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:.
Originally, it was something for her to do to reconnect with herself outside of being a mom. Sometimes, Stacy would exercise while Marcus would sleep and other times Marcus and his sisters would exercise alongside her. Her ultimate dream was to reach Ironman distance, and she knew that she wanted to set an example for Marcus and her three girls: that anything is possible.
Donate to NDSS. What is Down Syndrome? How Common is Down Syndrome? When Was Down Syndrome Discovered? Support NDSS The National Down Syndrome Society envisions a world in which all people with Down syndrome have the opportunity to enhance their quality of life, realize their life aspirations and become valued members of welcoming communities.
What Causes Down Syndrome? Does Down Syndrome Run in Families? How Is Down Syndrome Diagnosed? AT BIRTH 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.
Preferred Language Guide. Use this language when referring to Down syndrome and people who have Down syndrome: People with Down syndrome should always be referred to as people first. Using this word is hurtful and suggests that people with disabilities are not competent. Down syndrome is named for the English physician John Langdon Down, who characterized the condition, but did not have it. While Down syndrome is listed in many dictionaries with both popular spellings with or without an apostrophe s , the preferred usage in the United States is Down syndrome.
Brighter Tomorrows www. The physical movement of chromosomes could then be correlated with cells' patterns of genetic inheritance. The idea that genes were carried on cytological structures is now known as the chromosome theory.
Using such methods, researchers determined that although mitosis and meiosis are both forms of cell division , the results of these processes are actually quite different. Mitosis occurs in somatic cells; this means that it takes place in all types of cells that are not involved in the production of gametes. Prior to each mitotic division, a copy of every chromosome is created; thus, following division, a complete set of chromosomes is found in the nucleus of each new cell.
Indeed, apart from random mutations, each successive duplicate cell will have the same genetic composition as its parent, due to the inheritance of the same chromosome set and similar biological environment.
This works well for replacing damaged tissue or for growth and expansion from an embryonic state. Because the genes contained in the duplicate chromosomes are transferred to each successive cellular generation, all mitotic progeny are genetically similar.
However, there are exceptions. For example, there are genetic variations that arise in clonal species , such as bacteria , due to spontaneous mutations during mitotic division. Furthermore, chromosomes are sometimes replicated multiple times without any accompanying cell division. This occurs in the cells of Drosophila larvae salivary glands, for example, where there is a high metabolic demand. The chromosomes there are called polytene chromosomes, and they are extremely large compared to chromosomes in other Drosophila cells.
These chromosomes replicate by undergoing the initial phases of mitosis without any cytokinesis Figure 2. Therefore, the same cell contains thick arrangements of duplicate chromosomes side by side, which look like strands of very thick rope.
Scientists believe that these chromosomes are hyper-replicated to allow for the rapid and copious production of certain proteins that help larval growth and metamorphosis Gilbert, The greatest impact of Sutton's work has far more to do with providing evidence for Mendel's principle of independent assortment than anything else.
Specifically, Sutton saw that the position of each chromosome at the midline during metaphase was random, and that there was never a consistent maternal or paternal side of the cell division. Therefore, each chromosome was independent of the other. Thus, when the parent cell separated into gametes, the set of chromosomes in each daughter cell could contain a mixture of the parental traits, but not necessarily the same mixture as in other daughter cells. To illustrate this concept, consider the variety derived from just three hypothetical chromosome pairs, as shown in the following example Hirsch, Each pair consists of two homologues: one maternal and one paternal.
Here, capital letters represent the maternal chromosome, and lowercase letters represent the paternal chromosome:. When these chromosome pairs are reshuffled through independent assortment , they can produce eight possible combinations in the resulting gametes:.
A mathematical calculation based on the number of chromosomes in an organism will also provide the number of possible combinations of chromosomes for each gamete. In particular, Sutton pointed out that the independence of each chromosome during meiosis means that there are 2 n possible combinations of chromosomes in gametes, with "n" being the number of chromosomes per gamete.
Thus, in the previous example of three chromosome pairs, the calculation is 2 3 , which equals 8. Furthermore, when you consider all the possible pairings of male and female gametes, the variation in zygotes is 2 n 2 , which results in some fairly large numbers. But what about chromosome reassortment in humans?
Humans have 23 pairs of chromosomes. That means that one person could produce 2 23 different gametes. In addition, when you calculate the possible combinations that emerge from the pairing of an egg and a sperm, the result is 2 23 2 possible combinations. However, some of these combinations produce the same genotype for example, several gametes can produce a heterozygous individual. Of course, there are more than 23 segregating units Hirsch, While calculations of the random assortment of chromosomes and the mixture of different gametes are impressive, random assortment is not the only source of variation that comes from meiosis.
In fact, these calculations are ideal numbers based on chromosomes that actually stay intact throughout the meiotic process. In reality, crossing-over between chromatids during prophase I of meiosis mixes up pieces of chromosomes between homologue pairs, a phenomenon called recombination.
Because recombination occurs every time gametes are formed, we can expect that it will always add to the possible genotypes predicted from the 2 n calculation. In addition, the variety of gametes becomes even more unpredictable and complex when we consider the contribution of gene linkage.
