Genetic Disorder

Notes on Genetic Disorders

8.1 Chromosomal Abnormalities and Syndromes

Genetic disorders often stem from abnormalities in chromosomes, which can be structural or numerical. Structural abnormalities may occur due to deletions, duplications, inversions, or translocations of chromosome segments, while numerical abnormalities are categorized as aneuploidy (an abnormal number of chromosomes) or polyploidy (multiple complete sets of chromosomes).

8.1.1 Structural Chromosomal Abnormalities

• Deletion: A part of the chromosome is lost. Example: Retinoblastoma arises from a deletion on chromosome 13.
• Duplication: A segment is repeated, increasing the chromosome's length. Example: Charcot-Marie-Tooth disease from a gene duplication on chromosome 17.
• Inversion: A segment reverses its direction. Example: RCAD syndrome from an inversion on chromosome 17.
• Translocation: A segment moves to a different chromosome. Reciprocal translocation involves exchanges between two chromosomes, while Robertsonian translocation involves a segment attaching without exchange. Example: Burkitt’s lymphoma resulting from translocation between chromosomes 8 and 14.

8.1.2 Numerical Chromosomal Abnormalities

Numerical abnormalities can lead to syndromes characterized by distinct physical features and health challenges due to an extra or missing chromosome.

• Down's Syndrome (Trisomy 21): This syndrome is identified by an extra copy of chromosome 21, leading to distinct facial characteristics, low IQ, and various health issues.
• Klinefelter's Syndrome: Males with this condition possess an extra X chromosome (47, XXY), resulting in physical characteristics like tall stature and reduced fertility.
• Turner's Syndrome: Females with this disorder have a missing X chromosome (45, X), leading to short stature and other developmental issues.

8.2 Monogenic Disorders and Pedigree Mapping

Monogenic disorders arise from mutations in a single gene and can be classified according to their inheritance patterns:

• Autosomal Recessive: Two copies of a mutated gene are required to express the disorder. e.g., Sickle Cell Anemia caused by mutations in the hemoglobin gene.
• Autosomal Dominant: A single mutation leads to the disorder. e.g., Achondroplasia leads to dwarfism.
• X-linked Recessive: Typically affects males, with females being carriers. e.g., Hemophilia.
• X-linked Dominant: An affected male passes the mutant gene to all his daughters.

Pedigree analysis is crucial for tracing inheritance patterns and understanding genetic disorders within families.

8.3 Polygenic Disorders

Polygenic disorders arise from the interaction of multiple genes, making them complex. Examples include:

• Diabetes: A largely polygenic condition, especially prevalent in India, characterized by abnormal blood sugar levels.
• Hypertension: Elevated blood pressure, a significant risk factor for cardiovascular diseases.
• Coronary Heart Disease: A condition resulting from atherosclerosis, strongly linked with diabetes and hypertension.

8.3.1 Mitochondrial Inheritance and Diseases

Mitochondrial DNA mutations can lead to inherited conditions, since mitochondrial genes are passed from mother to offspring, emphasizing maternal lineage in mitochondrial disorders.

Summary**:

• Haploid vs. Diploid: Haploid cells have one set, while diploid cells have two sets of chromosomes.
• Aneuploidy: Refers to missing or extra chromosomes.
• Syndromes vs. Diseases: Syndromes are specific collections of symptoms, while diseases entail broader abnormal physiological responses.
• Chromosomal Abnormalities: Types include deletion, duplication, inversion, and translocation.
• Major Syndromes: Such as Down syndrome (trisomy 21), Klinefelter's syndrome (47, XXY) and Turner's syndrome (45, X).
• Monogenic and Polygenic Disorders: Monogenic disorders result from single gene mutations; polygenic disorders involve multiple genes.
• Mitochondrial Inheritance: Maternal inheritance of mitochondrial DNA affects energy production and function in many cells.

Exercises

1. Define dominant, recessive, homozygous, heterozygous, phenotype, and genotype.
2. Describe the origin, symptoms, and treatment of Down syndrome.
3. Describe the origin, symptoms, and treatment of Klinefelter syndrome.
4. Describe the origin, symptoms, and treatment of Turner syndrome.
5. Describe various structural chromosomal abnormalities.

Key Points

1. Chromosomal abnormalities can be structural (deletion, duplication, inversion, translocation) or numerical (aneuploidy, polyploidy).
2. Down's Syndrome is caused by trisomy of chromosome 21, leading to distinct physical and intellectual characteristics.
3. Klinefelter's syndrome (47, XXY) affects males; symptoms include tall stature and reduced fertility.
4. Turner's syndrome (45, X) leads to physical and developmental issues in females due to a missing X chromosome.
5. Monogenic diseases arise from mutations in a single gene and can follow various inheritance patterns: autosomal recessive, autosomal dominant, X-linked recessive, X-linked dominant.
6. Polygenic disorders are influenced by multiple genes; examples include diabetes and heart disease.
7. Mitochondrial DNA is inherited maternally and can lead to specific disorders when mutated.
8. Family pedigree analysis helps trace genetic disorders and inheritance patterns.