How are inherited disorders passed down through alleles?

SouravNovember 12, 2024

Answered step-by-step

Inherited disorders are passed down through alleles, which are different versions of a gene. The way these disorders are transmitted depends on the inheritance pattern associated with the specific alleles involved. Here’s an overview of how inherited disorders can be passed down through various genetic mechanisms:

1. Modes of Inheritance

Inherited disorders can follow several basic patterns of inheritance, each characterized by specific mechanisms:

Autosomal Dominant Inheritance

• Mechanism: In autosomal dominant disorders, only one copy of the altered gene (dominant allele) is sufficient to cause the disorder. This means that if an individual inherits the dominant allele from either parent, they will express the disorder.
• Transmission: Affected individuals typically have an affected parent, and the disorder can appear in every generation (vertical transmission). Each child of an affected parent has a 50% chance of inheriting the disorder.
• Examples: Huntington’s disease, neurofibromatosis.

Autosomal Recessive Inheritance

• Mechanism: Autosomal recessive disorders require two copies of the recessive allele (one from each parent) for the individual to express the disorder. Carriers (those with one normal and one recessive allele) do not show symptoms.
• Transmission: The disorder often skips generations because it may not be apparent in carrier parents. If both parents are carriers, there is a 25% chance that their child will inherit both recessive alleles and express the disorder.
• Examples: Cystic fibrosis, sickle cell anemia.

X-Linked Dominant Inheritance

• Mechanism: This pattern involves genes located on the X chromosome. A single copy of the mutated gene in females (who have two X chromosomes) or in males (who have one X chromosome) can cause the disorder.
• Transmission: Affected males pass the condition to all daughters but none of their sons. Affected females have a 50% chance of passing the disorder to each child, regardless of sex.
• Examples: Hypophosphatemic rickets.

X-Linked Recessive Inheritance

• Mechanism: For X-linked recessive disorders, males are more frequently affected because they have only one X chromosome. Females can be carriers if they have one affected X chromosome but usually do not express the disorder unless they inherit two copies.
• Transmission: Affected males pass the allele to all daughters (who become carriers) but not to sons. Carrier females have a 50% chance of passing on the affected X chromosome to their children.
• Examples: Hemophilia A, Duchenne muscular dystrophy.

Mitochondrial Inheritance

• Mechanism: Mitochondrial disorders are caused by mutations in mitochondrial DNA and are passed from mother to all her children since mitochondria are inherited maternally.
• Transmission: Both males and females can be affected, but only females can pass on mitochondrial traits.
• Examples: Leber’s hereditary optic neuropathy.

2. Genetic Variability and Expression

• The expression of inherited disorders can vary due to factors such as incomplete penetrance (not all individuals with a mutation express symptoms) and variable expressivity (the severity of symptoms can differ among individuals with the same mutation). These factors complicate predictions about inheritance patterns and outcomes.

3. Pedigree Analysis

• Family pedigrees are often used to track inheritance patterns across generations. By analyzing family history, genetic counselors can help identify whether a disorder follows a dominant or recessive pattern and assess risks for future offspring.