Why chromosome 21

Regardless of the type of Down syndrome a person may have, all people with Down syndrome have an extra, critical portion of chromosome 21 present in all or some of their cells. The cause of the extra full or partial chromosome is still unknown. Maternal age is the only factor that has been linked to an increased chance of having a baby with Down syndrome resulting from nondisjunction or mosaicism.

The additional partial or full copy of the 21st chromosome which causes Down syndrome can originate from either the father or the mother. Down syndrome occurs in people of all races and economic levels, though older women have an increased chance of having a child with Down syndrome. A 35 year old woman has about a one in chance of conceiving a child with Down syndrome, and this chance increases gradually to 1 in by age At age 45 the incidence becomes approximately 1 in The age of the mother does not seem to be linked to the risk of translocation.

Since many couples are postponing parenting until later in life, the incidence of Down syndrome conceptions is expected to increase. Therefore, genetic counseling for parents is becoming increasingly important. Still, many physicians are not fully informed about advising their patients about the incidences of Down syndrome, advancements in diagnosis, and the protocols for care and treatment of babies born with Down syndrome.

Heredity is not a factor in trisomy 21 nondisjunction and mosaicism. Most cases are sporadic — chance — events. However, in about one-third of cases, one parent is a carrier of a translocated chromosome.

Once a woman has given birth to a baby with trisomy 21 nondisjunction or translocation, it is estimated that her chances of having another baby with trisomy 21 is 1 in up until age Genetic counseling can determine the origin of translocation.

There are two categories of tests for Down syndrome that can be performed before a baby is born: screening tests and diagnostic tests. Prenatal screens estimate the chance of the fetus having Down syndrome. These tests do not tell you for sure whether your fetus has Down syndrome; they only provide a probability.

There is an extensive menu of prenatal screening tests now available for pregnant women. Most screening tests involve a blood test and an ultrasound sonogram. The blood tests or serum screening tests measure quantities of various substances in the blood of the mother.

New advanced prenatal screens are now able to detect chromosomal material from the fetus that is circulating in the maternal blood. These tests are not invasive like the diagnostic tests below , but they provide a high accuracy rate.

Humans normally have 46 chromosomes in each cell, divided into 23 pairs. Two copies of chromosome 21, one copy inherited from each parent, form one of the pairs. Chromosome 21 is the smallest human chromosome, spanning about 48 million base pairs the building blocks of DNA and representing 1. In , researchers working on the Human Genome Project announced that they had determined the sequence of base pairs that make up this chromosome. Chromosome 21 was the second human chromosome to be fully sequenced.

Identifying genes on each chromosome is an active area of genetic research. Because researchers use different approaches to predict the number of genes on each chromosome, the estimated number of genes varies.

Chromosome 21 likely contains to genes that provide instructions for making proteins. These proteins perform a variety of different roles in the body.

The following chromosomal conditions are associated with changes in the structure or number of copies of chromosome A genetic rearrangement translocation involving chromosome 21 is associated with a type of blood cancer known as core binding factor acute myeloid leukemia CBF-AML.

This rearrangement occurs in approximately 7 percent of acute myeloid leukemia cases in adults. This mutation is acquired during a person's lifetime and is present only in certain cells. This type of genetic change, called a somatic mutation, is not inherited.

This change in gene activity blocks the maturation differentiation of blood cells and leads to the production of abnormal, immature white blood cells called myeloid blasts. While t 8;21 is important for leukemia development, one or more additional genetic changes are typically needed for the myeloid blasts to develop into cancerous leukemia cells. Down syndrome is a chromosomal condition that is associated with intellectual disability, a characteristic facial appearance, and weak muscle tone hypotonia in infancy.

This may cause translocation Down syndrome. This is the only form of Down syndrome that may be inherited from a parent. A rare form is called mosaic trisomy This is when an error in cell division happens after the egg is fertilized.

People with this syndrome have both normal cells and some cells with an extra chromosome number This risk increases with each year of age, especially after age But younger women are more likely to have babies than older women. So most babies with Down syndrome are born to women younger than Chromosome problems such as Down syndrome can often be diagnosed before birth. This is done by looking at cells in the amniotic fluid or from the placenta. This is a noninvasive prenatal screening.

These tests are very accurate. Fetal ultrasound during pregnancy can also show the possibility of Down syndrome. Problems from Down syndrome may not be seen with ultrasound. After birth, your baby may be diagnosed with a physical exam. The healthcare provider may also take a blood sample. This is checked in a lab to find the extra chromosome. There is no cure for Down syndrome.

But a child with Down syndrome may need treatment for problems such as:. Heart defects. About half of babies with Down syndrome have heart defects. Some defects are minor and can be treated with medicines. Others may need surgery. All babies with Down syndrome should be looked at by a pediatric cardiologist. Babies with Down syndrome should also have an echocardiogram. This is a test that looks at the structure and function of the heart by using sound waves.

This exam and test should be done in the first 2 months of life. A team of expert clinicians — including world-class surgeons and physicians, pediatric nurses, physical and occupational therapists, psychologists and other specialists — will partner with you in the care of your child. Many children with Down syndrome are also diagnosed with a variety of secondary conditions that are present at birth and can be treated when your child is young. This is often true for conditions such as heart issues, hand and foot anomalies.

In other cases, the complications from Down syndrome may only become evident — or problematic — as your child grows. This is often true for spinal deformities such as scoliosis and hip conditions that might require surgical correction. Treatment may include therapy, surgery or additional support services for your child and family. A Care Binder can help you organize the most important information about your child's care in a central place.

Your child with Down syndrome should continue to be monitored by a physician into adulthood. Experts in the Trisomy 21 Program at CHOP are available to help you manage the care of your child with Down syndrome at all stages of growth.

The Trisomy 21 Program is one of the few programs at Children's Hospital that treats patients beyond age 18 and into old age. This toolkit outlines steps for individuals with Down syndrome who are transitioning from pediatric to adult healthcare. If your child had surgery, she will need to follow up with her surgeon about one to two weeks after surgery, then again at three and six months post-surgery. After that, annual monitoring by trained clinicians is strongly encouraged to ensure any problems are spotted and treated as soon as possible.

During follow-up visits, your child may need additional X-rays and other diagnostic testing done. The goal of continued monitoring is to help spot any irregularities in growth or development and to address health issues as they develop. Our team is committed to partnering with parents and referring physicians to provide the most current, comprehensive and specialized care possible for your child.