Lesson Explainer: Inherited Disorders Biology

In this explainer, we will learn how to use genetic diagrams to predict probabilities of offspring inheriting recessive or dominant genetic disorders.

Genes and their variations, called alleles, control a huge number of our characteristics. The combination and expression of your alleles will determine traits like your eye or hair color, whether you have freckles, and even whether you can roll your tongue! Unfortunately, there are also alleles that can cause disabling or deadly diseases. Here, we will take a look at some examples of these inherited disorders and how we can use Punnett squares to study the inheritance of their alleles.

Definition: Gene

A gene is a section of DNA that contains the information needed to produce a specific protein.

Definition: Allele

An allele is an alternative version of a gene.

Many genetic disorders are caused by recessive alleles. Recessive alleles are those that are only expressed in a person’s characteristics when there are two copies present, or there are no dominant alleles present, and are usually indicated by a lowercase letter.

Definition: Dominant Allele

A dominant allele is one that is always expressed in the phenotype if present in the genotype.

Definition: Recessive Allele

A recessive allele is one that is only expressed in the phenotype if two copies are present, or if a dominant allele is not present.

Let’s look at an example: The disease cystic fibrosis is caused by a recessive allele. People with cystic fibrosis will typically suffer from a buildup of mucus in their lungs and digestive system. They may have problems with breathing, suffer from recurring chest problems, and be more susceptible to malnutrition due to problems with digesting food. In 2017, it was estimated that over 48‎ ‎000 people in Europe had cystic fibrosis.

The cause of cystic fibrosis is a change in the DNA sequence of the CFTR gene. If we assume that the recessive allele that causes cystic fibrosis can be presented by a lowercase f, and the allele that allows the expression of the normal CFTR gene can be represented by an uppercase F, then a person must have the combination “ff” in their genotype to have cystic fibrosis.

Example 1: Recalling the Definition of Recessive Alleles in terms of Cystic Fibrosis

Cystic fibrosis is an inherited disorder caused by a recessive allele. Which of the following best explains what this means?

  1. A person must have two copies of the allele for cystic fibrosis to be expressed in the phenotype.
  2. Cystic fibrosis only affects offspring once every two generations.
  3. A person can carry more than one copy of the allele and not have cystic fibrosis.
  4. A person must have three copies of the allele for cystic fibrosis to be expressed in the phenotype.
  5. A person will express the disease in their phenotype even if only one allele is in their genotype.

Answer

Alleles are different versions of the same gene. They determine many of our genetically inherited characteristics, like eye color and natural hair color, but some alleles can also result in the inheritance of genetic diseases. Cystic fibrosis is caused by a mutation in the CFTR gene, and it leads to problems like a buildup of mucus in the respiratory and digestive systems.

For each gene, a person will have two alleles in their genetic makeup, or “genotype.” Recessive alleles are those alleles that can only be expressed in a person’s characteristics—or phenotype—if there are two copies present or a dominant allele is not present. Dominant alleles will be expressed even if only one copy is present in a person’s genotype. A person could have both a dominant allele that is expressed, which allows the correct function of the CFTR gene, and a recessive allele that causes cystic fibrosis but is not expressed; this person is known as a “carrier.” Diseases caused by recessive alleles will not necessarily “skip” generations. The following diagram shows how the offspring of a person affected by cystic fibrosis can also inherit the alleles and, therefore, the disease.

If the child inherits one cystic fibrosis allele from their father who does have cystic fibrosis, and one allele from their mother who is a carrier, they will have two recessive alleles in the genotype, so they will also be affected by the disease.

Therefore, the only correct choice from the options shown is that a person must have two copies of the allele for cystic fibrosis to be expressed in the phenotype.

We can study the inheritance of these alleles by using a Punnett square. Punnett squares are incredibly useful diagrammatic representations of the inheritance of alleles from a parent’s gamete, or sex cell, to the offspring. Figure 2 demonstrates the inheritance of the cystic fibrosis allele.

For each gene, a person will have two alleles. If these two alleles are the same, for instance, they are both recessive, the person is homozygous for these alleles. If they are different, so one is recessive but one is dominant, the person is heterozygous.

Definition: Homozygous

An individual is homozygous for a characteristic is they have a pair of identical alleles for a gene.

Definition: Heterozygous

An individual is heterozygous for a characteristic if they have two different alleles for a gene.

In figure 2, we can see that both parents are heterozygous. When they produce gametes, which are egg cells by the mother and sperm cells by the father, these gametes will only contain one allele for a specific gene. This is so when they combine during fertilization, the offspring inherits one allele for each trait from their mother and one allele from their father and therefore has the correct number of alleles.

If we look at the genotypes shown in the Punnett square in figure 2, they are 1×FF, 2×Ff, and 1×. Out of four possible combinations, two are heterozygous for the cystic fibrosis allele (Ff). One of these four possible combinations is homozygous dominant (FF) and one is homozygous recessive (ff). We know that if a dominant allele is present in the genotype, it will always be expressed. This means that three out of the four combinations, or 75%, will result in offspring that are unaffected by cystic fibrosis. Only one out of four of the combinations, or 25%, will result in the offspring having two recessive alleles and therefore inheriting the cystic fibrosis disease.

