SNP Shot: Genomic Insights

Down syndrome is a genetic condition arising from a chromosomal difference. It's characterized by intellectual disability, distinctive facial features, and reduced muscle tone (hypotonia) during infancy. While all individuals with Down syndrome experience some degree of cognitive delay, it typically ranges from mild to moderate.

Individuals with Down syndrome often share recognizable facial traits, including a flattened face, eyes that slant upwards (upslanting palpebral fissures), small ears, a short neck, and a protruding tongue. They may also have various congenital disabilities. Many have small hands and feet, and a single crease across the palm of one or both hands. Approximately half of children with Down syndrome are born with a heart defect. Less frequently, digestive system problems, like an intestinal blockage, may occur.

People with Down syndrome have a higher susceptibility to certain medical conditions. These include gastroesophageal reflux (GERD), where stomach acid flows back into the esophagus, and celiac disease, an intolerance to gluten. About 15% develop hypothyroidism, where the thyroid gland doesn't produce enough hormones. They also have an increased risk of hearing and vision impairments. In a small number of cases, children with Down syndrome may develop leukemia, a cancer of the blood-forming cells.

Developmental delays and behavioral challenges are commonly observed in children with Down syndrome. They may experience growth issues, and their speech and language skills tend to develop later and at a slower pace compared to other children. Speech intelligibility can also be a challenge. Common behavioral concerns include attention deficits, obsessive-compulsive tendencies, and stubbornness or temper tantrums. A small percentage may also be diagnosed with autism spectrum disorder, impacting communication and social interaction.

As individuals with Down syndrome age, they often experience a gradual decline in cognitive function, typically starting around age 50. Down syndrome is also linked to a higher risk of Alzheimer's disease, a neurodegenerative disorder leading to memory loss, impaired judgment, and functional decline. Roughly half of adults with Down syndrome develop Alzheimer's disease, often at an earlier age (in their 50s or 60s) compared to the general population.

Inheritance:

In most instances, Down syndrome is not inherited. When caused by trisomy 21, the extra chromosome 21 arises as a random occurrence during the creation of a parent's reproductive cells (eggs or sperm). This error, called nondisjunction, results in a reproductive cell having an abnormal chromosome number, such as an extra copy of chromosome 21. If this atypical cell contributes to fertilization, the child will have an extra chromosome 21 in each cell.

Translocation Down syndrome can be inherited from a parent who is a carrier of a balanced translocation. In this case, a section of chromosome 21 is attached to another chromosome. While the carrier parent has a normal amount of genetic material and typically no health problems, the translocation can become unbalanced when passed on. Offspring who inherit an unbalanced translocation involving chromosome 21 receive extra genetic material from chromosome 21, resulting in Down syndrome.

Similar to trisomy 21, mosaic Down syndrome is not inherited. It is caused by a random error in cell division during early fetal development. As a result, some cells have the usual two copies of chromosome 21, while others have three copies.