Autosomal recessive traits and disorders are caused by the inheritance of recessive traits on autosomes. Recall that autosomes are all the chromosomes in your body, excluding the sex chromosomes.
In order for a person to inherit an autosomal recessive trait or disorder, both parents must contribute recessive alleles. A person with an autosomal recessive trait or disorder has a genotype with two recessive alleles, which is called homozygous recessive.
What are the odds certain parents would end up with offspring that are homozygous recessive? As you have learned, Punnett squares can be used to help predict the chances:
The first set of squares shows the odds two people who are both heterozygous (carriers) for a particular trait of having an offspring that is homozygous recessive. Remember that a capital letter indicates that the trait is dominant, and a lowercase letter shows the recessive trait.
The second set of squares shows a predicted cross of a parent that is heterozygous (a carrier) for a trait with someone who is homozygous recessive (someone whose phenotype shows the recessive trait).
The third set of squares shows a predicted cross of two people that are homozygous recessive.
The fourth set of squares shows a predicted cross of two people who are homozygous dominant. There really is no need to make a square for two people that are both homozygous dominant because they will always produce children with the dominant gene.
When a genetic disorder is inherited in an autosomal recessive pattern, the disorder corresponds to the recessive phenotype. Heterozygous individuals will not display symptoms of this disorder because their unaffected gene will compensate. Such an individual is called a carrier. Carriers for an autosomal recessive disorder may never know their genotype unless they have a child with the disorder. This is one reason why it can be hard to remove recessive traits and disorders from a population in the absence of genetic testing.
terms to know
Autosomal Recessive
The pattern of recessive inheritance that corresponds to a gene on one of the 22 autosomal chromosomes.
Carrier
A heterozygous individual who does not display symptoms of a recessive genetic disorder but can transmit the disorder to their offspring.
1a. Autosomal Recessive Disorders
An autosomal recessive disorder is cystic fibrosis (CF). CF occurs in approximately 1 in 3,200 live births, and people of European ancestry are at highest risk. Recall that CF is characterized by the chronic accumulation of thick, tenacious mucus (mucus that is difficult to clear) in the lungs and digestive tract.
Decades ago, children with CF rarely lived to adulthood. With advances in medical technology, the average lifespan in developed countries has increased into middle adulthood. CF is a relatively common disorder that occurs in approximately 30,000 people in the United States. A child born to two CF carriers would have a 25% chance of inheriting the disease. This is the same as the 3:1 dominant:recessive ratio that Mendel observed in his pea plants. The pattern is shown in the image below using a diagram that tracks the likely incidence of an autosomal recessive disorder on the basis of parental genotypes.
On the other hand, a child born to a CF carrier and someone with two unaffected alleles would have a 0% probability of inheriting CF but would have a 50% chance of being a carrier.
Autosomal Recessive Inheritance—The inheritance pattern of an autosomal recessive disorder with two carrier parents reflects a 3:1 probability of expression among offspring. (credit: U.S. National Library of Medicine)
Some other examples of autosomal recessive disorders include the metabolic disorder phenylketonuria (PKU), the blood disorder sickle-cell anemia, and the fatal neurological disorder Tay-Sachs disease.
Phenylketonuria (PKU) is an autosomal recessive disorder in which the buildup of a certain amino acid (phenylalanine) will get too high in a person's body. If they get too much of this certain amino acid built up in their body, it can cause mental retardation. Diet can help lower the amount of phenylalanine and help prevent symptoms of this disorder. PKU occurs in approximately 1 in 10,000 to 15,000 live births.
Sickle-cell anemia is when red blood cells are shaped like a sickle (a small farm tool with a semicircular blade). Sickle-cell anemia can result in anemia, acute chest pain, pain episodes, and organ damage. This disease occurs in approximately 1 in 365 live births, and Black and African American individuals are at greatest risk.
