Autosomal Recessive Disorders - 28.01.26

Homozygous State: The condition where a mutated or pathogenic variant is present in both copies of a gene.
- Example: An infected individual has two abnormal mutant alleles, inherited from both heterozygous carrier parents (though this is not strictly necessary for all cases).

In the generation of a pedigree, carriers are often illustrated with half and half shading.
Autosomal Recessive Inheritance Pattern: Common characteristics include:
- Affected individuals often have unaffected parents.
- Many unaffected individuals in the pedigree.
- Key Characteristics:
- Equal likelihood of males and females being affected (since it's autosomal).
- Both parents must be carriers of at least one copy of the altered gene for the child to be affected.

Recurrence Risk: The likelihood of an affected child in each pregnancy from carrier parents is 1 in 4.
Probability details:
- Out of four children born, typically:
- 75% unaffected.
- 50% might be carriers without exhibiting the disorder.
- 25% affected.
- The chance of an unaffected child being a carrier is 2 in 3.

Cystic Fibrosis: Most prevalent in Ireland; one of the most common autosomal recessive conditions.
Sickle Cell Anemia: Common in West Africa; caused by a mutation in the beta-globin gene, leading to sickled, fragile red blood cells that block blood vessels, causing pain and chronic anemia.
Thalassemia: Reduced production of hemoglobin causing chronic anemia, can be classified as alpha or beta thalassemia depending on which globin gene cluster is affected.

Cystic Fibrosis: Characterized by mucus build-up affecting lungs and digestive system; the CFTR gene located on chromosome 7 is mutated, affecting chloride and sodium transport across cell membranes.
- Most common mutation: Deletion of phenylalanine at position 508 (F508del).
- Symptoms: Excessive sodium in sweat, lung infections, digestive issues, potential complications include liver cirrhosis.

Diagnosis through:
- Sweat test: Measures sweat chloride levels.
- Genetic testing: Identifying known mutations.
Treatment involves:
- Antibiotics for infections, bronchodilators, digestive enzyme supplements, high-calorie diets, and potential lung transplants.
- Emerging therapies targeting specific mutations in CFTR protein function.
New therapies include inhaled lentiviral gene therapy aimed at restoring normal CFTR function in patients.

Compound Heterozygous: An individual possesses two different pathogenic variants within the same gene but is affected because no functional gene copy exists.
Double Heterozygote: A healthy individual carries different autosomal recessive disorders from different genes without showing symptoms.

Defines conditions caused by mutations in more than one gene, exemplified in conditions like Oculocutaneous Albinism.

Assortative Mating: A term describing the increased likelihood of individuals with similar phenotypes/genotypes mating, resulting in higher incidences of autosomal recessive disorders, as seen in profound childhood deafness.
Consanguinity: Marriage between related individuals increases the risk of autosomal recessive disorders due to shared ancestral pathogenic variants.
- Probability examples:
- Risk for cousin marriages yielding affected offspring is estimated at 1 in 64.

Some recessive disorders offer survival advantages in certain environments, influencing carrier frequency:
- Sickle Cell Disease: Carrier status provides resistance to malaria, enhancing survival rates in malaria-endemic regions.
- Cystic Fibrosis: Historical resistance to certain gastrointestinal infections among carriers may have provided a survival advantage.

Understanding autosomal recessive inheritance characteristics, gene functions, common disorders, and distinctions between compound and double heterozygotes.
Recognizing genetic factors influencing the prevalence of certain diseases based on ethnic backgrounds and mating practices.