5.15 Genetic Disorders

Created by: CK-12/Adapted by Christine Miller

Example of polydactyly
Figure 5.15.1 Bilateral polydactyly with short fingers in a baby with Ellis-van Creveld syndrome.

Polly Who?

Each hand in the Figure 5.15.1 photo has an extra pinky finger. This is a condition called polydactyly, which literally means “many digits.” People with polydactyly may have extra fingers and/or toes, and the condition may affect just one hand or foot, or both hands and feet. Polydactyly is often genetic in origin and may be part of a genetic disorder associated with other abnormalities.

What Are Genetic Disorders?

 are diseases, syndromes, or other abnormal conditions caused by  in one or more genes, or by chromosomal alterations. Genetic disorders are typically present at birth, but they should not be confused with , a category that includes any disorder present at birth, regardless of cause. Some congenital disorders are not caused by genetic mutations or chromosomal alterations. Instead, they are caused by problems that arise during embryonic or fetal development, or during the process of birth. An example of a nongenetic congenital disorder is fetal alcohol syndrome. This is a collection of birth defects, including facial anomalies and intellectual disability, caused by maternal alcohol consumption during pregnancy.

Genetic Disorders Caused by Mutations

Table 5.15.1 lists several genetic disorders caused by mutations in just one gene. Some of the disorders are caused by mutations in autosomal genes, others by mutations in X-linked genes. Which disorders would you expect to be more common in males than females?

Table 5.15.1: Types of Genetic Disorders, Their Effects and Mode of Inheritance
Genetic Disorder Direct Effect of Mutation Signs and Symptoms of the Disorder Mode of Inheritance
Marfan syndrome Defective protein in connective tissue Heart and bone defects and unusually long, slender limbs and fingers Autosomal dominant
Sickle cell anemia Abnormal hemoglobin protein in red blood cells Sickle-shaped red blood cells that clog tiny blood vessels, causing pain and damaging organs and joints Autosomal recessive
Hypophosphatemic  (Vitamin D-resistant) rickets Lack of a substance needed for bones to absorb minerals Soft bones that easily become deformed, leading to bowed legs and other skeletal deformities X-linked dominant
Hemophilia A Reduced activity of a protein needed for blood clotting Internal and external bleeding that occurs easily and is difficult to control X-linked recessive

Very few genetic disorders are controlled by mutant [pHypophosphatemicb_glossary id=”2119″]alleles[/pb_glossary]. A dominant allele is expressed in every individual who inherits even one copy of it. If it causes a serious disorder, affected people may die young and fail to reproduce. Therefore, the mutant dominant allele is likely to die out of a population.

A recessive mutant allele — such as the allele that causes sickle cell anemia or cystic fibrosis — is not expressed in people who inherit just one copy of it. These people are called . They do not have the disorder themselves, but they carry the mutant allele and their offspring can inherit it. Thus, the allele is likely to pass on to the next generation, rather than die out.

Genetic Disorders Caused by Chromosomal Alterations

Mistakes may occur during  that result in . This is the failure of replicated  to separate properly during meiosis. Some of the resulting will be missing all or part of a chromosome, while others will have an extra copy of all or part of the chromosome. If such gametes are fertilized and form , they usually do not survive. If they do survive, the individuals are likely to have serious genetic disorders.

Table 5.15.2 lists several genetic disorders that are caused by abnormal numbers of chromosomes. Most chromosomal disorders involve the X chromosome. The X and Y chromosomes are the only chromosome pair in which the two chromosomes are very different in size. This explains why nondisjunction tends to occur more frequently in sex chromosomes than in autosomes.

Table 5.15.2: Genetic Disorders, Their Genotypes, and Phenotypic Effects
Genetic Disorder Genotype Phenotypic Effects
Down syndrome Extra copy (complete or partial) of chromosome 21 (see Figure 5.15.3) Developmental delays, distinctive facial appearance, and other abnormalities (see Figure 5.15.2)
Turner syndrome One X chromosome but no other sex chromosome (XO) Female with short height and infertility(inability to reproduce)
Triple X syndrome Three X chromosomes (XXX) Female with mild developmental delays and menstrual irregularities
Klinefelter syndrome One Y chromosome and two or more X chromosomes (XXY, XXXY) Male with problems in sexual development and reduced levels of the male hormone testosterone
Image shows a family comprised of a woman (mom), a young adult with Down Syndrome and a man (dad).
Figure 5.15.2 Family with down syndrome child.
Down Syndrome Karyotype
Figure 5.15.3 Trisomy 21 (Down Syndrome) Karyotype.

A karyotype is a picture of a cell’s chromosomes. In Figure 5.15.3, note the extra chromosome 21. In Figure 5.15.2, a young man with Down syndrome exhibits the characteristic facial appearance.

