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Genetics is the study of genes and chromosomes, which is the hereditary material that is passed from one generation to the next.  Diagnostic testing looks for changes in genetic material that can cause disease or disorder, which may not always be discernible upon diagnostic imaging.

What are chromosomes?


chromosomes

Our bodies are made of billions of cells.  Within each cell are chromosomes, which are tiny packages that carry all of our genetic information (or DNA).   DNA is organized into smaller parts called genes which carry specific instructions that tell our body how to grow, develop and function.   Normally, we each have 46 chromosomes, in 23 pairs, where one set is inherited from the mother (or egg), and the other, from the father (or sperm).  To see if a cell has the normal number of chromosomes, and that the general structure of them is correct, a laboratory can create an arranged picture of the chromosomes called a karyotype.

In a karyotype, the 23 pairs are arranged according to size and pattern.

The first 22 pairs of chromosomes are called autosomes and are labeled numerically (i.e. “chromosome 1”, “chromosome 2”, etc.)  The 23rd pair is called the sex chromosomes which determine whether an individual is male or female.  Two X chromosomes (XX) make a female, and one X and one Y (XY) make a male.

What are chromosome abnormalities?

chromosomes-abnormal

Chromosomal abnormalities usually result from an error that occurs when an egg or sperm cell develops. It is not known why these errors occur. As far the research shows, nothing that a parent does or doesn’t do before or during pregnancy can cause a chromosomal abnormality in his or her child.  Egg and sperm cells each typically contain 23 chromosomes. When they join together, they form a fertilized egg with 46 chromosomes. But sometimes something goes wrong before fertilization.  When there are too many or too few chromosomes, it is called aneuploidy.

In addition to having too many or too few chromosomes, another type of chromosome abnormality  has to do with the structure of the chromosomes.  Individuals with structural chromosomal abnormalities usually have the normal number of chromosomes. However, small pieces of a chromosome (or chromosomes) may be deleted, duplicated, inverted, misplaced or exchanged with part of another chromosome.

What is microarray?

Prenatal microarray is designed to detect cytogenetic (chromosome) imbalances that are smaller than what can be detected through routine chromosome analysis. Prenatal array testing is typically offered when there is a suspicion of a chromosomal imbalance or suspicion of a genetic syndrome, especially due to abnormal ultrasound findings. A prenatal microarray can evaluate over 200 genetic syndromes and over 900 gene regions which are important for human development and function. It is not, however, designed to detect point mutations or other changes associated with single gene disorders. If a specific fetal genetic diagnosis/syndrome is suspected, specific genetic testing should be arranged through your genetic counselor.

What are genes?

Genes are contained in chromosomes and are made of DNA.  They are units of hereditary and typically correspond to various different biological traits, some of which are obvious, such as eye or hair color, and some of which are not, such as blood type or increased risk for specific diseases.  Ultimately, genes are instructions for our cells to carry out the biological processes that are necessary for life.  Humans have approximately 25,000-30,000 genes.

genes

What are genetic mutations?

A gene mutation is a permanent change in the DNA sequence that makes up a gene and can occur in two ways: they can be inherited or acquired.  Mutations that are passed from parent to child are called hereditary mutations. This type of mutation is present throughout a person’s life in virtually every cell in the body.

Mutations that occur only in an egg or sperm cell, or those that occur just after fertilization, are called new (de novo) mutations. De novo mutations may explain genetic disorders in which an affected child has a mutation, but no one else in the family does.

What are birth defects?

Birth defects are structural or functional problems that occur during pregnancy, which affect fetal growth and development. These problems can cause physical or mental disabilities, and in some cases, may be fatal. There are thousands of different birth defects.   The most common birth defects include congenital heart defects, cleft lip and palate and spina bifida.

Birth defects have a variety of underlying causes, such as a chromosome abnormality, genetic syndrome (caused by a specific gene mutation) or an environmental factor (such as exposure to harmful medications or alcohol during pregnancy).  However, most birth defects are caused by a complex combination of factors, including genetics and the environment.  This is known as multifactorial inheritance.  While there has been much research, the causes of many birth defects remain unknown.

What is genetic counseling?
Genetic counselors are health care professionals who combine their knowledge of basic science, medical genetics and counseling theory with their skills in genetic risk assessment, education and interpersonal communication to help patients understand the implications of genetic disease. This includes interpretation of the family medical history, the chance of disease occurrence and risk of recurrence, education about inheritance, testing, management, prevention and counseling about informed choices.

Prenatal genetic counseling provides information and support to people whose unborn baby is at risk for a genetic disorder.  A prenatal genetic counselor meets with you to discuss specific genetic risks, the possibility of chromosome abnormalities or genetic syndromes, as well as genetic testing.   There are many reasons to seek prenatal genetic counseling. You may consider it if you have or are concerned that you may have an inherited disorder, are pregnant or planning to be pregnant after age 35, already have a child with a genetic disorder or birth defect, have had two or more miscarriages, or have already experienced the loss of a child.

At the Center for Prenatal Pediatrics, a genetic counselor is available to discuss any of these situations, as well as discuss what an abnormal ultrasound or screening results could mean for your unborn baby and the pregnancy.