Genetics is the study of the pattern of inheritance. A geneticist studies birth disorders and their cause. Normally each individual has 46 chromosomes in every cell. Most fetuses receive 23 chromosomes from each parent. Chromosomes are structures found in every cell of the body. Typically there are 22 non-sex chromosome pairs called autosomes, and one sex chromosome pair. If you are male, the most common sex chromosome pair is XY and if you are female, the most common pair is XX. On each chromosome are hundreds of gene pairs. Genes are the blueprint for human growth and development and tell the cell how to function. Single gene disorders may be autosomal (not sex linked) or X-linked (related to the sex chromosome). Another common classification for genetic disorders is dominant or recessive. Some common genetic disorder classifications are:

• An autosomal dominant disorder may be inherited from one parent who is affected. Dominant means the nonfunctional gene of the gene-pair is able to override the normal gene. A person affected by an autosomal dominant trait has a 50% chance of passing it on to their offspring.
• An autosomal recessive disorder will most commonly occur when both parents carry the trait and the offspring receives the nonfunctional gene from each parent. Recessive means the traits of the nonfunctional gene are hidden by the normal gene or the normal gene traits override the nonfunctioning gene. A person may carry the nonfunctioning gene but have no symptoms. With an autosomal recessive trait the offspring have a 25% chance of inheriting the disorder and having symptoms, a 50% chance of receiving a defective gene and thus being a carrier with no symptoms, and a 25% chance of not having the disorder and also not being a carrier of the nonfunctioning gene.  
  With an X-linked dominant disorder, affected fathers will transmit the disorder only to their daughters. Affected mothers will have a 50% chance of transmitting the disorder to her offspring.
• Finally, an X-linked recessive disorder will typically affect males. A nonaffected female is a carrier who will pass on the disorder to 50% of her sons, and 50% of her daughters will be carriers. With an affected male, all his daughters will be carriers.

A chromosomal disorder may be inherited or may be sporadic, meaning there is no family history. The most common types of chromosomal disorders can be classified in the following way:

• Aneuploidy means there is the wrong number of chromosomes. This can be too many or too few. Trisomies 13, 18 and 21 are examples of an extra chromosome. These three are the more common ones seen, and the number relates to which extra chromosome they have. For example the Trisomy 21 has three number 21 chromosomes rather than the usual pair. Turner syndrome is an example of too few chromosomes. With Turner syndrome, one of the sex chromosomes is not transferred, leaving a single X chromosome, or 45 rather than the usual 46.
• Deletion means that a part of a chromosome is missing, which ultimately means that the genetic material on the missing section of chromosome is also missing. An example of a deletion syndrome is the Cri du Chat syndrome in which a portion of chromosome 5 is deleted.
• Inversion refers to a chromosome that has broken. The broken piece turns upside down and reattaches itself. Inversions may or may not cause birth defects depending on their exact structure.
• Translocation refers to a rearrangement of a chromosomal segment from one location to another. A balanced translocation has an equal exchange, so no genetic material is added or lost and this person may never be aware of the translocation. For a person with a balanced translocation there is, however, risk to their offspring. The risk is that the chromosome may be passed on as an unbalanced chromosome. An unbalanced translocation means genetic material is added or lost and can cause a variety of problems in varying degrees for the offspring.
• Mosaicism is the presence of two or more chromosome patterns in the cells of a person. This results in two or more cell lines, which means some cells will have 46 chromosomes and some will have a number other than 46 (either greater than or less than 46).
• 60% of all birth defects do not have any known cause. Chromosomal defects account for only about 6% of all birth defects. Multifactoral (or a combination of genetics and environmental) inheritance accounts for approximately 20% of congenital anomalies. Teratogens are agents that are known to cause birth defects. These can be medications (legal and/or illegal), something in the environment, or a maternal infection. Teratogens cause about 4-5% of all birth defects.

Listed below is a brief description of some of the more common genetic/chromosomal defects that may be diagnosed prenatally.

Trisomy 13

Trisomy 13 means there are three number 13 chromosomes rather than the usual two. Trisomy 13 has a reported incidence of one in 2,200 to 7,600 live births. This chromosomal defect is associated with major congenital anomalies. The most common of which include: holoprosencephaly (the two cerebral hemispheres are fused) or other central nervous system abnormalities, abnormal midface development including clefting, and congenital heart defect. Many fetuses with Trisomy 13 die before they reach term and/or are miscarried. 50-80% of infants with Trisomy 13 that are born alive will die by the age of 1 month and 75-90% have died by age 6 months. Very rarely, affected persons have survived to adulthood.

