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8.1: Single Gene Disorders

Difficulty Level: At Grade Created by: CK-12
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What are some genetic conditions that research might be able to help treat or cure?

One reason that geneticists are interested in human traits and variations is because some are recognized as diseases, disorders, or defects. Geneticists, working to control, cure, or prevent these genetic conditions, hope to improve the quality of life.

There are thousands of human genetic conditions caused by single genes. In these conditions, for some reason, the gene does not produce a protein that functions properly. To study these single gene conditions more easily, geneticists have classified them into three groups-dominant, recessive, and sex-linked-based on their pattern of inheritance.

Did You Know?

There are thousands of disorders or handicaps that scientists know are caused by single gene defects. Although some of these genetic defects have symptoms that can be treated, scientists are still unable to treat the defects, let alone to cure the people who have the condition.

Dominant Inheritance Pattern

In those conditions that have the dominant inheritance pattern, the allele that causes the defect is a dominant allele. In this case, one of the parents has the condition and has the dominant allele. If only one parent has the condition, the children in that family will each have either a 50% or a 100% chance of inheriting it depending on the genotype of the affected parent. If the afflicted parent is heterozygous, the children have a 50% chance of receiving the dominant allele and a 50% chance of receiving the recessive allele. Heterozygous means that the person has one dominant allele and one recessive allele. If the parent is homozygous, all the children will receive the dominant allele. Homozygous means the person has the same alleles. In this case, the parent has two dominant alleles.

A homozygous parent for the dominant allele can be represented by DD and a homozygous parent for the recessive allele can be represented by dd. A heterozygous parent can be represented by Dd. Remember that an uppercase letter always stands for the dominant allele and a lowercase letter always stands for the recessive allele. Now use D for the dominant allele and d for the recessive allele to sort alleles according to the possible genotypes of parents. Then you can find out what possible genotypes the different combinations of alleles might produce in their offspring.

There are 1,500 confirmed or suspected dominant abnormal conditions in humans. The names of some of them are

  • achondroplasia, which is a form of dwarfism,
  • polydactyly, which means having extra fingers or toes, and
  • Huntington's disease, which causes progressive dementia later in life.

Figure 7.1 In a dominant inheritance pattern, if a heterozygous parent has the allele, there is a 50% chance that any child born will receive the defective gene and have the condition. Also, there is a 50% chance that a child will receive the normal gene and not have the condition.

Recently, the gene for Huntington's disease has been found. A group of researchers in the early 1990s was able to identify the area of the specific chromosome (Chromosome 4) where the gene for Huntington's disease is located by studying a population of affected people in Central America. Huntington's disease is progressive, involving the destruction of brain cells. Usually, death occurs 10 to 20 years after the onset of symptoms. One of the characteristics of Huntington's disease is that affected people usually do not get sick until after they are in their mid-30s. In the future, it may be possible for people to know they will develop the symptoms of a disorder before they actually get the disease, as researchers have been able to do for those with Huntington's disease.

Recessive Inheritance Pattern

Another class of inherited abnormal conditions is recessive conditions. In these conditions, both parents have a single recessive allele for the disorder. Because each of the parents has one recessive allele and one dominant allele, the condition is not apparent in them. It is hidden. Neither parent has the disease, but both are carriers of the disease. There is one chance in four that both parents will pass on the recessive allele to their child resulting in the child having both recessive alleles. The recessive allele will then be expressed and the child will have the disease. If the child received the recessive allele from only one parent, the child would be like the parents, that is, he or she would be a carrier but have no symptoms of the disease. The child could pass the allele on to his or her children.

There is a common mistake that people often make about recessive conditions. Sometimes people mistakenly think that if parents have had one child with a condition, the next child will not have the condition. This conclusion is incorrect. Since each birth is a new event, the chances of having a child with the condition is one in four each time a child is conceived by these parents.

There are over 1,220 confirmed or suspected recessive, abnormal conditions. Some of the common recessive conditions are

  • cystic fibrosis, which affects the mucus and sweat glands,
  • phenylketonuria, which is a deficiency of an enzyme in the liver,
  • sickle-cell disease, which is a disorder of the red blood cells, and
  • Tay-Sachs disease, in which the brain is damaged.

Because geneticists are learning much more about recessive disorders, you may read about them in the newspaper or hear about them on television. If you are a good observer, you may see something about a genetic disease almost every day. Have you read anything recently in the newspaper or in a magazine about genetic diseases and some possible new treatments or cures?

Activity 7-1: Exploring a Single Gene Disorder


How can a single gene be responsible for causing a genetic condition in humans? What are some examples of single gene disorders? What kinds of treatments are available? In this activity you use models to demonstrate how dominant and recessive alleles interact with each other to cause genetic disorders. Then you select an example of a single gene disorder to investigate.