Diagnosing and Treating Genetic Disorders

A genetic disorder that is caused by a mutation can be inherited. Therefore, people with a genetic disorder in their family may be concerned about having children with the disorder. A genetic counselor can help them understand the risks of their children being affected. If they decide to have children, they may be advised to have prenatal (“before birth”) testing to see if the fetus has any genetic abnormalities. One method of prenatal testing is . In this procedure, a few fetal cells are extracted from the fluid surrounding the fetus in utero, and the fetal chromosomes are examined. Down syndrome and other chromosomal alterations can be detected in this way.

Image shows a how a PKU test is conducted.
Figure 5.15.3 The PKU test is conducted shortly after birth in order to determine if an infant has higher than normal levels of phenylalanine.

The symptoms of genetic disorders can sometimes be treated or prevented. In the genetic disorder called phenylketonuria (PKU), for example, the amino acid phenylalanine builds up in the body to harmful levels. PKU is caused by a in a gene that normally codes for an needed to break down phenylalanine. When a person with PKU consumes foods high in phenylalanine (including many high-protein foods), the buildup of PKU can lead to serious health problems. In infants and young children, the build-up of phenylalanine can cause intellectual disability and delayed development, along with other serious problems. All babies in Canada and the United States and many other countries are screened for PKU soon after birth.  As shown in Figure 5.15.3, the PKU test involves collecting a small amount of blood from the infant, typically from the heel using a small lancet.  The blood is collected on a special type of filter paper and then brought to a laboratory for analysis. If PKU is diagnosed, the infant can be fed a low-phenylalanine diet, which prevents the buildup of phenylalanine and the health problems associated with it, including intellectual disability. As long as a low-phenylalanine diet is followed throughout life, most symptoms of the disorder can be prevented.

Curing Genetic Disorders

Cures for genetic disorders are still in the early stages of development. One potential cure is gene therapy.  is an experimental technique that uses genes to treat or prevent disease. In gene therapy, normal are introduced into to compensate for abnormal genes. If a mutated gene causes a necessary to be nonfunctional or missing, gene therapy may be able to introduce a normal copy of the gene to produce the needed functional protein.

A gene inserted directly into a cell usually does not function, so a carrier called a is genetically engineered to deliver the gene (see Figure 5.15.4 illustration). Certain viruses, such as adenoviruses, are often used as vectors. They can deliver the new gene by infecting cells. The viruses are modified so they do not cause disease when used in people. If the treatment is successful, the new gene delivered by the vector will allow the synthesis of a functioning protein. Researchers still must overcome many technical challenges before gene therapy will be a practical approach to curing genetic disorders.


Image shows how adenoviruses are used in gene therapy.
Figure 5.15.4 Gene therapy is an experimental technique for curing a genetic disorder by changing the patient’s genetic makeup. Typically, gene therapy involves introducing a normal copy of a mutant gene into the patient’s cells.

Feature: Human Biology in the News

Image shows an image of a young boy with Down Syndrome.
Figure 5.15.5 A Boy With Down Syndrome.

Down syndrome is the most common genetic cause of intellectual disability. It occurs in about one in every 700 live births, and it currently affects nearly half a million Americans. Until recently, scientists thought that the changes leading to intellectual disability in people with Down syndrome all happen before birth.

Even more recently, researchers discovered a genetic abnormality that affects brain development in people with Down syndrome throughout childhood and into adulthood. The newly discovered genetic abnormality changes communication between nerve cells in the brain, resulting in slower transmission of nerve impulses. This finding may eventually allow the development of strategies to promote brain functioning in Down syndrome patients, and it may also be applicable to other development disabilities, such as autism. The results of this promising study were published in the March 16, 2016 issue of the scientific journal Neuron.

5.15 Summary

  • are diseases, syndromes, or other abnormal conditions that are caused by in one or more , or by chromosomal alterations.
  • Examples of genetic disorders caused by single-gene mutations include Marfan syndrome (autosomal dominant), sickle cell anemia (autosomal recessive), vitamin D-resistant rickets (X-linked dominant), and hemophilia A (X-linked recessive). Very few genetic disorders are caused by dominant mutations because these alleles are less likely to be passed on to successive generations.
  • Nondisjunction is the failure of replicated chromosomes to separate properly during meiosis. This may result in genetic disorders caused by abnormal numbers of chromosomes. An example is Down syndrome, in which the individual inherits an extra copy of chromosome 21. Most chromosomal disorders involve the X chromosome. An example is Klinefelter’s syndrome (XXY, XXXY).
  • Prenatal genetic testing (by , for example) can detect chromosomal alterations in utero. The symptoms of some genetic disorders can be treated or prevented. For example, symptoms of phenylketonuria (PKU) can be prevented by following a low-phenylalanine diet throughout life.
  • Cures for genetic disorders are still in the early stages of development. One potential cure is , in which normal genes are introduced into cells by a vector such as a virus to compensate for mutated genes.