Some genes will always cosegregate into gametes if they are tightly linked, and they will therefore show a very low recombination rate. While linkage is a force that tends to reduce independent assortment of certain traits, recombination increases this assortment.
In fact, recombination leads to an overall increase in the number of units that assort independently, and this increases variation. While in mitosis, genes are generally transferred faithfully from one cellular generation to the next; in meiosis and subsequent sexual reproduction , genes get mixed up. Sexual reproduction actually expands the variety created by meiosis, because it combines the different varieties of parental genotypes.
Thus, because of independent assortment, recombination, and sexual reproduction, there are trillions of possible genotypes in the human species. During cell division, chromosomes sometimes disappear.
This occurs when there is some aberration in the centromere , and spindle fibers cannot attach to the chromosome to segregate it to distal poles of the cell. Consequently, the lost chromosome never properly groups with others into a new nuclear envelope , and it is left in the cytoplasm , where it will not be transcribed. Also, chromosomes don't always separate equally into daughter cells.
This sometimes happens in mitosis, when sister chromatids fail to separate during anaphase. One daughter cell thus ends up with more chromosomes in its nucleus than the other. Likewise, abnormal separation can occur in meiosis when homologous pairs fail to separate during anaphase I. This also results in daughter cells with different numbers of chromosomes. The phenomenon of unequal separation in meiosis is called nondisjunction. If nondisjunction causes a missing chromosome in a haploid gamete, the diploid zygote it forms with another gamete will contain only one copy of that chromosome from the other parent, a condition known as monosomy.
Conversely, if nondisjunction causes a homologous pair to travel together into the same gamete, the resulting zygote will have three copies, a condition known as trisomy Figure 3.
The term " aneuploidy " applies to any of these conditions that cause an unexpected chromosome number in a daughter cell. Nondisjunction can occur during either meiosis I or II, with different results Figure 7.
If homologous chromosomes fail to separate during meiosis I, the result is two gametes that lack that chromosome and two gametes with two copies of the chromosome. If sister chromatids fail to separate during meiosis II, the result is one gamete that lacks that chromosome, two normal gametes with one copy of the chromosome, and one gamete with two copies of the chromosome.
An individual with the appropriate number of chromosomes for their species is called euploid; in humans, euploidy corresponds to 22 pairs of autosomes and one pair of sex chromosomes. An individual with an error in chromosome number is described as aneuploid, a term that includes monosomy loss of one chromosome or trisomy gain of an extraneous chromosome.
Monosomic human zygotes missing any one copy of an autosome invariably fail to develop to birth because they have only one copy of essential genes. Most autosomal trisomies also fail to develop to birth; however, duplications of some of the smaller chromosomes 13, 15, 18, 21, or 22 can result in offspring that survive for several weeks to many years.
Trisomic individuals suffer from a different type of genetic imbalance: an excess in gene dose. Cell functions are calibrated to the amount of gene product produced by two copies doses of each gene; adding a third copy dose disrupts this balance. The most common trisomy is that of chromosome 21, which leads to Down syndrome. Individuals with this inherited disorder have characteristic physical features and developmental delays in growth and cognition.
The incidence of Down syndrome is correlated with maternal age, such that older women are more likely to give birth to children with Down syndrome Figure 7. Concept in Action Visualize the addition of a chromosome that leads to Down syndrome in this video simulation.
Humans display dramatic deleterious effects with autosomal trisomies and monosomies. Therefore, it may seem counterintuitive that human females and males can function normally, despite carrying different numbers of the X chromosome.
In part, this occurs because of a process called X inactivation. Early in development, when female mammalian embryos consist of just a few thousand cells, one X chromosome in each cell inactivates by condensing into a structure called a Barr body. The genes on the inactive X chromosome are not expressed. The particular X chromosome maternally or paternally derived that is inactivated in each cell is random, but once the inactivation occurs, all cells descended from that cell will have the same inactive X chromosome.
By this process, females compensate for their double genetic dose of X chromosome. Females heterozygous for an X-linked coat color gene will express one of two different coat colors over different regions of their body, corresponding to whichever X chromosome is inactivated in the embryonic cell progenitor of that region. When you see a tortoiseshell cat, you will know that it has to be a female. In an individual carrying an abnormal number of X chromosomes, cellular mechanisms will inactivate all but one X in each of her cells.
As a result, X-chromosomal abnormalities are typically associated with mild mental and physical defects, as well as sterility. If the X chromosome is absent altogether, the individual will not develop. Several errors in sex chromosome number have been characterized. Individuals with three X chromosomes, called triplo-X, appear female but express developmental delays and reduced fertility. The XXY chromosome complement, corresponding to one type of Klinefelter syndrome, corresponds to male individuals with small testes, enlarged breasts, and reduced body hair.
The extra X chromosome undergoes inactivation to compensate for the excess genetic dosage. Turner syndrome, characterized as an X0 chromosome complement i. An individual with more than the correct number of chromosome sets two for diploid species is called polyploid.
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