Mosaic Down syndrome is a form of Down syndrome where a person has a mixture of two types of cells: some cells have the usual 46 chromosomes, while others have 47 chromosomes, with the extra chromosome being chromosome 21.
Here's a more detailed explanation:
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Understanding Chromosomes: Human cells typically contain 46 chromosomes arranged in 23 pairs. These chromosomes carry genetic information.
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Down Syndrome (Trisomy 21): Classic Down syndrome, also known as Trisomy 21, occurs when a person has three copies of chromosome 21 instead of the usual two in all their cells.
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Mosaicism: Mosaicism, in general, refers to a situation where different cells within the same person have different genetic makeups.
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Mosaic Down Syndrome Specifically: In mosaic Down syndrome, not all cells have the extra chromosome 21. Instead, there is a mix:
- Some cells have the normal 46 chromosomes.
- Other cells have 47 chromosomes, including the extra chromosome 21.
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Percentage Representation: The proportion of cells with the extra chromosome 21 can vary from person to person. This mosaicism is typically described as a percentage (e.g., "5% mosaic Down syndrome," meaning that 5% of the cells examined had the extra chromosome 21).
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Variability in Symptoms: Because mosaic Down syndrome involves a mix of cells, individuals with this condition may experience a wide range of characteristics and developmental outcomes. The severity of symptoms often depends on the percentage of cells with the extra chromosome. Some individuals may have milder characteristics compared to those with classic Down syndrome.
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Diagnosis: Diagnosis typically involves chromosome analysis (karyotyping) of a sample of cells (usually blood, but sometimes skin or bone marrow). Multiple cells are examined to determine the proportion of cells with the extra chromosome.
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Cause: Mosaic Down syndrome is not usually inherited. It typically occurs as a random event during cell division early in development after fertilization.