5.15 Review Questions

  1. Define genetic disorder.
  2. Identify three genetic disorders caused by mutations in a single gene.
  3. Why are single-gene genetic disorders more commonly controlled by recessive than dominant mutant alleles?
  4. What is nondisjunction? Why can it cause genetic disorders?
  5. Explain why genetic disorders caused by abnormal numbers of chromosomes most often involve the X chromosome.
  6. How is Down syndrome detected in utero?
  7. Use the example of PKU to illustrate how the symptoms of a genetic disorder can sometimes be prevented.
  8. Explain how gene therapy works.
  9. Compare and contrast genetic disorders and congenital disorders.
  10. Explain why parents that do not have Down syndrome can have a child with Down syndrome.
  11. Hemophilia A and Turner’s syndrome both involve problems with the X chromosome. In terms of how the X chromosome is affected, what is the major difference between these two types of disorders?
  12. Can you be a carrier of Marfan syndrome and not have the disorder? Explain your answer.

5.15 Explore More

How CRISPR lets you edit DNA – Andrea M. Henle, TED-Ed, 2019.


What you need to know about CRISPR | Ellen Jorgensen, TED, 2016.


The ethical dilemma of designer babies | Paul Knoepfler, TED, 2017.


Figure 5.15.1

Polydactyly_ECS by Baujat G, Le Merrer M. on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 5.15.2

Downs/ All the Family [photo] by Nathan Anderson on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 5.15.3

Phenylketonuria_testing by U.S. Air Force photo/Staff Sgt Eric T. Sheler in the US Air Force National Archives on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.15.4

Gene_therapy by National Institutes of Health (NIH) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.15.5

Boy_with_Down_Syndrome by Vanellus Foto on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license.


Baujat, G., Le Merrer, M. (2007, January 23). Ellis-Van Creveld syndrome. Orphanet Journal of Rare Diseases, 2, 27. https://doi.org/10.1186/1750-1172-2-27

Hecht, M. (2019, June 26). What is polydactyly? [online article]. Healthline. https://www.healthline.com/health/polydactyly

Genetic and Rare Diseases Information Center (GARD). (2016). Hypophosphatemic rickets (previously called vitamin D-resistant rickets) [online article]. NIH. https://rarediseases.info.nih.gov/diseases/6735/hypophosphatemic-rickets [last updated 7/1/2020]

Mayo Clinic Staff. (n.d.). Cystic fibrosis [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/cystic-fibrosis/symptoms-causes/syc-20353700

Mayo Clinic Staff. (n.d.). Down syndrome [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/down-syndrome/diagnosis-treatment/drc-20355983

Mayo Clinic Staff. (n.d.). Hemophilia [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/hemophilia/symptoms-causes/syc-20373327

Mayo Clinic Staff. (n.d.). Klinefelter syndrome [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/klinefelter-syndrome/symptoms-causes/syc-20353949

Mayo Clinic Staff. (n.d.). Marfan syndrome [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/marfan-syndrome/symptoms-causes/syc-20350782

Mayo Clinic Staff. (n.d.). Phenylketonuria (PKU) [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/phenylketonuria/symptoms-causes/syc-20376302

Mayo Clinic Staff. (n.d.). Sickle cell anemia [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/sickle-cell-anemia/symptoms-causes/syc-20355876

Mayo Clinic Staff. (n.d.). Turner syndrome [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/turner-syndrome/symptoms-causes/syc-20360782

Mayo Clinic Staff. (n.d.). Triple X syndrome [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/triple-x-syndrome/symptoms-causes/syc-20350977

National Center on Birth Defects and Developmental Disabilities. (2020). Fetal alcohol spectrum disorders (FASDs): Basics about FASDs [webpage]. Centers for Disease Control and Prevention (CDC). https://www.cdc.gov/ncbddd/fasd/facts.html

TED-Ed. (2019, January 24). How CRISPR lets you edit DNA – Andrea M. Henle. YouTube. https://www.youtube.com/watch?v=6tw_JVz_IEc

TED. (2016, October 24). What you need to know about CRISPR | Ellen Jorgensen. YouTube. https://www.youtube.com/watch?v=1BXYSGepx7Q&feature=youtu.be

TED. (2017, February 10). The ethical dilemma of designer babies | Paul Knoepfler. YouTube. https://www.youtube.com/watch?v=nOHbn8Q1fBM&t=3s


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Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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