Understanding Mendelian vs. Non-Mendelian Disorders

The question asks to identify the disorder from the given options that is not classified as a Mendelian disorder.

What Defines a Mendelian Disorder?

A Mendelian disorder is a genetic condition caused by a mutation in a single gene. These disorders typically follow specific patterns of inheritance as described by Gregor Mendel, such as autosomal dominant, autosomal recessive, or X-linked inheritance.

Analyzing the Given Options

Let's examine each option:

Haemophilia

Haemophilia is a genetic disorder that impairs the body's ability to control blood clotting. It is caused by a mutation in a single gene located on the X chromosome that codes for clotting factors. Haemophilia follows an X-linked recessive pattern of inheritance, making it a classic example of a Mendelian disorder.

Sickle Cell Anemia

Sickle cell anemia is a blood disorder characterized by abnormally shaped red blood cells. It results from a mutation in a single gene (HBB) that codes for the beta-globin chain of haemoglobin. This disorder follows an autosomal recessive inheritance pattern and is considered a Mendelian disorder.

Phenylketonuria (PKU)

Phenylketonuria is a metabolic disorder where the body cannot process the amino acid phenylalanine. It is caused by a mutation in a single gene (PAH) that provides instructions for making the enzyme phenylalanine hydroxylase. PKU is inherited in an autosomal recessive manner, classifying it as a Mendelian disorder.

Turner Syndrome

Turner Syndrome is a condition that affects only females. It is not caused by a mutation in a single gene, but rather by a chromosomal abnormality. Specifically, it occurs when a female is missing all or part of one of her X chromosomes (resulting in a karyotype typically 45,X). Because it involves a change in chromosome number rather than a single gene defect following Mendelian rules, Turner Syndrome is considered a chromosomal disorder, not a Mendelian disorder.

Conclusion

Haemophilia, Sickle cell anemia, and Phenylketonuria are all examples of Mendelian disorders caused by mutations in single genes that follow clear inheritance patterns. Turner Syndrome, however, is a chromosomal abnormality.

Identifying the Non-Mendelian Disorder

Based on the analysis, Turner Syndrome is the option that is not a Mendelian disorder.

Revision Table: Comparing Disorder Types

Additional Information on Genetic Conditions

Genetic conditions can arise from different types of genetic alterations:

• Single-gene (Mendelian) disorders: Caused by changes in one gene. Follow predictable inheritance patterns (autosomal dominant/recessive, X-linked, etc.).
• Chromosomal disorders: Result from changes in the number or structure of entire chromosomes. Affect multiple genes. Examples include aneuploidies (like trisomy or monosomy).
• Multifactorial disorders: Caused by complex interactions of multiple genes and environmental factors. Inheritance is less predictable than Mendelian disorders.
• Mitochondrial disorders: Caused by mutations in the DNA found in mitochondria, inherited maternally.

Understanding the distinction between these types of genetic disorders is fundamental in genetics and medical diagnosis.