3.7: Genetic Disorders

When is a cold not just a cold?

At some point in your life, you're bound to catch a cold. And there are ways to prevent catching a cold. But what if you couldn't prevent an illness? What if you were born with a disease? What if having a disease was actually due to your DNA? These are genetic diseases, and they can be very serious.

Human Genetic Disorders

Many genetic disorders are caused by mutations in one or a few genes. Others are caused by chromosomal mutations. Some human genetic disorders are X-linked or Y-linked, which means the faulty gene is carried on these sex chromosomes. Other genetic disorders are carried on one of the other 22 pairs of chromosomes; these chromosomes are known as autosomes or autosomal (non-sex) chromosomes. Some genetic disorders are due to new mutations, others can be inherited from your parents.

Autosomal Recessive Disorders

Some genetic disorders are caused by recessive alleles of a single gene on an autosome. An example of autosomal recessive genetic disorders are Tay-Sachs disease and cystic fibrosis. Children with cystic fibrosis have excessively thick mucus in their lungs, which makes it difficult for them to breathe. The inheritance of this recessive allele is the same as any other recessive allele, so a Punnett square can be used to predict the probability that two carriers of the disease will have a child with cystic fibrosis. Recall that carriers have the recessive allele for a trait but do not express the trait. What are the possible genotypes of the offspring in the following table (Table below)? What are the possible phenotypes?

	F	f
F	FF (normal)	Ff (carrier)
f	Ff (carrier)	ff (affected)

According to this Punnett square, two parents that are carriers (Ff) of the cystic fibrosis gene have a 25% chance of having a child with cystic fibrosis (ff). The affected child must inherit two recessive alleles. The carrier parents are not affected.

Tay-Sachs disease is a severe genetic disorder in which affected children do not live to adulthood, so the gene is not passed from an affected individual. Carriers of the Tay-Sachs gene are not affected. How does a child become affected with Tay-Sachs?

Autosomal Dominant Disorders

Huntington’s disease is an example of an autosomal dominant disorder. This means that if the dominant allele is present, then the person will express the disease. A child only has to inherit one dominant allele to have the disease.

The disease causes the brain’s cells to break down, leading to muscle spasms and personality changes. Unlike most other genetic disorders, the symptoms usually do not become apparent until middle age. You can use a simple Punnett square to predict the inheritance of a dominant autosomal disorder, like Huntington’s disease. If one parent has Huntington’s disease, what is the chance of passing it on to the children? If you draw the Punnett square, you will find that there is a 50 percent chance of the disorder being passed on to the children.

Can you have too many chromosomes?

Yes, it's not a good thing to have extra chromosomes. An extra chromosome can be fatal to an embryo, in fact. In the case of a few chromosomes, however, a baby may be born with an extra chromosome. This child will have a chromosomal disorder.

Some children are born with genetic defects that are not carried by a single gene. Instead, an error in a larger part of the chromosome or even in an entire chromosome causes the disorder. Usually the error happens when the egg or sperm is forming. Having extra chromosomes or damaged chromosomes can cause disorders.

Extra Chromosomes

One common example of an extra-chromosome disorder is Down syndrome (Figure below). Children with Down syndrome are mentally disabled and also have physical deformities. Down syndrome occurs when a baby receives an extra chromosome 21 from one of his or her parents. Usually, a child will receive one chromosome 21 from the mother and one chromosome 21 from the father. In an individual with Down syndrome, however, there are three copies of chromosome 21 (Figure below). Therefore, Down syndrome is also known as Trisomy 21. These people have 47 total chromosomes.

Figure \(\PageIndex{1}\): A child with Down syndrome.

Figure \(\PageIndex{2}\): Chromosomes of a person with Down Syndrome. Notice the extra chromosome 21.

Another example of a chromosomal disorder is Klinefelter syndrome, in which a male inherits an extra “X” chromosome. These individuals have an XXY genotype. They have underdeveloped sex organs and elongated limbs. They also have difficulty learning new things.

Outside of chromosome 21 and the sex chromosomes, most embryos with extra chromosomes do not usually survive. Because chromosomes carry many, many genes, a disruption of a chromosome can cause severe problems with the development of a fetus. Individuals with one (or more) fewer chromosome usually don't survive either. Can you explain why?

Damaged Chromosomes

Chromosomal disorders also occur when part of a chromosome becomes damaged. For example, if a tiny portion of chromosome 5 is missing, the individual will have cri du chat (cat’s cry) syndrome. These individuals have misshapen facial features, and the infant’s cry resembles a cat’s cry.

Summary

• Autosomal recessive genetic disorders, such as cystic fibrosis, are caused by recessive alleles of a single gene on an autosome.
• Autosomal dominant genetic disorders, such as Huntington's disease, are caused by dominant alleles of a single gene on an autosome.
• Changes in chromosome number can lead to disorders like Down syndrome.
• Chromosomal disorders also occur when part of a chromosome becomes damaged.

Explore More

Use the resources below to answer the questions that follow.

I. What are Genetic Disorders? at http://learn.genetics.utah.edu/content/disorders/

1. What are multifactorial disorders? What is an example of a multifactorial disorder?
2. What are single-gene disorders? What is an example of a single-gene disorder?
3. What causes galactosemia? How is it diagnosed? How is it treated?
4. What causes Colon Cancer? What is a tumor suppressor gene?
5. What is newborn genetic screening? How is it carried out?

II. Cracking the Code: Rare Chromosome Disorders at https://www.youtube.com/watch?v=k4Lps1kIyR0

1. What do all people diagnosed with a chromosome disorder share?
2. What is a clinical geneticist?
3. What are they trained to do that is different from a regular doctor?
4. What is a karyotype? Do chromosomal disorders always involve extra genetic material? Explain your answer.

Review

1. Can you be a carrier of an autosomal recessive genetic disorder?
2. Can you be a carrier of an autosomal dominant genetic disorder?
3. One parent is a carrier of the cystic fibrosis gene, while the other parent does not carry the allele. Can their child have cystic fibrosis?
4. What is a chromosomal disorder?
5. Explain what causes Down Syndrome.
6. When do chromosomal defects occur?
7. What happens to most embryos with extra chromosomes? Explain your answer.