Trisomy 18

Trisomy 18 is a term used to describe the presence of three number 18 chromosomes rather than the usual two. Trisomy 18 has a reported incidence of 1 in 3,000 to 7,000 live births. This chromosomal defect is associated with major congenital anomalies. The most commonly associated abnormalities include: intrauterine growth restriction, cardiac defects, club foot/feet or rocker bottom feet, and omphalocele. Cranial abnormalities are seen with trisomy 18. These abnormalities include: an unusually shaped head with a wide occipitoparietal and narrow frontal diameter, which has also been described as the "strawberry sign" because of the shape of the head view on ultrasound. Several studies reviewed demonstrated 50% of babies with Trisomy 18 had died by one week of age, and 90% by six months. There is a small percentage that will survive beyond infancy. Females are more likely to survive (although not for an extended length of time) than males and the disorder is seen more often in females than males (3:1).

Trisomy 21

Trisomy 21 is a term used to describe the presence of three number 21 chromosomes rather than the usual two. Another name for trisomy 21 is Down syndrome. Down syndrome has a reported incidence of one in 800 live births, however the risk of incidence increases with advanced maternal age. The most commonly seen congenital abnormalities include: cystic hygroma (abnormal fluid accumulation around the neck area), nuchal-fold thickness (skin on the back of the neck is thicker than normal), hydrops (abnormal fluid accumulation in two areas of the body such as around the heart, around the lungs, in the abdomen or under the skin), cardiac defects, renal hydronephrosis (part of the kidney has abnormal collection of fluid) and skeletal (bone) abnormalities.

Klinefelter Syndrome (47,XXY karyotype)

These are male infants with an extra X chromosome. Many will go undiagnosed until maturity as adults undergoing infertility workups. Fetuses are typically identified during amniocentesis performed for advanced maternal age. They are not normally identified because of an abnormal ultrasound finding. Some information typical of Klinefelter syndrome includes:

• Sons are typically taller than normal (In the 75th percentile on growth charts).
• Puberty will be entered normally but may consider testosterone supplementation therapy after mid-adolescence.
• Sons are infertile.
• Sons are at risk for developmental problems and speech, neuromotor and learning delays.

Achondroplasia

Achondroplasia is an autosomal dominant genetic disorder of bone growth. It affects one in 25,000 live births and occurs equally in both sexes and all races. Affected individuals have short arms and legs with a normal torso size. The head is usually large, sometimes due to hydrocephalus, and the forehead is prominent. Achondroplasia is the result of an abnormal gene located on one of the chromosome 4 pair. 80% of these cases are not inherited. Both parents are normal size with normal chromosome 4, but a new mutation occurs for an unknown reason. If one parent has the condition and the other parent does not, their offspring have a 50% chance of being affected. If both parents have achondroplasia, they have a 50% chance of their offspring inheriting the condition, 25% chance of not inheriting the condition, and a 25% chance of inheriting the abnormal gene from both parents, which results in severe skeletal abnormalities that lead to an early death.

Tay-Sachs Disease

Tay-sachs is an inherited disease of the central nervous system that is incompatible with life. A Tay-Sachs carrier has one normal gene and one Tay-Sachs gene making it an autosomal recessive disease. It occurs most frequently in central and eastern European descendants, (Ashkenazi) Jews. Approximately one in every 30 American Jews is a carrier of the Tay-Sachs gene. The carrier leads a normal, healthy and full life. Both parents would need to be carriers and each would contribute the affected gene to have an affected baby. A baby with Tay-sachs will appear normal at birth and for approximately 4 to 6 months. The affected baby lacks an enzyme necessary for breaking down certain fatty substances in the brain and nerve cells. At a few months of age the baby will gradually stop smiling, crawling or turning over. Eventually they lose their ability to grasp or reach out and become blind and paralyzed. Death occurs by age 5. If both parents are carriers of Tay-Sachs, their offspring have a 50% chance of being a carrier for Tay-sachs, a 25% chance of contracting the disease by receiving a defective gene from each parent and a 25% chance of inheriting normal genes from each parent.

Phenylketonuria (PKU)

Phenylketonuria (PKU) is an autosomal recessive, inherited disorder of body metabolism. Metabolism is the process of breaking down food to be used by the body. An affected individual lacks an enzyme necessary to process the protein, phenylalanine. Without treatment this protein builds up in the bloodstream and causes mental retardation. Treatment involves following a special diet that is low in phenylalanine. Since the 1960s, all newborns born in the United States are screened for this disorder shortly after birth. Approximately one in every 15,000 babies born is affected. Both parents of these newborns are carriers, but are unaffected. Their offspring have a 50% chance of being carriers, a 25% chance of being affected or inheriting the defective gene from both parents and a 25% chance of inheriting only normal genes from their parents. The affected baby appears normal at birth but will develop symptoms of listlessness and lose interest in their surroundings by age 3 to 6 months. The issue today is females who have been diagnosed in infancy with the disease who are now having babies. These women may have begun to eat a normal diet and no longer follow the phenylalanine restrictions. The result is a very high level of phenylalanine in their blood, which can be devastating to their offspring. These women should be counseled prenatally to resume the special diet for at least three months prior to conception and throughout their pregnancy to prevent defects in their babies.

