Question

What is required for an autosomal recessive disorder to appear? Name four autosomal recessive disorders. What is required for an autosomal dominant disorder to appear? Name two autosomal dominant disorders. (pages \(474-77\) )

Short answer

For an autosomal recessive disorder to appear, an individual must inherit two copies of the recessive gene, one from each parent. Four examples of autosomal recessive disorders are cystic fibrosis, sickle cell anemia, Tay-Sachs disease, and phenylketonuria (PKU). An autosomal dominant disorder appears when an individual inherits just one copy of the dominant gene. Two examples of autosomal dominant disorders are Huntington's disease and Marfan syndrome.

Step-by-step solution

Conditions for an Autosomal Recessive Disorder to Appear

Autosomal recessive disorders occur when an individual inherits two copies of the recessive gene associated with the disorder, one from each parent. In order to have the autosomal recessive disorder, the person must be homozygous for the recessive gene (i.e., carry two copies of the recessive allele).

Four Autosomal Recessive Disorders

Here are four examples of autosomal recessive disorders: 1. Cystic fibrosis: a genetic disorder that affects the lungs, pancreas, and other organs, resulting in difficulty breathing and frequent lung infections 2. Sickle cell anemia: a genetic disorder that affects the shape and functionality of red blood cells, leading to anemia and other complications 3. Tay-Sachs disease: a rare and fatal genetic disorder that progressively destroys nerve cells in the brain and spinal cord 4. Phenylketonuria (PKU): a genetic disorder that prevents the body from breaking down the amino acid phenylalanine, leading to intellectual disabilities if not treated with a strict diet.

Conditions for an Autosomal Dominant Disorder to Appear

Autosomal dominant disorders occur when an individual inherits just one copy of the dominant gene associated with the disorder. If a person has one dominant allele and one recessive allele (heterozygous), they will express the dominant trait and have the autosomal dominant disorder. Thus, inheriting one copy of the abnormal gene from either parent is sufficient for the disorder to appear.

Two Autosomal Dominant Disorders

Here are two examples of autosomal dominant disorders: 1. Huntington's disease: a genetic disorder that affects the brain, leading to the progressive loss of muscle coordination and cognitive decline 2. Marfan syndrome: a genetic disorder that affects the body's connective tissue, leading to problems with the skeletal system, cardiovascular system, and eyes.

Key concepts

Autosomal Recessive

To understand autosomal recessive genetic disorders, it's important to know that these disorders occur when both copies of a gene in an individual are mutated. This means you need to inherit one defective gene from each parent. In genetic terms, this makes the individual homozygous for the recessive allele.

These parents might not show any symptoms of the disorder themselves. They are commonly referred to as carriers; they have one normal and one mutated allele but do not exhibit any significant symptoms of the disorder. For a child to be affected by an autosomal recessive disorder:

• Both parents must be carriers of the mutated gene.
• The child must inherit two mutated genes, one from each parent.

The chances of two carrier parents having an affected child is 25%. However, there is also a 50% chance the child will be a carrier like the parents, and a 25% chance the child will inherit two normal alleles.

Autosomal Dominant

Autosomal dominant disorders are a bit different from their recessive counterparts. They can appear when a person inherits just one mutated allele from one parent. This is because the mutated allele is dominant over the normal one.

In this scenario, the affected individual will display the disorder symptoms. This means you only need one copy of the abnormal gene to express the condition. Such traits are said to be heterozygous, having one normal and one mutated allele.

Key points about autosomal dominant disorders:

• A single copy of the mutant allele is enough for the disorder to be present.
• There is a 50% chance an affected parent will pass the trait to their child.
• Both males and females are equally likely to be affected.

Cystic Fibrosis

Cystic Fibrosis (CF) is one of the well-known autosomal recessive disorders. It's caused by mutations in the CFTR gene which leads to the production of thick and sticky mucus. This mucus primarily affects the lungs and pancreas, causing severe respiratory and digestive problems.

While CF primarily affects the lungs, leading to repeated infections and difficulties in breathing, it also interferes with the pancreas's ability to secrete necessary enzymes, leading to nutrient absorption problems. This results in:

• Breathing difficulties due to clogged airways.
• Chronic lung infections.
• Digestive issues.

There is no cure for CF currently, but treatment focuses on managing symptoms and preventing complications.

Sickle Cell Anemia

Sickle Cell Anemia is an autosomal recessive disorder affecting red blood cells. It's caused by mutations in the HBB gene responsible for producing hemoglobin. This mutation results in hemoglobin molecules forming an abnormal "sickle" shape.

These crescent-shaped cells are less flexible and can block blood flow, leading to pain and organ damage. Individuals with sickle cell anemia face challenges such as:

• Chronic anemia due to the destruction of sickle cells.
• Painful crises due to blocked blood vessels.
• Increased risk of infection and organ damage.

Managing sickle cell anemia involves preventing sickle cell crises and treating symptoms.

Huntington's Disease

Huntington's Disease is an autosomal dominant disorder caused by mutations in the HTT gene, leading to the progressive breakdown of nerve cells in the brain. Typically, the onset of symptoms occurs in middle adulthood, leading to both physical and cognitive decline.

Individuals with this disorder experience:

• Uncontrolled movements known as chorea.
• Cognitive deterioration including memory loss.
• Emotional changes like depression or irritability.

There is no cure for Huntington's Disease. Treatments focus on managing symptoms and improving the quality of life. Given its dominant nature, offspring of an affected parent have a 50% chance of inheriting the mutated allele.