Genetic Disorder

8.1 Chromosomal Abnormalities and Syndromes

Chromosomal abnormalities refer to alterations in the structure or number of chromosomes within the cells. These abnormalities can arise due to external environmental factors such as radiation exposure, dietary influences, or internal genetic conditions affecting chromosome integrity. When the structure of a chromosome changes, it is termed a structural chromosomal abnormality or aberration. Conversely, changes in the number of chromosomes are called numerical chromosomal abnormalities. Numerical abnormalities include conditions such as monosomy, where one chromosome of a pair is missing (2n-1), and trisomy, where a chromosome is present in three copies (2n+1). Both monosomy and trisomy fall under the broader category of aneuploidy, which is any deviation from the normal chromosome number that is not a whole set multiple. Polyploidy, on the other hand, refers to the multiplication of the entire set of chromosomes, such as triploidy (3n), tetraploidy (4n), etc. Polyploidy is common in plants and has been exploited in artificial breeding to develop varieties with desirable traits. For example, bread wheat is hexaploid (6n), cabbages and mustards are tetraploid (4n), bananas and apples are triploid (3n), and strawberries and sugarcane are octoploid (8n). Both structural and numerical chromosomal abnormalities can lead to significant phenotypic changes, often manifesting as diseases or syndromes characterized by a specific set of symptoms occurring together. Understanding these abnormalities is crucial for diagnosing genetic disorders and for genetic counseling.

• Chromosomal abnormalities can be structural or numerical.
• Monosomy (2n-1) and trisomy (2n+1) are types of aneuploidy.
• Polyploidy involves multiplication of the entire chromosome set (e.g., triploid, tetraploid).
• Polyploidy is common in plants and used in breeding for desirable traits.
• Chromosomal abnormalities can cause diseases or syndromes with characteristic symptoms.
• Environmental factors like radiation and diet can induce chromosomal damage.
• 📌 Chromosomal abnormality: Alteration in chromosome number or structure.
• 📌 Monosomy: Loss of one chromosome from a pair (2n-1).
• 📌 Trisomy: Presence of an extra chromosome (2n+1).

8.1.1 Structural chromosomal abnormalities

Structural chromosomal abnormalities involve changes in the physical structure of chromosomes. These abnormalities can be classified into four main types: 1. Deletion: A segment of a chromosome breaks away and is lost, leading to shortening of the chromosome. For example, deletion of a portion of chromosome 13 causes retinoblastoma, a type of eye cancer. Sometimes, when both ends of a chromosome are deleted, the chromosome can form a ring structure called a ring chromosome. 2. Duplication: A segment of a chromosome is repeated, resulting in a longer chromosome. This can lead to diseases such as Charcot-Marie-Tooth disease, caused by duplication of genes on chromosome 17. 3. Inversion: A chromosome segment breaks away, reverses its orientation by 180 degrees, and reattaches to the same chromosome. The overall length remains the same but the gene order is reversed. An example is RCAD syndrome caused by inversion on chromosome 17. 4. Translocation: A segment of one chromosome breaks away and attaches to another chromosome. If two chromosomes mutually exchange segments, it is called reciprocal translocation, such as in Burkitt's lymphoma involving chromosomes 8 and 14. If a segment attaches to another chromosome without mutual exchange, it is Robertsonian translocation, which may reduce chromosome number. These structural changes can disrupt gene function and lead to various genetic disorders or syndromes.

• Deletion causes loss of chromosome segments, shortening the chromosome.
• Duplication results in repeated chromosome segments, lengthening the chromosome.
• Inversion reverses a chromosome segment's orientation without changing length.
• Translocation involves exchange or transfer of chromosome segments between chromosomes.
• Reciprocal translocation is mutual exchange; Robertsonian translocation is one-way attachment.
• Structural abnormalities can cause diseases like retinoblastoma and Burkitt's lymphoma.
• 📌 Deletion: Loss of a chromosome segment.
• 📌 Duplication: Repetition of a chromosome segment.
• 📌 Inversion: Reversal of a chromosome segment's orientation.