  • 2 strips of colored construction paper
  • 2 strips of clear plastic
  • Activity Report
  • Computer with Internet connection (Optional)


Step 1 Obtain 2 strips of colored construction paper and 2 strips of clear plastic. Place the strips on the table in front of you. Imagine that each strip of paper and plastic is an allele (gene) for the same trait. The colored strips of paper represent dominant alleles and the clear plastic strips represent recessive alleles.

Step 2 Place a colored strip on top of another colored strip. What do you observe? These alleles contain the same information. The variation represented by this information will be expressed in the individual. dominant alleles and the clear plastic strips represent recessive alleles.

Step 3 Place a colored strip on top of a clear strip of plastic. What do you observe? These alleles contain different information from one another. The variation caused by the dominant allele will be expressed in the individual.

Step 4 Place a clear plastic strip on top of the colored strip. What do you observe? These alleles contain different information from one another. Which variation will be expressed in the individual?

Step 5 Place one clear plastic strip on top of the other clear plastic strip. What do you observe? In this pair of alleles, the variation caused by the recessive alleles is expressed.

Step 6 Return the strips.

Step 7 Select one of the following single-gene disorders or another one of your choice to investigate. Use the library, local college or university resources, and the Internet, if available, to research the selected single-gene disorder.

  • Cystic fibrosis
  • Phenylketonuria
  • Tay-Sachs
  • Polydactyly

What is known about the disorder you selected? On which chromosome is the defective gene located? Are there treatments for this disorder, and, if so, what are they? What do geneticists need to find out in order to treat or cure the disorder?

Step 8 Complete the Activity Report.

X-linked Inheritance Pattern

For some traits, there is a difference in the variations that are expressed (seen) among males and females. For example, many more boys than girls are color-blind. Some of these traits are called X-linked or sex linked. The genes for these traits are part of the X chromosome but are not on the Y chromosome. This is because the X chromosome is larger and possesses more genes.

The Transmission of X-linked Traits

The gene for an X-linked genetic disease is controlled by two alleles. If the expression of the recessive allele results in the genetic disease, then we represent this recessive allele by the lowercase letter n. The normal condition results when the normal allele is present because it is dominant. We represent this allele by the uppercase letter N. The symbols n and N, indicating the X-linked trait are shown on the X chromosome but not the Y chromosome.

Chromosome/Gene Pattern Expression
\begin{align*}\mathrm{X}^\mathrm{N}\mathrm{X}^\mathrm{N}\end{align*}XNXN Normal female
\begin{align*}\mathrm{X}^\mathrm{N}\mathrm{X}^\mathrm{n}\end{align*}XNXn Normal female carrying recessive gene
\begin{align*}\mathrm{X}^\mathrm{n}\mathrm{X}^\mathrm{N}\end{align*}XnXN Normal female carrying recessive gene
\begin{align*}\mathrm{X}^\mathrm{n}\mathrm{X}^\mathrm{n}\end{align*}XnXn Female having the genetic disease
\begin{align*}\mathrm{X}^\mathrm{n}\mathrm{Y}\end{align*}XnY Male having the genetic disease
\begin{align*}\mathrm{X}^\mathrm{N}\mathrm{Y}\end{align*}XNY Normal male

Figure 7.2

A mother carries a defective gene on one of her X-chromosomes. Neither the mother nor the father shows evidence of the genetic disease.

\begin{align*}\mathrm{X^N}\end{align*}XN \begin{align*}\mathrm{X^n}\end{align*}Xn
\begin{align*}\mathrm{X^N}\end{align*}XN \begin{align*}\mathrm{X^N X^N}\end{align*}XNXN \begin{align*}\mathrm{X^N X^n}\end{align*}XNXn
\begin{align*}\mathrm{Y}\end{align*}Y \begin{align*}\mathrm{X^N Y}\end{align*}XNY \begin{align*}\mathrm{X^n Y}\end{align*}XnY

Figure 7.3

Look at Figures 7.2 and 7.3. What are the chances that the daughters of the couple will have the genetic disease? What are the chances that the sons of the couple will have the genetic disease? What proportion of the daughters will carry but not express the genetic disease? Why do more males than females have X-linked genetic diseases?

