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 Very few families are known who have more than one child with Down's syndrome, so the real chance is probably less than this.
There are differences of opinion as to how the much-quoted figure of 1 in should be interpreted for older mothers who already have one child with Down's syndrome. Others feel that, when the age-related chance is more than 1 in , this alone gives the more realistic estimate so for a woman of 43, the chance remains 1 in This usually arises when the small arms of chromosome 21 and another chromosome break, and the two remaining long arms join together at their exposed ends.
This process of chromosomes breaking and rejoining to other chromosomes is known as translocation because the chromosome material has transferred its location. People with Down's syndrome which has arisen in this way still have an extra copy of a large part of chromosome Their features of Down's syndrome are no different from those in a child with regular trisomy How does translocation occur?
In two-thirds of people with Down's syndrome due to a translocation, the translocation was an isolated event during the formation of the individual egg or sperm involved in their conception. As with regular trisomy 21, there is no known reason why this occurs.
It cannot be predicted and it is not a result of anything the parents or other family members have done. The egg or sperm contains the usual number of chromosomes i. Thus there is one free, whole chromosome 21 and most of a second chromosome 21 attached to another chromosome. Mosaicism or mosaic Down syndrome is diagnosed when there is a mixture of two types of cells, some containing the usual 46 chromosomes and some containing Those cells with 47 chromosomes contain an extra chromosome Research has indicated that individuals with mosaic Down syndrome may have fewer characteristics of Down syndrome than those with other types of Down syndrome.
However, broad generalizations are not possible due to the wide range of abilities people with Down syndrome possess. The presence of the extra full or partial chromosome 21 causes the characteristics of Down syndrome. 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 are ongoing studies to find out if there are any differences in the health of people with mosaic Down syndrome, depending on how the mosaicism occurred. Mosaic Down syndrome cases can vary greatly. They can range from having very mild features to having most of the features of Down syndrome.
It increases yearly with increasing age. Facts about Down Syndrome. Down Syndrome Education International. About us. Children's Hospital of Philadelphia. Data and Statistics on Down Syndrome. Department of Health and Humans Services. National Institutes of Health. What conditions or disorders are commonly associated with Down syndrome? American Academy of Orthopaedic Surgeons. Down Syndrome: Musculoskeletal Effects.
National Down Syndrome Society. American Cancer Society. Risk Factors for Childhood Leukemia. Trisomy 21 and facial developmental instability.
Am J Phys Anthropol. National Organization for Rare Disorders. Self-perceptions from people with Down syndrome. Am J Med Genet. Jan 31, Down Syndrome: Data and Statistics.
Updated June 27, Sept 26, Holmes, G. Gastrointestinal Disorders in Down Syndrome. Gastroenterol Hepatol Bed Bench. Winter, ; Mayo Clinic Staff. Down Syndrome. Mayo Clinic. June 27, Your Privacy Rights. To change or withdraw your consent choices for VerywellHealth. At any time, you can update your settings through the "EU Privacy" link at the bottom of any page.
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