Question

Thalassemia is an inherited anemic disorder in humans. Affected individuals exhibit either a minor anemia or a major anemia. Assuming that only a single gene pair and two alleles are involved in the inheritance of these conditions, is thalassemia a dominant or recessive disorder?

Short answer

Answer: Thalassemia is a recessive disorder.

Step-by-step solution

Understanding Dominant and Recessive Traits

A dominant trait is one that will be expressed when an individual has at least one dominant allele. A recessive trait is expressed only when an individual has two recessive alleles. In the case of thalassemia, the two conditions mentioned are minor anemia and major anemia. We will represent the dominant allele with "A" and the recessive allele with "a."

Scenario 1: Thalassemia as a Dominant Trait

If thalassemia were a dominant trait, then a person with at least one dominant allele (A) would exhibit either minor or major anemia, and a person with two recessive alleles (aa) would be unaffected. Possible genotypes: - AA: Major anemia - Aa: Minor anemia - aa: No anemia This scenario assumes that the severity of the anemia (major or minor) depends on the presence of one or two dominant alleles. However, it does not clearly explain why those with a single dominant allele (Aa) exhibit only minor anemia, while those with two (AA) have major anemia.

Scenario 2: Thalassemia as a Recessive Trait

If thalassemia were a recessive trait, then a person with two recessive alleles (aa) would exhibit either minor or major anemia, while those with at least one dominant allele (A) would be unaffected or minimally affected carriers. Possible genotypes: - AA: No anemia or minor anemia (carrier) - Aa: No anemia or minor anemia (carrier) - aa: Major anemia In this scenario, it is clear that those with the recessive genotype (aa) have major anemia, while those with the dominant allele (A) are either unaffected or minimally affected carriers, exhibiting minor anemia. This scenario is consistent with the given information about the inheritance of thalassemia.

Conclusion

Based on the analysis of the two scenarios, it can be concluded that thalassemia is a recessive disorder. The inheritance pattern of thalassemia is consistent with those with the recessive genotype (aa) exhibiting major anemia, while those with at least one dominant allele (A) are either unaffected or minimally affected carriers, exhibiting minor anemia.

Key concepts

Dominant and Recessive Traits

Understanding the difference between dominant and recessive traits is fundamental to grasp how certain characteristics are passed down through generations. A dominant trait is one that appears in the phenotype, or the observable characteristics of an organism, when at least one copy of the dominant allele is present in the genotype, which is the genetic makeup. For example, if 'A' represents a dominant trait, having either the genotype AA or Aa would result in the trait being expressed.

A recessive trait, on the other hand, is only expressed in the phenotype when two copies of the recessive allele are present, as in the genotype aa. Therefore, an individual with the Aa genotype would not display the recessive trait, but could still pass the recessive allele 'a' to their offspring. In the context of thalassemia, understanding which form of the trait is dominant or recessive helps predict inheritance patterns and risk of expression in future generations.

Genotype and Phenotype Correlations

The terms genotype and phenotype are central to the study of genetics. The genotype refers to the actual genetic information carried by an organism. This includes the alleles inherited from its parents. Phenotype, however, refers to how these genetic traits manifest physically or physiologically.

In cases where a disease is influenced by one gene pair, such as thalassemia, the connection between genotype and phenotype becomes particularly illuminating. It explains why individuals with certain genotypes exhibit specific characteristics (like minor or major anemia). For instance, in the scenario where thalassemia is considered recessive, the phenotype (major anemia) only appears when the genotype is homozygous recessive (aa). Meanwhile, heterozygous carriers (Aa) and homozygous dominant individuals (AA) may be symptom-free or have only minor anemia, illustrating the impact of the genotype on the phenotype.

Genetic Disorders

A genetic disorder is a disease or condition that arises due to abnormalities in an individual’s DNA. These disorders can be single-gene, like thalassemia, or they may be caused by complex interactions of multiple genes and environmental factors. Single-gene disorders follow the simple inheritance patterns such as those seen with dominant and recessive traits.

Thalassemia is an example of a genetic disorder that is inherited in an Mendelian pattern, where mutations affect hemoglobin production, leading to anemia. The pattern in which thalassemia is inherited helps determine the risk factors for offspring and can influence decisions related to family planning and genetic counseling. Understanding the inheritance of genetic disorders like thalassemia is crucial for managing the disease and providing appropriate care to those affected.