Half are from the father and half are from the mother. But sometimes an error occurs when the 46 chromosomes are being divided in half. An egg or sperm cell may keep both copies of chromosome number 21, instead of just 1 copy.
If this egg or sperm is fertilized, then the baby will have 3 copies of chromosome number This is called trisomy Sometimes the extra number 21 chromosome or part of it is attached to another chromosome in the egg or sperm.
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.
During pregnancy, an error in mitosis can occur. A teratogen ter-AT-uh-jen is something that can cause or raise the risk for a birth defect in a baby. They are things that a mother may be exposed to during her pregnancy. Teratogens include:.
Animal studies. Animal studies are the main way to find out if a medicine or other substance is safe during human pregnancy. Observations from human exposure. These are the reports of problems about a substance over time. Search Encyclopedia. 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 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.
For each meiotic group, there was an increasing risk with an increasing number of low SES factors. However, the odds ratios was much larger for the MMII cases than for the MMI cases, but with much larger confidence intervals due to smaller numbers.
All comparisons are highly significant, although the wide confidence intervals indicate low precision. In a previous analysis of this population that includes live births, spontaneous fetal deaths, and elective terminations, we observed an increased risk for a recognized pregnancy with DS for mothers with a cumulative life-exposure to low SES factors.
A concern would be that the sample used for the determination of the origin of NDJ would not be representative of DS cases in general. Because the inclusion of families in the subsample was not based on their type of NDJ, which was unknown at the time, there could not have been a selection bias favoring one or the other type of NDJ. This is corroborated by the distribution of the types of NDJ among the sampled cases: The inclusion of three pregnancy terminations in the sampled case group should not have changed the distribution of the meiotic origin of NDJ as others have found no differences between live births and pregnancy terminations in the distribution of MMI and MMII.
In other respects, the sample of cases in this analysis resembles other DS samples from the literature. The distribution of cases by parental origin of the NDJ, In the present study, which includes primarily a sample of live born infants with DS, we found an association of SES with the risk of having a DS that is similar to that found in our analysis of the group that includes all outcomes.
Although our sample was not a random selection from all DS cases, but a selection based on the availability of the family for a blood draw, the type of NDJ was not known to us or to the families that participated.
Our choice of variables captures the life-course SES of the case mother, from the time of formation of her oocyte pool and her youth, as represented by her father's occupation, to the time of conception of her offspring with DS, as represented by her education, the case father's occupation or education, and the family income.
However, many young mothers did not know their family income. When considered separately, unknown income was strongly associated with MMII. With one exception, it was also correlated with low socioeconomic factors. By using the conservative approach of including unknown income with the high-income group, we biased the results toward the null hypothesis of no association between the type of meiotic NDJ and low SES.
When the cases with an unknown income were included with the cases with low income, the odds ratios for trends were even higher, confirming that our measure of association was conservative. Although the SES variables were highly correlated, in multivariate analysis they were individually associated with the risk for a pregnancy with a DS.
This precludes confounding that could have been introduced if all the SES variables had been included in the analysis as separate entities. The reason for an association of low SES with the risk for a live born with DS, independently of maternal age, ethnicity, and gravidity or parity, is not explained, but has been reported by others. Because age is the most important risk factor for DS, one hypothesis would imply that environmental risk factors could modify the aging of the oocyte pool of the mother.
Several authors have reported that women of lower SES have an earlier menopause. Those risk factors could increase the level of spontaneous deletions of mitochondrial DNA, which are known to increase exponentially at approximately the same age as the exponential risk for a DS birth.
Other researchers have suggested that genetic factors could influence the type of meiotic error contributed to the DS offspring. Petersen et al. The authors suggest a function of presenilin proteins in chromosome segregation because the expression of these proteins has been localized to the nuclear membrane, kinetochores, and centromeres.
Avramopoulos et al. To our knowledge, an interaction between potential genetic and environmental risk factors for NDJ has not been explored to date. Yang et al. Their study is the only one that has evaluated environmental factors that might influence differentially the susceptibility to one or the other meiotic error.
In our study, none of the mothers of MMII cases both smoked and took contraceptive pills, and only 3 of the mothers of MMI cases did so. The large majority of mothers in our mostly Hispanic population did not smoke. Neither smoking nor use of oral contraceptive pills, either separately or together, had any association with DS in our analyses results not shown ; consequently these variables were not entered in subsequent analyses.
Analysis of our data according to mothers' age groups corresponding to those of the Yang study showed results similar to those of our main analyses data not shown. Furthermore, the increasing trend in risk with the increasing number of low SES factors and the high values of the ORs suggest that the association is real. Recently, an environmental factor, bisphenol A, has been shown experimentally to cause meiotic aneuploidy in oocytes of exposed mice.
Although evidence is accumulating for a possible effect of deleterious environmental factors on the meiotic process, the mechanism by which this effect would be obtained has not been elucidated. As the process of maturation of the oocyte involves many pathways, different environmental factors may impact different pathways.
Clearly NDJ is a multifactorial disorder. The pattern of chiasmata along the chromosome established during the fetal life of a female is thought to predispose the bivalent to NDJ. Although intriguing, our results need confirmation. Further explorations of the interaction of environmental and genetic susceptibility factors for NDJ are warranted. Epidemiologic study of Down syndrome in a racially diverse California population, — Am J Epidemiol ; : — Jones KL.
Smith's recognizable patterns of human malformations, 4 ed. Philadelphia: WB Saunders; Susceptible chiasmate configurations of chromosome 21 predispose to non-disjunction in both maternal meiosis I and meiosis II. Nat Genet ; 14 : — Down syndrome: toward a molecular definition of the phenotype. Am J Med Genet Suppl ; 7 : 91— Antonarakis SE.
Parental origin of the extra chromosome in trisomy 21 as indicated by analysis of DNA polymorphisms. N Engl J Med ; : — Advanced maternal age and the risk of Down syndrome characterized by the meiotic stage of the chromosomal error: a population-based study. Am J Med Genet ; 58 : — CAS Google Scholar. The meiotic stage of nondisjunction in trisomy Determination by using DNA polymorphisms.
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