Down syndrome occurs when babies are born with an extra copy of chromosome 21. Normally, each cell in the body has 23 chromosomes.
People with Down syndrome typically live longer than others. But they may have more health problems as they age. So it’s important for them to get regular health checkups.
What is Down syndrome?
Down syndrome is a genetic disorder caused by an extra copy of chromosome 21. This extra chromosome changes how a person’s body and brain develop. Down syndrome affects every person differently. Some people with Down syndrome have very mild symptoms, while others have more serious ones. People with Down syndrome can live happy and fulfilling lives, but they do have to face some challenges, such as learning to roll over, sit up, stand, walk, talk and reach certain milestones at a slower pace than other people their age. They may also have some physical health issues, such as heart defects and conditions that affect the bones and muscles.
The most common form of Down syndrome is trisomy 21, which means that a person has three copies of chromosome 21 in each cell instead of the usual two copies. This type of Down syndrome is not inherited from parents and happens by accident during the development of a sperm or egg cell before fertilization. It accounts for about 95% of all cases of Down syndrome.
Mosaic Down syndrome is rarer. It occurs when abnormal cell division in the embryo causes some cells to have an extra copy of chromosome 21, while other cells don’t have it. This results in a mixture of normal and abnormal cells, which is why this type of Down syndrome is sometimes called mosaic Down syndrome. Mosaic Down syndrome accounts for about 1% of all cases of Down syndrome.
Translocation Down syndrome is another type of Down syndrome that accounts for about 4% of all cases of Down syndrome. This occurs when part of chromosome 21 becomes attached to another chromosome, usually chromosome 13 or 16, before or at conception. This type of Down syndrome can be passed down from one parent to a child, but it isn’t inherited the way that trisomy 21 is.
There are many tests that can be done before a baby is born to see if it has Down syndrome. The most accurate test is a karyotype, which checks for an extra copy of chromosome 21. These tests can be done in a laboratory during an ultrasound exam or through a procedure called amniocentesis or chorionic villus sampling (CVS), which are performed after 20 weeks of pregnancy. These procedures carry up to a 1% risk of causing a miscarriage.
Trisomy 21
When a baby is born with Down syndrome, they have an extra copy of chromosome 21. Chromosomes are a type of DNA that carry genes that determine many important characteristics, including physical traits, intelligence, and personality. Normally, each cell in an individual has two copies of each chromosome. But during cell division, one of the two copies of chromosome 21 fails to separate properly (this is called non-disjunction). As a result, each daughter cell gets an extra copy of chromosome 21, which causes Down syndrome.
Most of the time, a genetic mutation causes Down syndrome. But there are also cases where a problem occurs during pregnancy. A birth defect occurs when something goes wrong during a mother’s pregnancy, usually in the first three months of life. Most birth defects occur in the first trimester of pregnancy, and Down syndrome is no exception.
There are several different kinds of Down syndrome, which affect how a person looks and learns. Most people with Down syndrome have a mild to moderate intellectual disability, making it difficult for them to learn and retain information. They may also have physical disabilities such as slanted eyes, short ears and an open mouth with a protruding tongue. Some individuals with Down syndrome have heart problems and other health issues such as hearing loss or attention deficit hyperactivity disorder (ADHD).
The earliest known case of a human with Down syndrome was documented by a French physician, Jerome Lejeune, in 1959. He described an infant who had three copies of chromosome 21 as a “mongoloid,” which became the term for Down syndrome when he published his work. Unfortunately, Lejeune did not have the tools that we now use to diagnose Down syndrome during a prenatal screening.
Researchers have learned that mice can have trisomy in chromosome 16 and actually genetically create this phenotype to study the effects of Down syndrome. Despite the fact that this condition does not naturally occur in a wild animal, it is important to understand how an extra chromosome impacts behavior and development in humans. This can help scientists develop treatments that will improve quality of life for individuals with Down syndrome.
Translocation
Normally, the genetic material stored in cells is grouped along rod-like structures called chromosomes. Chromosomes contain genes, which carry the codes that determine inherited traits. A gene is a segment of deoxyribonucleic acid (DNA) that is responsible for making a particular protein. Each cell has 23 pairs of chromosomes, and one copy of each pair is inherited from each parent. Down syndrome occurs when an extra copy of chromosome 21 is present, giving the developing fetus three copies of the chromosome instead of the typical two. This type of Down syndrome, known as trisomy 21, accounts for 95% of all cases. A small percentage of Down syndrome cases occur when a partial or full extra copy of chromosome 21 is attached to another chromosome — a situation called translocation. Some of these cases are balanced and can be passed from parents to children, but most unbalanced translocations are spontaneous or caused by mistakes in the specialized cell division that produces eggs or sperm.
Diagnosis
Although it is not true that animals can only develop Down syndrome because of the difference in our chromosomes, monkeys can experience similar problems. Over the years, a few of these animals have been mistakenly diagnosed with Down syndrome. Monkeys are known to have a variety of symptoms that can be confused with the condition, including slow growth, intellectual challenges of varying degrees, and features like crossed eyes.
While scientists have documented a few cases of this genetic condition in other animals, monkeys are the closest animal to humans that can develop Down syndrome. Unlike human fetuses, animals don’t undergo routine prenatal testing to see if they will develop the condition, which is why it is so rare to find a monkey with Down syndrome.
A monkey named Kanako has been diagnosed with trisomy 22, a condition characterized by an extra copy of chromosome 22. She has slowed growth and intellectual challenges, as well as several other symptoms such as a congenital heart disease, crossed eyes, and thinning corneas. Her caretakers at Kyoto University’s Kumamoto Sanctuary didn’t initially suspect she had Down syndrome, despite her stunted growth and other symptoms.
Researchers were able to diagnose Kanako’s trisomy 22 after she developed cataracts and other eye problems, which made her difficult to see. They then ran a series of tests, including an echocardiogram, which showed that she had a hole in her heart wall that separates the top two chambers of the organ. This prompted them to run a chromosomal test, which confirmed that she has trisomy 22.
The finding isn’t necessarily surprising, however. It is thought that chimpanzees, gorillas and orangutans share the same genetic mutations as humans with Down syndrome. These apes also have 24 pairs of chromosomes, which means that they can develop a third copy of chromosome 22, which is the condition characterized by Down syndrome in humans. In fact, a chimpanzee named Tarou was born with a third copy of chromosome 22 in 1969. He died at an early age and was the only chimpanzee with trisomy 22 to be studied.