Question

About 80 percent of cases of Edward syndrome are caused by trisomy \(18 ; 10\) percent are caused by mosaic trisomy \(18,\) and 10 percent are attributed to translocation. Distinguish among these three chromosome aberrations.

Short answer

Trisomy 18, or Edward's syndrome, occurs when there are three copies of chromosome 18 instead of the usual two in all cells. Mosaic Trisomy 18 is a variant where only some cells carry the extra chromosome 18. Translocation describes a situation where pieces from chromosome 18 are rearranged or attached to another chromosome.

Step-by-step solution

Understanding Trisomy 18

Trisomy 18, also known as Edward Syndrome is a type of chromosomal disorder caused by the presence of an extra chromosome 18. That means there are three copies of chromosome 18 instead of the normal two copies in each cell.

Understanding Mosaic Trisomy 18

Mosaic Trisomy 18 is a variant of Edward syndrome where not all cells contain three copies of chromosome 18. In this case, some cells have the normal two copies and some cells have three copies. Hence, the term 'mosaic' referring to a mixed cell population.

Understanding Translocation

Translocation refers to a chromosomal condition in which a piece of chromosome gets relocated. This fragment can get attached to another chromosome, it can be inverted, or it can be duplicated. A translocation is responsible for 10% of Edward syndrome cases. This means that some material from chromosome 18 has been rearranged, or translocated, to another chromosome.

Key concepts

Trisomy 18

Trisomy 18, also known as Edwards Syndrome, is a rare and severe genetic condition where an individual has an extra copy of chromosome 18. This additional chromosome disrupts normal development, leading to severe intellectual and developmental disabilities. Most affected infants present with a complex set of birth defects, including heart defects, kidney problems, and issues with other organs and systems.

Since individuals with Trisomy 18 have three copies of chromosome 18, it causes a disruption in the normal pattern of development in the earliest stages of fetal growth. This can result in life-threating complications and sadly, many infants with Trisomy 18 do not survive their first year of life.

Unfortunately, currently, there is no cure for Trisomy 18. Care for individuals with this condition focuses on providing comfort, managing symptoms, and supporting the family.

Mosaic Trisomy 18

Mosaic Trisomy 18, a less common variant of Edwards Syndrome, involves the presence of a mixture of two types of cells: some with the usual pair of chromosome 18, and some with an extra chromosome, totaling three. Unlike the classic Trisomy 18 where all cells have three copies, in Mosaic Trisomy 18, the percentage of cells with the extra chromosome can vary greatly from person to person.

As a result, the severity of the symptoms can also vary. Some individuals may have mild to moderate physical and intellectual disabilities, while others might experience more severe conditions. Diagnosis is made through genetic testing, which can identify the different cell lines. The variability in the cell population is the basis for the 'mosaic' naming convention.

Chromosomal Translocation

Chromosomal Translocation occurs when a segment of one chromosome breaks off and attaches to another chromosome. There are two main types: reciprocal and Robertsonian. In a reciprocal translocation, segments from two different chromosomes have been exchanged. In Robertsonian translocation, an entire chromosome attaches to another at the centromere; this kind of translocation is often implicated in cases of Edwards Syndrome where translocation involves chromosome 18.

Translocations can be balanced, where no genetic material is lost or gained and usually cause no health problems, or unbalanced, where the genetic exchange leads to a loss or gain of genetic material, potentially causing developmental issues or disorders. In some cases, a parent may carry a balanced translocation without any symptoms but can have children with an unbalanced translocation, leading to genetic disorders such as Edwards Syndrome.

Chromosomal Disorders

Chromosomal Disorders arise due to abnormalities in the structure or number of chromosomes. These disorders can manifest as genetic syndromes, like Down syndrome (Trisomy 21), Turner syndrome, and Klinefelter syndrome, each associated with a specific set of clinical features and developmental challenges.

A commonality among these disorders is that they can profoundly affect an individual's quality of life, with impacts ranging broadly in severity. Diagnostic methods, such as karyotyping and genomic sequencing, are used to detect these chromosomal abnormalities. Prenatal testing gives the possibility to diagnose such chromosomal disorders before birth, providing valuable information for managing the pregnancy and preparing for potential healthcare needs of the child.