Girls have two X chromosomes and boys have an X and a Y. The sex chromosomes are different from the others. In chromosome tests, the chromosomes usually from blood cells are stained with dye. Each pair has a different staining pattern.
When seen down a microscope the chromosomes lie in a haphazard manner, but when they are photographed, cut out and arranged in pairs, a picture of an individual's chromosomes known as a karyotype is obtained. The genes, thousands on each chromosome, are spaced along the length of the DNA molecule, in a definite order.
Apart from the X and Y chromosomes, the order of the genes on one chromosome exactly matches the order on the other chromosome of the same pair and on chromosomes of the same number in everyone else. In general, there are only two copies of each gene and in all people they are always on the same chromosome pair.
The two copies of each gene may not be identical. Any changes may be small "misprints" or bigger alterations, with a corresponding effect of the "readability" of the instructions. When this happens, variation occurs between people - this variation is sometimes advantageous, sometimes neutral and sometimes unhelpful.
Often a mistake in the one copy of a gene can be "cancelled out" when there is no mistake is the other copy of the same gene. Geneticists believe that everyone carries a number of defective genes which are partnered by a normal copy which "cancels" the defect.
They also believe we all carry a number of genes with small variations which are not cancelled out, and which account for some of the differences in appearance or health among people in the general population. Ordinary cell division The body grows by making new cells.
New cells are made by one existing cell dividing into two. In order to work, each new cell needs a set of 46 chromosomes. Before a cell divides an exact copy of each chromosome is made, creating a complete new set. The two sets of chromosomes move to opposite ends of the cell. The cell then divides across the middle producing two new cells, each with 46 chromosomes.
This type of cell division is known as mitosis. Eggs and Sperm Eggs and sperm are different from all other cells in that they contain only a single set of 23 chromosomes. This means that when the egg and sperm come together at fertilisation, the usual number of 46 is restored, ready for the baby's development. There is a special type of cell division used only in the making of eggs and sperm.
This special type of cell division is called meiosis. Meiosis involves two rounds of cell division. The first round is very specialised; it is called reduction division, because it reduces the number of chromosomes to The second meiotic division is similar to ordinary cell division mitosis.
Men make millions of fresh sperm, all the time, from puberty onwards. It is quite different in women. Long before birth, the female embryo has already started making the cells which will form her eggs when she is a mature woman. After puberty, a woman's ovaries release one egg or occasionally more per month until menopause.
All eggs made by the mother will contain one copy each not a pair of chromosomes , and an X chromosome. All sperm made by the father will contain one copy each not a pair of chromosomes and either an X or a Y chromosome.
If an X-carrying sperm fertilises the egg the baby will be a girl, whilst if a Y-carrying sperm fertilises the egg the baby will be a boy. When a chromosome egg is fertilised by a chromosome sperm, the first cell of a 46 chromosome embryo, fetus and baby is made. All that baby's cells will have the same 46 chromosomes which were in the original fertilised egg. Overview Down's syndrome occurs in babies born with extra chromosome 21 material in their cells. Down's syndrome is also known as trisomy Trisomy 21 means there are 3 tri copies of chromosome somy Down's syndrome therefore arises from a change in gene quantity rather than gene quality.
We do no know exactly why Down's syndrome occurs. This makes it different from other genetic conditions, such as cystic fibrosis or sickle cell disease, whose inheritance can be traced through families. These people all have rare types of translocation. It is very unusual for parents to have more than one child with Down's syndrome, or for relatives of these parents to have a child with the condition.
Individuals with regular trisomy 21 have an extra chromosome 21 in every cell. They therefore have 47 chromosomes in each cell instead of the usual How does regular trisomy 21 occur? Regular trisomy 21 occurs because of an unusual cell division which has produced either an egg or a sperm with 24 chromosomes instead of When this egg or sperm fuses with an ordinary egg or sperm, the first cell of the developing baby has 47 chromosomes instead of 46, and all that baby's cells will have 47 chromosomes.
No one knows why this happens. There is no way or predicting whether a person is more or less likely to make eggs or sperm with 24 chromosomes. The extra chromosome can come from uneven division of the chromosomes called non-disjunction at either the first or the second meiotic division, in either parent.
Where does the extra chromosome come from? The extra chromosome comes from either the mother or the father. It makes no difference to the person with Down's syndrome which parent the extra chromosome came from. Why does regular trisomy 21 happen? Despite much research this question remains unanswered. There is no evidence that any nationality, ethnic group, diet, medicines, illnesses or upbringing have any influence on whether or not a parent can or will have a child with Down's syndrome.