Tay-Sachs disease is a progressive neurological disorder that produces muscle degeneration and blindness, and it is usually fatal. This disease occurs in 1 in 320,000 live births, and it is more frequent in people of central or eastern European Jewish descent.
terms to know
Cystic Fibrosis (CF)
An autosomal recessive disorder that results in buildup of mucus in the lungs.
Phenylketonuria (PKU)
An autosomal recessive disorder that results in the buildup of phenylalanine (an amino acid) in the body that can lead to mental retardation if levels exceed a certain point.
Sickle-Cell Anemia
An autosomal recessive disorder that results in sickle-shaped red blood cells.
Tay-Sachs Disease
An autosomal recessive neurological disorder that produces muscle degeneration and blindness and is usually fatal.
2. Autosomal Dominant Inheritance
If an abnormality is dominant and the allele is located on one of the 22 pairs of autosomes (non-sex chromosomes), we refer to its inheritance pattern as autosomal dominant.
If a parent possesses at least one dominant allele, that parent will have that trait. The only way that 100% of their potential children would be normal is if both parents were homozygous recessive. In the Punnett square below, you can see that if both parents are homozygous recessive, meaning neither possesses a single dominant allele, then they have a 100% chance of having all normal children. In this case, the normal trait would be homozygous recessive; however, you only need one dominant allele to possess a dominant trait.
Say both parents are heterozygous, meaning they have an autosomal trait or disorder because they both have at least one dominant allele. If you look at the Punnett Square below, you'll notice that because they also each possess a recessive allele for the normal condition, they actually have a 25% chance of having a child who is unaffected by the disease.
Now say one parent is heterozygous, and the other parent is homozygous recessive. The first parent contains one allele for that autosomal dominant trait—that parent possesses the trait—whereas the second parent is completely normal. As the Punnett square below shows, this combination of parent alleles means the parents have a 50% chance of having normal children.
term to know
Autosomal Dominant
The pattern of dominant inheritance that corresponds to a gene on one of the 22 autosomal chromosomes.
2a. Autosomal Dominant Disorders
Neurofibromatosis type I is a disease that induces tumor formation within the nervous system, leading to skin and skeletal deformities. Neurofibromatosis is caused by a gene mutation that is autosomal dominant, as one copy of this mutation causes this disorder.
IN CONTEXT
If there is a couple in which one parent is heterozygous for this disorder (and who therefore has neurofibromatosis), Nn, and one parent is homozygous for the normal gene, nn, the heterozygous parent would have a 50% chance of passing the dominant allele for this disorder to their offspring, and the homozygous parent would always pass the normal allele. Therefore, four possible offspring genotypes are equally likely to occur: Nn, Nn, nn, and nn. That is, every child of this couple would have a 50% chance of inheriting neurofibromatosis.
Inheritance of Neurofibromatosis—Inheritance pattern of an autosomal dominant disorder, such as neurofibromatosis, is shown in a Punnett square.
Other genetic diseases that are inherited in this pattern are Marfan syndrome, Huntington’s disease, achondroplasia, and familial hypercholesterolemia. Because autosomal dominant disorders are expressed by the presence of just one gene, an individual with the disorder will know that they have at least one faulty gene.
Huntington's Disease: A neurodegenerative disorder that causes motor and cognitive impairment and eventually becomes fatal. It is inherited, but the symptoms don't show up until adulthood. Often, people don't realize that they have Huntington's disease until they've already reproduced and passed it on to their children.
Marfan Syndrome: A genetic disorder of connective tissue that causes people to have a certain appearance, being abnormally tall with long limbs and digits; it can also affect other connective tissues such as heart valves and can be fatal. This disorder causes a weakening of the aorta, meaning that the aorta can rupture over time. Generally, people with this disorder are very, very tall and lanky. However, the weakening of the aorta is the most serious aspect of Marfan syndrome because rupturing can occur with intense physical activity.
