What causes many genetic diseases in humans, and how do dominant and co-dominant alleles play a role?
What causes many genetic diseases in humans, and how do dominant and co-dominant alleles play a role?
Answer
Genetic diseases in humans arise from various genetic mutations and inheritance patterns, with dominant and co-dominant alleles playing significant roles in their expression. Here’s an overview of the causes of genetic diseases and how these alleles influence phenotypes.
Causes of Genetic Diseases
- Monogenic Disorders: These are caused by mutations in a single gene. Such mutations can be inherited in different ways:
- Autosomal Dominant Inheritance: A single copy of a mutated dominant allele is sufficient to cause the disorder. For instance, Huntington’s disease occurs when an individual inherits one mutated allele (A) from an affected parent, leading to the expression of the disease phenotype. This results in a 50% chance that offspring will inherit the disorder if one parent is affected.
- Autosomal Recessive Inheritance: Affected individuals must inherit two copies of the recessive allele (aa) to express the disorder. Carriers (Aa) do not show symptoms but can pass the allele to their offspring. Cystic fibrosis is a classic example, where both parents must be carriers for a child to be affected.
- Multifactorial Disorders: These involve multiple genes and environmental factors contributing to the disease. Conditions like heart disease and diabetes fall into this category, where both genetic predispositions and lifestyle choices play a role in disease manifestation.
- Chromosomal Disorders: These result from structural or numerical abnormalities in chromosomes, such as Down syndrome (trisomy 21), which arises from an extra copy of chromosome 21.
Role of Dominant Alleles
Dominant alleles can mask the effects of recessive alleles due to their expression in heterozygous conditions (one dominant allele present). This means that if an individual carries one dominant allele associated with a genetic disorder, they will exhibit symptoms regardless of the presence of a recessive allele. For example:
- In familial hypercholesterolemia, inheriting just one copy of the mutated gene leads to high cholesterol levels, demonstrating how dominant alleles can lead to disease even when paired with normal alleles.
Role of Co-Dominant Alleles
Co-dominant alleles are expressed equally in heterozygous individuals, resulting in phenotypes that reflect both alleles without one masking the other. This can lead to unique traits or conditions:
- An example is seen in blood types within the ABO system, where individuals with genotype AB express both A and B antigens on their red blood cells due to co-dominance.
- In sickle cell trait, individuals with one normal hemoglobin allele and one sickle cell allele produce both types of hemoglobin, leading to a mix of normal and sickle-shaped red blood cells. This illustrates how co-dominance can influence health outcomes by allowing for partial expression of both alleles
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