Also, because Down's syndrome is present from the time of conception, nothing a woman does in pregnancy will influence whether or not her baby has Down's syndrome. Nothing is known which could have stopped the parent giving an extra chromosome. There is nothing "different" about the parents of a child with the condition. The parents' ages? Any couple can have a baby with Down's syndrome, but it is well known that older women are more likely to have a baby with the condition than younger women.
There is controversy about the influence of the father's age. Most researchers consider that the father's age does not appear to affect the chance of having a child with Down's syndrome. At the present time about one baby with Down's syndrome is born for every 1, total births.
More babies with Down's syndrome are conceived than are born, because the chance of miscarriage is higher if the foetus has Down's syndrome. The reason why older women are more likely to have children with Down's syndrome is unknown. There are two main current theories. One theory suggests that all women have some eggs with an extra chromosome, and that these eggs are more likely to be used last, towards the end of a woman's reproductive life.
The other theory suggests that the rate of trisomic conceptions is the same at all maternal ages, but that affected pregnancies are more likely to continue less likely to end in miscarriage in older women. The assumption is that the body recognises that this is a late pregnancy, perhaps the last or only one, and thus tries harder to make sure the pregnancy comes to term.
What is the chance of having another baby with the condition? Parents with one baby with regular trisomy 21 are usually told that the chance of having another baby with Down's syndrome is 1 in 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.
Still, all of these screens will not definitively diagnose Down syndrome. Prenatal screening and diagnostic tests are now routinely offered to women of all ages. The diagnostic procedures available for prenatal diagnosis of Down syndrome are chorionic villus sampling CVS and amniocentesis.
Amniocentesis is usually performed in the second trimester between 15 and 20 weeks of gestation, CVS in the first trimester between 9 and 14 weeks. Down syndrome is usually identified at birth by the presence of certain physical traits: low muscle tone, a single deep crease across the palm of the hand, a slightly flattened facial profile and an upward slant to the eyes. Because these features may be present in babies without Down syndrome, a chromosomal analysis called a karyotype is done to confirm the diagnosis.
They photograph the chromosomes and then group them by size, number, and shape. By examining the karyotype, doctors can diagnose Down syndrome. Another genetic test called FISH can apply similar principles and confirm a diagnosis in a shorter amount of time. Individuals with Down syndrome are becoming increasingly integrated into society and community organizations, such as school, health care systems, work forces, and social and recreational activities.
Individuals with Down syndrome possess varying degrees of cognitive delays, from very mild to severe. Most people with Down syndrome have cognitive delays that are mild to moderate. Due to advances in medical technology, individuals with Down syndrome are living longer than ever before.
In , children with Down syndrome were expected to survive to age nine. With the discovery of antibiotics, the average survival age increased to 19 or More and more Americans are interacting with individuals with Down syndrome, increasing the need for widespread public education and acceptance. These downloadable versions of the Preferred Language Guide are available to print and distribute:.
Being an Ambassador helps me to understand other people with disabilities and how I can help get more rights for people with Down Syndrome. Donate to NDSS. What is Down Syndrome? How Common is Down Syndrome? When Was Down Syndrome Discovered? Do you have updated information on this disease? Cause Cause. There are 3 possible genetic causes of Down syndrome : [2] [3] [5] Trisomy Most often, Down syndrome is caused by an extra chromosome 21 in all cells of the affected person.
In these cases, the chromosome 21 pair fails to separate during the formation of an egg or sperm ; this is called " nondisjunction. The extra chromosome is then copied in every cell of the baby's body, causing the features of Down syndrome. The cause of nondisjunction is unknown, but research has shown that it happens more often as women age.
Nondisjunction is not known to be caused by anything in the environment or anything that parents do or don't do before or during pregnancy. Mosaic trisomy In this situation, the fertilized egg may have the right number of chromosomes, but due to a cell division error early in the development of the embryo, some cells "acquire" an extra chromosome A person with mosaic trisomy 21 typically has 46 chromosomes in some cells, and 47 chromosomes with the extra chromosome 21 in others.
The features and severity in people with mosaic trisomy 21 may vary widely. Translocation trisomy For parents of a child with Down syndrome due to a translocation, there may be an increased chance of Down syndrome in future pregnancies. This is because one of the two parents may be a carrier of a balanced translocation.