Achondroplasia: This disorder results in a person who has short arms and legs and who is short overall. Most people with this type of disorder only get to be about 4 to 4.5 feet tall, so achondroplasia affects a person's height, and then—as a result—causes shortness of the arms and legs.
Familial Hypercholesterolemia: This disorder leads to high blood cholesterol. This is because the cholesterol in the blood of people with this disorder won't carry low-density lipoproteins (LDLs), which is the first step in removing the cholesterol from the body. Therefore, most people with familial hypercholesterolemia often don't have as long of a lifespan as other people as a result of their very high cholesterol.
Even though autosomal dominant disorders occur if there is just one faulty allele, it can be hard to remove these disorders from populations if they present after individuals have already reproduced. For example, Huntington’s disease manifests after the childbearing years. The onset of Huntington’s disease may not occur until age 40, at which point the afflicted persons may have already passed the allele to 50% of their offspring.
terms to know
Neurofibromatosis
An autosomal dominant disease that induces tumor formation within the nervous system that leads to skin and skeletal deformities.
Huntington's Disease
An autosomal dominant disorder which does not show up until later in life, often after the gene has been passed onto offspring; it is a neurodegenerative disorder that causes motor and cognitive impairment and eventually becomes fatal.
Marfan Syndrome
A genetic disorder of connective tissue that causes people to have a certain appearance, being abnormally tall with long limbs and digits; it can also affect other connective tissues such as heart valves and can be fatal.
Achondroplasia
An autosomal dominant disorder in which the person is abnormally short in stature with short arms and legs.
Familial Hypercholesterolemia
An autosomal dominant disorder in which a person has chronic high cholesterol.
summary
In this lesson, you learned about how traits and disorders can be inherited in autosomal dominant and recessive patterns. First, you explored autosomal recessive inheritance, in which the trait of interest is expressed in a recessive manner and on one of the 22 autosomal chromosomes, and you learned about some relatively common autosomal recessive disorders, including cystic fibrosis (CF), phenylketonuria (PKU), sickle-cell anemia, and Tay-Sachs disease. Then, you examined autosomal dominant inheritance, in which the trait of interest is expressed in a dominant manner and on one of the 22 autosomal chromosomes, and you learned about some common autosomal dominant disorders, including neurofibromatosis, Huntington’s disease, Marfan syndrome, achondroplasia, and familial hypercholesterolemia.
Terms to Know
Achondroplasia
An autosomal dominant disorder in which the person is abnormally short in stature with short arms and legs.
Autosomal Dominant
The pattern of dominant inheritance that corresponds to a gene on one of the 22 autosomal chromosomes.
Autosomal Recessive
The pattern of recessive inheritance that corresponds to a gene on one of the 22 autosomal chromosomes.
Carrier
A heterozygous individual who does not display symptoms of a recessive genetic disorder but can transmit the disorder to their offspring.
Cystic Fibrosis (CF)
An autosomal recessive disorder that results in buildup of mucus in the lungs.
Familial Hypercholesterolemia
An autosomal dominant disorder in which a person has chronic high cholesterol.
Huntington's Disease
An autosomal dominant disorder which does not show up until later in life, often after the gene has been passed onto offspring; it is a neurodegenerative disorder that causes motor and cognitive impairment and eventually becomes fatal.
Marfan Syndrome
A genetic disorder of connective tissue that causes people to have a certain appearance, being abnormally tall with long limbs and digits; it can also affect other connective tissues such as heart valves and can be fatal.
Neurofibromatosis
An autosomal dominant disease that induces tumor formation within the nervous system that leads to skin and skeletal deformities.
Phenylketonuria (PKU)
An autosomal recessive disorder that results in the buildup of phenylalanine (an amino acid) in the body that can lead to mental retardation if levels exceed a certain point.
Sickle-Cell Anemia
An autosomal recessive disorder that results in sickle-shaped red blood cells.
Tay-Sachs Disease
An autosomal recessive neurological disorder that produces muscle degeneration and blindness and is usually fatal.