The cells of the Punnett square show the possible combination of alleles for each offspring born to these parents. It is important to remember that it is not showing how many children out of four are going to inherit the disease, but rather the chance of each child born inheriting the disease. This means that the couple could have four children all unaffected by cystic fibrosis or have four children all affected by it.

Example 2: Interpreting a Punnett Square to Show Inheritance of a Genetic Disease

Tay–Sachs is an inherited disease caused by a recessive allele (t). The Punnett square shows the genotypes of a male and a female and the predicted genotypes for their offspring. What is the probability, in percent, that a child born to these parents will inherit Tay–Sachs disease?

FatherMother
Tt
TTTTt
tTttt

Answer

To answer this question, we need to first understand some of the key terms. Alleles are different versions of the same gene, and for each characteristic that is controlled by a gene, a person will have two alleles. Recessive alleles are those that can only be expressed if there are two copies present or there is no dominant allele present, whereas dominant alleles are expressed even when there is only one copy present. A child will inherit one allele for a particular characteristic from their mother and one allele from their father. The genotype of a person is the combination of alleles that they have, which in Punnett squares is shown in the central boxes.

A Punnett square is an incredibly useful diagram to show the probability of the offspring born to a couple inheriting certain alleles. It is important to note that this probability is given for each child born to these parents, not out of every four children born.

Looking at the Punnett square provided, we can see that the possible genotypes for each child born to these parents are TT, Tt, Tt, and tt. To simplify this, each child born has a 1-in-4 chance of having the genotype TT, a 2-in-4 chance of having the genotype Tt, and a 1-in-4 chance of having the genotype tt.

The question states that Tay–Sachs is caused by a recessive allele (t). As we mentioned, for a recessive allele to be expressed, two copies must be present in the genotype. That means the only combination shown in the Punnett square that would mean the child inheriting Tay–Sachs is tt. As we saw previously, there is a 1-in-4 chance of this happening. The question asks us to give our answer as a percent, so we need to convert 1 in 4 to a percent; we can do this by using the following calculation: (1÷4)×100%=25%.

Alternatively, we could remember that a quarter of 100% is 25%.

Therefore, the probability of a child born to these parents inheriting Tay–Sachs disease is 25%.

Let’s have a look at another example of an inherited disorder. Huntington’s disease is a genetic disease that impairs a person’s ability to move, negatively affects their memory and cognition, and can impact their control of their emotions. The first signs of Huntington’s disease will usually not appear in a person until they are over 30 years of age. This means that people can grow up and have children before they even know they have inherited this disease. Unlike cystic fibrosis, Huntington’s disease is caused by a dominant allele, which means a person only has to have one copy of the allele present in their genotype to be affected by this disease. Figure 3 outlines how the Huntington’s allele can be inherited from parents to offspring.

Figure 3 demonstrates how the presence of a single dominant allele (H) will result in a child inheriting the disease, even if the other allele present is recessive. The probability a child born to the parents in figure 3 will inherit Huntington’s is 50%; this is very high considering only one parent has the allele, but this is a typical trait of a dominant genetic disease.

Example 3: Completing a Punnett Square to Show Inheritance of a Dominant Genetic Disease

Polydactyly is an inherited disorder, and in most cases, it is caused by a dominant allele (D). Use this partially completed Punnett square, which shows the genotypes of a male and a female, to predict the probability, in percent, that a child born to these parents would inherit polydactyly.

FatherMother
dd
D
d

Answer

To answer this question, we need to first understand some of the key terms. Alleles are different versions of the same gene, and for each characteristic that is controlled by a gene, a person will have two alleles. Recessive alleles are those that can only be expressed if there are two copies present, whereas dominant alleles are expressed even when there is only one copy present. A child will inherit one allele for a particular characteristic from their mother and one allele from their father. The genotype of a person is the combination of alleles that they have, which in Punnett squares is shown in the central boxes.

A Punnett square is an incredibly useful diagram to show the probability of the offspring born to a couple inheriting certain alleles. It is important to note that this probability is given for each child born to these parents, not out of every four children born.

Let’s complete the Punnett square to show the possible combination of alleles that the offspring may inherit.

FatherMother
dd
DDdDd
ddddd

We can complete each of the cells in the table by taking the allele from the mother (the column head) and the allele from the father (the row head). Each cell to show the possible genotypes should always have two alleles. The possible combinations are, therefore, 2×Dd and 2×dd.

Looking back at the question, we can see that polydactyly is caused by a dominant allele (D). We know that if a dominant allele is present in the genotype, it is expressed, even if the other allele is recessive. This means out of our possible 4 combinations, 2 are going to express the dominant allele D, and the child will be affected by polydactyly. The question has asked for our answer in percent, which we can calculate using the following calculation: (2÷4)×100%=50%.

Alternatively, we can remember that half of 100% is 50%.

Therefore, the probability of a child born to these parents inheriting polydactyly is 50%.