Hemophilia Hemophilia is an X-linked trait. This is a condition in which the blood fails to clot or clots very slowly after an injury. Like color blindness, hemophilia is caused by a recessive gene. H represents normal clotting and h represents abnormal clotting. Then \begin{align*}\mathrm{X}^\mathrm{h}\mathrm{X}^\mathrm{h}\end{align*}XhXh and \begin{align*}\mathrm{X}^\mathrm{h}\mathrm{Y}\end{align*}XhY would suffer from hemophilia while \begin{align*}\mathrm{X}^\mathrm{H}\mathrm{X}^\mathrm{H}\end{align*}XHXH, \begin{align*}\mathrm{X}^\mathrm{H}\mathrm{X}^\mathrm{h}\end{align*}XHXh, and \begin{align*}\mathrm{X}^\mathrm{H}\mathrm{y}\end{align*}XHy would be normal. Suppose a non hemophiliac man marries a non hemophiliac woman, and they have two children. A son suffers from hemophilia and a daughter is a non hemophiliac.

  1. What is the chromosome/gene pattern of the parents?
  2. What is the chromosome/gene pattern of the son and daughter?

The reason X-linked traits are more likely to be seen in males than in females is because the male gets only one X chromosome and the female gets two X chromosomes. Therefore, if a male has a defective gene on his X chromosome, it will be expressed. But if a female has a defective gene on one of her X chromosomes, she still is likely to have a normal allele on her other X chromosome. Remember that a gene codes for a protein. If the X-linked gene is defective in a male, he will have only the defective protein. The female with this gene may also have the defective protein in her cells, but she will have the correct protein as well.

There are over 200 confirmed or suspected X-linked conditions. Some of them are

  • red-green color blindness, in which the person cannot distinguish red from green,
  • hemophilia, in which blood does not clot properly, and
  • some forms of muscular dystrophy, in which the muscles waste away.

You may have heard of hemophilia in your study of history because Alexis, the son of Nicholas II of Russia, had hemophilia. The interference of Rasputin, who promised to cure the disease, is considered by some historians to be one of the causes of the Russian Revolution. More recently, in the 1980s, hemophilia was in the news because some hemophiliacs contracted AIDS from the blood transfusions that they needed then to control their disease. At that time, the blood banks did not have the excellent HIV virus screening procedure that is in use today. Because of these screening techniques for the HIV virus that causes AIDS, the blood supplies in the United States are very safe today.

X-linked Inheritance Pattern-Color Blindness The gene for color blindness is a recessive allele. This allele can be represented by the lowercase letter c. Normal color vision is a dominant allele that can be represented by the uppercase letter C. X-linked genes are part of the X chromosome but not the Y chromosome. Therefore, the symbols C and c, representing the color vision trait, are shown on the X chromosome, not on the Y chromosome.

The reason that hemophiliacs need blood transfusions is a good example of the fact that genes code for proteins. There are certain proteins that circulate in your blood that can be activated by an injury to a blood vessel. When activated, these proteins cause blood to clot. That clotting stops bleeding from the injured blood vessel. Hemophilia results from a defective gene that cannot make a normal clotting protein. Hemophiliacs used to get that normal protein by transfusions. Many are now able to get the necessary clotting factors or proteins by production in the laboratory through a process called genetic engineering. You will learn more about genetic engineering later in this unit.

Another recessive X-linked trait is color blindness. Look at the table below. Then answer the following questions about the genetics of color blindness.

  1. What will be the chromosome/gene pattern for a color-blind female? What will be the chromosome/gene pattern for a colorblind male?
  2. Do more males than females express color blindness? Explain.
  3. Two parents who have normal color vision parents produce a color-blind son.
    1. What is the chromosome/gene pattern of the parents?
    2. Can they produce a color-blind daughter? Explain.

Imagine that you are completely colorblind. You see a world of black and white all the time. Write a poem about your impressions of the world-a world where color has no meaning but which exists in shades of gray.

Chromosome/Gene Pattern Expression
\begin{align*}\mathrm{X}^{\mathrm{C}} \mathrm{X}^{\mathrm{C}}\end{align*}XCXC Normal female
\begin{align*}\mathrm{X}^{\mathrm{C}} \mathrm{X}^{\mathrm{c}}\end{align*}XCXc Normal female carrying the recessive gene
\begin{align*}\mathrm{X}^{\mathrm{c}} \mathrm{X}^{\mathrm{c}}\end{align*}XcXc Color-blind female
\begin{align*}\mathrm{X}^{\mathrm{C}} \mathrm{Y}\end{align*}XCY Normal male
\begin{align*}\mathrm{X}^{\mathrm{c}} \mathrm{Y}\end{align*}XcY Color-blind male

Pedigree Analysis

The following link is to a pedigree analysis activity. Autosomal dominant, autosomal recessive and sex-linked recessive inheritance is explored through an interactive activity. CK-12 Pedigree Analysis Animation

Review Questions

  1. What is the difference between a dominant inheritance pattern and a recessive inheritance pattern?
  2. What is the chance that a son and daughter will be color-blind if their parents are not color-blind, but their mother's father is color-blind?

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Date Created:
Feb 23, 2012
Last Modified:
Jan 30, 2016
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