The most common reason a physician might recommend that a woman have maternal serum screening and a karyotype analysis is concern that her fetus may have Down syndrome. Go to the OMIM website at www.ncbi.nlm.nih.gov/omim and look up Down syndrome (OMIM 190685).

How might those genes lead to the main symptoms of Down syndrome?

Key Concepts

Chromosomal Abnormalities

Down syndrome, also known as Trisomy 21, is primarily caused by the presence of an extra copy of chromosome 21. This chromosomal abnormality results from nondisjunction during meiosis, where chromosomes fail to separate properly, leading to gametes with an abnormal number of chromosomes. When such a gamete contributes to fertilization, the resulting zygote has three copies of chromosome 21, which is the hallmark of Down syndrome.

Gene Expression and Phenotype

The extra genetic material in Down syndrome affects gene expression, leading to the characteristic features and health issues associated with the condition. Genes on chromosome 21 are involved in various biological processes, and their overexpression can disrupt normal development. This dysregulation contributes to the physical traits, cognitive impairments, and increased risk of certain medical conditions seen in individuals with Down syndrome.

Maternal Serum Screening

Maternal serum screening is a prenatal test that measures specific substances in a pregnant woman's blood to assess the risk of certain genetic conditions, including Down syndrome. This screening typically occurs in the first or second trimester and can indicate whether further diagnostic testing, such as karyotype analysis, is warranted. It is a non-invasive method that helps inform expectant parents about potential genetic concerns.