There are some key differences that we can see when a genetic disease is caused by a dominant allele rather than a recessive allele. If the disorder is caused by a dominant allele, the offspring only need one copy of the allele in their genotype to inherit the disorder. This means that the likelihood of them inheriting and developing the disease is higher than if it was caused by a recessive allele. Let’s compare two Punnett squares, one showing the inheritance of the cystic fibrosis allele and one showing the inheritance of the Huntington’s disease allele.

In figure 4, we can compare how these alleles are inherited and expressed in the phenotype when both parents are heterozygous for the disease-causing alleles. Both parents that are heterozygous for cystic fibrosis are not affected by the disease, as it is caused by a recessive allele and in both cases a dominant allele is present to “mask” the expression of the recessive allele. However, both parents heterozygous for Huntington’s are affected by the disease. In the example showing the inheritance of cystic fibrosis, each offspring produced by these parents has a 25% chance of being affected with the disease. However, in the case of Huntington’s, any offspring produced will have a 75% chance of inheriting the dominant allele and being affected with the disease.

Inheritance of a recessive disease-causing allele can also result in some people being “carriers.” This happens when a person is heterozygous for the allele. As we can see in the cystic fibrosis Punnett square of figure 4, there is a 50% chance of the offspring having the combination “Ff”—they will not be affected by the disease because they need two copies of the recessive allele for it to be present in the phenotype. However, they do carry this recessive allele and could potentially pass it on to their offspring; therefore, they are known as “carriers.” We can see the action of carriers in the first Punnett square of figure 4, as both parents are heterozygous for the cystic fibrosis allele and are therefore carriers themselves. Carriers do not appear in inheritance of dominant alleles because people heterozygous for these diseases will be affected by them.

The inheritance of some alleles can cause disorders so severe that they can be fatal before the organism has even been born. The yellow fur of mice is determined by the presence of a dominant allele (Y), with the recessive allele (y) giving the mouse brown fur. However, if a mouse in the uterus of its mother has inherited two Y alleles, it will die while still in the embryonic stage. The allele for yellow fur is known as a “lethal gene.” Figure 5 outlines how these alleles are inherited from heterozygous parents and the phenotypes produced in the offspring.

Example 4: Analyzing Punnett Squares to Determine Potential Impacts of Lethal Alleles

The allele that gives mice yellow fur (Y) is dominant to the allele that gives brown fur (y). However, if a mouse embryo inherits two dominant alleles, it will die in utero. A student views the genetic diagram provided. They conclude that no mice will inherit the lethal alleles and die in utero. Is this student correct? Why?

Answer

The question is asking us to determine some genotypes from the provided phenotypes to establish if the student is correct in stating that no mice in the offspring will inherit two dominant yellow fur alleles (YY) and therefore die before birth.

We can see from the phenotypes given that the dominant : recessive phenotypic trait (so number of yellow mice : number of brown mice) is occurring in the ratio 31. Therefore, we can assume that the single brown mouse has inherited two recessive alleles (yy). We know that offspring inherit one allele for a characteristic from their mother and one from their father. This means that the genotype of both parents must include a recessive allele. As they both display the yellow phenotype, we can also determine that they must have a dominant yellow allele. The genotypes of the parents must therefore be Yy.

Let’s use their genotypes to complete a Punnett square and show the probability of the offspring inheriting a certain genotype.

As we can see, there are three possible genotypes that the offspring can have: There is a 2-in-4 chance that they will have the genotype Yy and have yellow fur but will be unaffected by the lethal alleles. There is a 1-in-4 chance that they will have the genotype yy and have brown fur. Finally, there is a 1-in-4 chance that they will have the genotype YY, which are the lethal alleles, and die before birth.

Therefore, the student is incorrect. There is a 25% chance of the offspring born to these parents inheriting two dominant Y alleles and dying in utero.

Lethal genes can negatively affect plants and well as animals. The green color of plant leaves comes from the production of the pigment chlorophyll, and chlorophyll is crucial in trapping the sunlight that plants need for photosynthesis. If a plant cannot photosynthesize, it cannot produce its own food, and it will struggle to survive. The gene for chlorophyll production in corn plants (C) is dominant over the gene for the absence of chlorophyll (c). When corn plants that were heterozygous for these alleles were self-pollinated, there were some seedlings that had complete absence of chlorophyll. These plants would initially grow but very quickly wilt and die. The plants completely absent of chlorophyll were homozygous recessive, so they had the genotype cc, and hence this is an example of a recessive lethal gene.

Let’s review some of the key things we have learned about genetically inherited disorders.

Key Points

  • Alleles are variants of a gene that control certain characteristics.
  • Some recessive and dominant alleles can cause genetic disorders, for example, cystic fibrosis and Huntington’s disease respectively.
  • If the disease is caused by a recessive allele, offspring could be carriers, but if it is caused by a dominant allele, there will be no carriers.
  • Some alleles can be lethal when present in an organism’s genotype, for example, those that code for yellow fur in mice or the absence of chlorophyll in plants.
  • Punnett squares can be used to study inheritance of alleles and predict the probability of offspring inheriting genetic diseases.

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