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?

Verified step by step guidance

1. Step 1: Understand that Down syndrome is caused by trisomy 21, meaning there is an extra copy of chromosome 21, which leads to overexpression of genes located on this chromosome.
2. Step 2: Visit the OMIM website and look up the entry for Down syndrome (OMIM 190685) to identify specific genes on chromosome 21 that are implicated in the condition.
3. Step 3: Review the functions of these genes, focusing on how their increased dosage (due to the extra chromosome) can disrupt normal developmental processes and cellular functions.
4. Step 4: Connect the overexpression of these genes to the characteristic symptoms of Down syndrome, such as intellectual disability, distinct facial features, and congenital heart defects, by understanding the biological pathways they influence.
5. Step 5: Summarize how the gene dosage imbalance caused by trisomy 21 leads to the phenotypic manifestations of Down syndrome through altered gene expression and developmental abnormalities.

Key Concepts

Genetic Basis of Down Syndrome

Down syndrome is primarily caused by trisomy 21, where an individual has three copies of chromosome 21 instead of two. This extra genetic material disrupts normal development and leads to the characteristic physical and cognitive symptoms associated with the syndrome.

Gene Dosage Effect

The gene dosage effect refers to how having an extra copy of genes on chromosome 21 increases the expression of those genes. This overexpression can alter cellular processes and development, contributing to the various symptoms seen in Down syndrome, such as intellectual disability and distinct facial features.

Maternal Serum Screening and Karyotype Analysis

Maternal serum screening measures specific proteins and hormones in the mother's blood to assess the risk of chromosomal abnormalities like Down syndrome. Karyotype analysis examines the fetus's chromosomes directly to confirm the presence of trisomy 21, providing a definitive diagnosis.