Down syndrome (DS) is an inherited genetic disorder. It occurs when a child has an extra copy of chromosome 21.
Chromosomes are rod-like structures that package genetic information in cells. Normally, people have 23 pairs of chromosomes. Each pair has one chromosome from the mother and one from the father.
Trisomy 21 is a genetic disorder that affects 1 in every 700 babies. It is caused when an infant has three copies of chromosome 21 instead of the usual two, for a total of 47 chromosomes. This extra chromosome can cause different medical issues and may lead to developmental delays.
Symptoms of trisomy 21 include a distinct facial appearance, intellectual disability, developmental delays and thyroid/heart disease. In addition to these physical problems, individuals with trisomy 21 have a higher risk of developing Alzheimer’s disease later in life than those without the syndrome.
One of the most intriguing cases of a monkey with trisomy 21 is Kanako, a 24-year-old chimp that was born in captivity and is now living at Kyoto University. Her caretakers didn’t suspect she had the condition until an echocardiogram showed a “hole” in her heart that separated its top two chambers.
But before that, she had been diagnosed with a range of health issues that are common in people with trisomy 22, including stunted growth, cataracts as a baby, cross eyes and a disorder that causes her corneas to thin. Her behavior was also normal, except for the fact that she had a hard time standing up, according to researchers who recently analyzed her chromosomes.
The researchers used a new technique to determine the presence of trisomy 21, called Q-banding and multicolor fluorescence in situ hybridization (FISH). They found that Kanako had an extra chromosome 17 — a type of translocation — but no other chromosomal rearrangements, such as mosaicism or heterozygous variants.
Another method of determining trisomy 21 is by measuring the level of a gene that produces a protein that helps control cell growth, called PLAC4 (platelet-activating C4; 314670). Plasma PLAC mRNA concentrations were significantly increased in pregnancies affected with trisomy 21. This method is more accurate than using real-time or digital PCR to detect trisomy 21, but it requires additional testing by a laboratory.
Mosaicism is when someone is born with two or more genetically different sets of chromosomes (twenty-three pairs) in some of their cells. This is very rare, but it can cause certain health problems and some genetic disorders.
People with mosaicism can have babies with an increased risk of certain genetic diseases such as Down syndrome, Turner syndrome and Klinefelter syndrome. It also makes it harder to treat some diseases, such as inherited skin cancers and some types of arthritis.
To make a long story short, mosaicism can be diagnosed either prenatally using chorionic villus sampling (CVS) or amniocentesis, or after birth by testing a sample of DNA from blood or a cheek swab. This can help to identify mosaicism and determine whether it is a cause of an increased risk for future pregnancies or not.
It is also important to understand that some chromosomal abnormalities found in the mother’s blood may not actually be present in the baby – this is called placental mosaicism and can lead to false positive results on non-invasive prenatal screening tests. This is particularly true for trisomies and is why the test should always be confirmed by a diagnostic test.
One example of placental mosaicism is the extra chromosome 22 in a cynomolgus monkey with down syndrome, Kanako. This extra chromosome affects her brain and body, leading to microcephaly, mental retardation and myoclonic seizures.
Interestingly, the chromosomal change also causes her to have clubbed digits, a wide 1-2 toe gap and a shortened stature. This can affect the way she walks and how well she gets around.
The frequency of mosaicism and aneuploidy varies depending on the patient’s age and the underlying disorder. For example, aneuploidy on chromosome 16 occurs at higher frequencies in females with advanced maternal age than in those of young age.
A similar pattern is found for chromosomal mosaicisms involving individual chromosomes. For example, a patient with trisomy 21 has aneuploidy on chromosomes 22, 16, and 15. In this study, the same patterns were seen for patients with Down syndrome or Turner syndrome.
Nondisjunction is a genetic disorder that occurs during meiosis, the process of cell division. It is the result of a mistake during the process that causes some of the daughter cells to lose one of their chromosomes and gain another. The chromosomes are supposed to separate during the anaphase and telophase stages of meiosis.
The process is complicated, so mistakes are common. These mistakes may occur in both sexes. They can be caused by a single error or several errors that are repeated over and over again.
In most cases, these chromosomal errors result in abnormalities called aneuploidies. These are a type of genetic disorder that can cause serious problems in children or adults. These chromosomal abnormalities can be identified by seeing a karyogram, which is a picture of a human’s chromosomes.
These chromosomal abnormalities can include the duplication or loss of entire chromosomes, as well as changes in the number of complete sets of chromosomes. They can also involve an increase in the number of chromosomes within each set, which is known as mosaicism.
This can lead to many different kinds of disorders. Some of these disorders are very severe, while others can be milder and have no symptoms.
The most common of these chromosomal abnormalities is trisomy 21, which is the result of an extra chromosome in one or both sexes. It accounts for about 95 percent of all down syndromes.
There are two other types of trisomy 21 that occur less often: mosaicism and translocation. Mosaicism is the result of chromosomes that are not properly separated during meiosis, which can lead to aneuploidies in the zygotes. These zygotes are usually not viable, and do not grow or develop normally.
These zygotes will have a range of developmental problems, including rocker-bottom feet, microphthalmia (abnormally small eyes), microcephaly (abnormally small head), polydactyly, holoprosencephaly (abnormally long legs), cleft lip and palate, congenital heart disease, and severe intellectual disability.
Some of these chromosomal abnormalities can be diagnosed prenatally, and some can be identified in the embryo before birth. This can help determine whether the baby is born with aneuploidies or normal chromosome numbers.
Translocation is a genetic disorder in which a part of one chromosome breaks off and attaches to another. Sometimes it is a large piece of chromosome, but it can also be a small deletion or addition of autosomal material that is too small to be detected by normal karyotype testing techniques.
Some people with Down syndrome have a large piece of chromosome 21 (trisomy 21) attached to another chromosome before or at the time of conception. These children are called mosaics because they have both normal and abnormal cells with chromosome 21 attached. Mosaic Down syndrome is the rarest form of this disorder.
A rarer type of this disorder is translocation that creates a duplication event in the region of chromosome 21. This is known as t(21;21) or 46,XX,dup(21q).
This extra copy of the chromosome is responsible for some of the physical and mental characteristics of Down syndrome. Individuals with this type of chromosome abnormality have a high risk for developing certain mental health problems, such as dementia or depression.
Although trisomy 21 is the most common cause of Down syndrome, about 5% of all cases are caused by another abnormality. This abnormality is called translocation, and it is more likely to be found in boys than girls.
Most people who have a balanced translocation are silent carriers, meaning that they don’t have any symptoms or health problems because of the extra chromosome they carry. However, if they have children, these children will be at a much higher risk for Down syndrome than the rest of their family.
These children have the balanced chromosomes from the parent and an extra chromosome from the other parent, usually chromosome 14. The child can have any number of different chromosome combinations (known as segregants) because of this mis-segregation during gametogenesis.
Some people who have a balanced translocation are able to have children without any problems. But these babies are more likely to have a miscarriage than other children with balanced chromosomes.
The best way to know if you have a chromosome translocation is to get tested. This can be done by taking a blood sample and having the cells in your blood examined in a laboratory. If your test shows that you have a chromosome translocation, you can talk to your doctor about how you can reduce your risk of having a baby with Down syndrome.