAS stands for Myopathy, Encephalopathy, Lactic Acidosis, and Strokes. These four features are the classic features of MELAS syndrome. This disorder is unique in that it is mitochondrially inherited, and the genes and mutations for this disorder occur on the circular mitochondrial chromosome (not one of the 46 nuclear chromosomes). Mitochondrial DNA is only maternally inherited, so a mother passes down this disorder to her children. Typically, MELAS presents with muscular symptoms, like weakness, temporary paralysis, loss of coordination, and neurological symptoms like seizures, vision loss, and headaches.

Common Genetic Disorders

Which genetic disorders are considered common? In genetics, common is relative. Cystic Fibrosis occurs commonly in people of North European ancestry, so it is common to this group. But to people with solely West African ancestry, cystic fibrosis incidence is extremely rare. The reverse could be said about sickle cell disease, which is rather common in people with West African ancestry, but very rare in people with solely North European ancestry.

Remember that common genetic diseases are usually common to a particular population or ethnicity.

Nevertheless, genetic disorders typically classified as common include cystic fibrosis, sickle cell anemia, Tay-Sachs (in Ashkenazi Jewish people), alpha and beta-thalassemia (in people of Mediterranean and Asian ancestry), Down Syndrome, and Huntington's disease.

Rare Genetic Disorders

Thankfully, the vast majority of genetic disorders are uncommon. But which are considered especially rare?

Heritable mitochondrial disorders, like DAD syndrome or LHON, are very rare. Disorders that lead to dangerously decreased immunity to viruses and bacteria, like SCID and Agammaglobulinemia, are quite rare. Heritable polydactyly, a genetic condition that leads an individual to have more than the normal number of fingers or toes, is an example of a spectrum of phenotype and rareness. An extra digit on the pinky finger, for example, is relatively common, while polydactyly of the thumb is rare, and polydactyly of the middle finger is extremely rare!

Genetic Disorders - Key takeaways

  • Genetic disorders are diseases or syndromes that arise because of mutations in the DNA of the affected individual.
  • To test or analyze a genetic disorder, we can do pedigrees, karyotyping, CVS, or amniocentesis (for fetuses).
  • Broadly, genetic disorders include autosomal recessive, autosomal dominant, X-linked, chromosomal, and mitochondrially inherited.
  • Common genetic disorders include sickle cell anemia and cystic fibrosis.
  • Rare genetic disorders include mitochondrially inherited disorders like Leber's Hereditary Optic Neuropathy.

Genetic Disorders

Autism is a disorder with multifactorial causes; both genetic and environmental

Bipolar disorder is known to have a genetic component (more likely in related family members) but it also has idiopathic or environmental causes.

Genetic disorders are diseases or syndromes that arise because of mutations in the DNA of the affected individual

Borderline personality disorder is a psychiatric disorder that has a genetic component, but also an environmental component. 

Research has shown significant genetic component to eating disorders, however they have a multifactorial etiology.

Final Genetic Disorders Quiz

Question

What kind of inheritance pattern does sickle cell anemia have?

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Answer

autosomal recessive

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Question

Which of these inheritance patterns is the rarest for genetic disorders?

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Answer

y-linked

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Question

What is the name of the disorder that causes severe respiratory illnesses, is very common in North European individuals, and is due to a defect in chloride channels?

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Answer

Cystic Fibrosis

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Question

What two tools are used frequently to test for the presence of trisomy in a fetus while they're still in utero?

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Answer

Chorionic Villus Sampling and Amniocentesis

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Question

What is the name of the tool/method that allows us to visualize chromosomes to examine their size, number, and shape?

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Answer

Karyotyping

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Question

What is the inheritance pattern of Hemophilia?

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Answer

x-linked recessive

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Question

What kind of inheritance pattern leads to a mother passing down her disorder to all her children, boys and girls?

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Answer

Mitochondrial inheritance

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Question

Rett Syndrome is an X-linked dominant disorder. How many mutated chromosomes does a girl need to have this disorder?

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Answer

One

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Question

What is a trisomy?

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Answer

A chromosomal disorder where there are three copies of a chromosome instead of two.

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Question

If a father has a disease and passes it down to all his daughters, but none of his sons, what is the inheritance pattern of this disease?

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Answer

X-linked dominant.

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Question

How many copies of the hemophilia mutation do girls need to have the disorder?

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Answer

Two copies

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Question

What tool can be used to look at the incidence of a disease in a family over several generations?

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Answer

A pedigree.

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Question

True or False: X-linked recessive disorders are much more common in boys.

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Answer

True

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Question

What ethnic background has a higher incidence of sickle cell anemia?

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Answer

West Africans

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Question

What ethnic background has a higher incidence of Tay Sachs Disease?

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Answer

Ashkenazi Jewish people

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