Cystic Fibrosis (CF)

Cystic fibrosis (CF) is an autosomal recessive, inherited disorder that affects breathing and digestion. There is no cure. Both parents are carriers but are unaffected. A child with CF has inherited a defective gene from each parent. Future offspring have a 50% chance of becoming a carrier or inheriting the defective gene from one parent a 25% chance of being affected and a 25% chance of inheriting only normal genes. With cystic fibrosis the pancreas and lungs are most affected. Mucus and other secretions become thick and sticky. This thick mucous can clog the lungs and causes breathing problems and frequent lung infections, which eventually damages the lungs. The thickened secretions made by the pancreas for digestion of food, are unable to reach the small intestine. This can lead to digestive problems including inability to gain weight or grow at a normal rate. The symptoms of cystic fibrosis will range from mild to severe. Some will attend school and college and participate in some exercise, while others are too ill to attend school regularly. Males affected by CF are commonly infertile and females have reduced fertility. Cystic fibrosis does not affect a person's appearance or intelligence. Most affected individuals survive to 30 or 40 years of age.

Fragile X Syndrome

Fragile X syndrome is the most common inherited form of mental retardation in males. It affects one in 4,000 males and one in 8,000 females. In 1991, a researcher discovered a "mutation" in a gene located on the X chromosome, as the cause of the Fragile X syndrome. Mutation means the addition or deletion of genetic material. A small section of the genetic material at this location is repeated too many times. Normally there are six to 40 repeats of this section. If there are 60 to 200 repeats, this is a pre-mutation, and greater than 200 repeats is a mutation. With a mutation the gene will "turn off" and not produce the protein that it normally would make. The lack of this specific protein causes the symptoms of fragile X syndrome. A pre-mutation carrier mother has a 50% chance of passing on the abnormal gene to her offspring. Males are generally more severely affected because they have only one X chromosome and one Y chromosome, as compared to a female who has two X chromosomes. As this defective gene is passed on it is likely to expand in the number of repeats and become a full mutation. A man can also be an unaffected carrier of a pre-mutation fragile X gene. This male will pass on the pre-mutation (does not usually expand) to all his daughters but to none of his sons. These daughters generally have no symptoms, but are carriers and may pass it on to their children. Affected individuals have varying degrees of mental retardation or learning disabilities and behavioral and emotional problems, including autistic-like features.

Turner Syndrome

Turner syndrome affects only females. It occurs when one of the two X chromosomes normally found in females is missing or incomplete. This, at present, appears to be a random event with no known cause. Turner syndrome is among the most common chromosomal abnormalities affecting one in every 2,500 live female births. Most affected females will have normal intelligence. However, 10% will have substantial delays and as many as 70% will have some mild delays such as learning disabilities. It is believed these women can lead a full and productive life with regular, competent medical care. The most common characteristics of a female with Turner syndrome includes short stature and lack of ovarian development. They are also prone to cardiovascular, kidney and thyroid problems as well as skeletal disorders (scoliosis) and ear and/or hearing disturbances.

Yes! Please ask your baby's nurse about ways to interact with and care for your baby.

If you had planned on breastfeeding your baby, and the baby is unable to eat immediately after birth, you can express your milk while you are still in the hospital. A lactation consultant can assist in answering your questions. Your milk will be frozen until your baby is ready for it. The Birth Center and Neonatal Intensive Care Unit (NICU) have breast pumps available to you when you are in the hospital or visiting. You can bring in pictures, small toys, booties, and blankets for your baby if they in the NICU.

Your baby will go home when you and the medical professionals have deemed it appropriate. Some of these genetic disorders have a short life expectancy. If you are ready to go home and desire to take your baby home with you, knowing they will not have a long-term survival, arrangements can be made for you to continue to care for your baby at home. There are home health services or hospice care that can assist you with care in the home. Some of these babies may have conditions that require surgical repair before they can go home. This will prolong the hospital stay for your baby. Some genetic disorders may go undiagnosed for months to years if symptoms are mild. Many of these babies go home from the newborn nursery without any problems until later in life.

Learn more about the Genetics Center at Children's Wisconsin, which works in partnership with the Fetal Concerns Center.