However, not all parents of people with translocation trisomy 21 have a translocation. Regardless of the type of Down syndrome a person has, all people with Down syndrome have an extra, critical portion of chromosome 21 present in all or some of their cells.
This extra genetic material disrupts the normal course of development, causing the characteristic features of Down syndrome. Diagnosis Diagnosis. During pregnancy, a woman can opt to have specific tests that may either screen for, or diagnosis, Down syndrome in a fetus.
A screening test poses no risks to the fetus and can determine the likelihood that a fetus has Down syndrome. It may show that a fetus is at an increased risk to be affected, but cannot determine whether it is definitely affected. Depending on the type of screening tests a woman has, they may be done during the 1st trimester, the 2nd trimester, or both. If a screening test shows an increased risk for Down syndrome, a woman may then choose to have a diagnostic test.
Examples of diagnostic tests include chorionic villus sampling in the 1st trimester and amniocentesis in the 2nd trimester. During these tests, a small sample of genetic material is obtained from the amniotic fluid or placenta, and the fetus' chromosomes are then analyzed in a laboratory. However, women who have a positive NIPT result should then have invasive diagnostic testing to confirm the result. People with questions about the different options for prenatal screening or diagnostic testing should speak with a genetic counselor.
A genetic counselor can discuss the benefits, limitations and risks of each test, and help each person decide which test if any is best for them. If a diagnosis of Down syndrome is not made prenatally, the diagnosis can be made in the newborn.
Down syndrome may be suspected if a newborn has characteristic physical features of the condition. The diagnosis can then be confirmed by obtaining a karyotype a blood test to look at a picture of the newborn's chromosomes. Treatment Treatment. Early intervention services , quality educational programs, a stimulating home environment, good health care, and positive support from family and friends can help people with Down syndrome develop to their full potential.
The Research Down syndrome Foundation have a webpage with information about active reseach projects. Committee on Genetics. J Pediatr Health Care. Find a Specialist Find a Specialist. Healthcare Resources To find a medical professional who specializes in genetics, you can ask your doctor for a referral or you can search for one yourself.
You can also learn more about genetic consultations from MedlinePlus Genetics. Related Diseases Related Diseases. Conditions with similar signs and symptoms from Orphanet. Differential diagnosis includes Zellweger syndrome, 9qter deletion or other chromosomal abnormalities.
Visit the Orphanet disease page for more information. Research Research. Clinical Research Resources ClinicalTrials. Click on the link to go to ClinicalTrials. Please note: Studies listed on the ClinicalTrials. We strongly recommend that you talk with a trusted healthcare provider before choosing to participate in any clinical study. Patient Registry A registry supports research by collecting of information about patients that share something in common, such as being diagnosed with Down syndrome.
The type of data collected can vary from registry to registry and is based on the goals and purpose of that registry. Some registries collect contact information while others collect more detailed medical information. Learn more about registries. Organizations Organizations. Organizations Supporting this Disease. Association for Children with Down Syndrome, Inc. Do you know of an organization? Living With Living With. Their Web site offers general and condition-specific education resources to help teachers and parents better understand the needs of students who have genetic conditions.
Learn More Learn More. Click on the link to view information on this topic. March of Dimes has a factsheet on Down syndrome. MedlinePlus was designed by the National Library of Medicine to help you research your health questions, and it provides more information about this topic.
This website is maintained by the National Library of Medicine. The Merck Manual provides information on this condition for patients and caregivers. A Positive Exposure program called FRAME has an educational film about Down syndrome that was created to change how medical information is presented to healthcare professionals. Positive Exposure is an organization that uses photography, film, and narrative to transform public perceptions of people living with genetic, physical, intellectual, and behavioral differences.
In-Depth Information Medscape Reference provides information on this topic. You may need to register to view the medical textbook, but registration is free. Each entry has a summary of related medical articles.
It is meant for health care professionals and researchers. Orphanet is a European reference portal for information on rare diseases and orphan drugs. Access to this database is free of charge. PubMed is a searchable database of medical literature and lists journal articles that discuss Down syndrome. Click on the link to view a sample search on this topic. Have a question? References References. Down syndrome. Genetics Home Reference Web site.
Chen H. Down Syndrome. Medscape Reference. March of Dimes. Genetic Science Learning Center. What Is Down Syndrome?.
National Down Syndrome Society. How do health care providers diagnose Down syndrome?. Nina N. Merck Manuals. DS Down Syndrome Research and Treatment Foundation. Do you know of a review article? Share this content:.
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