Humans typically have 46 chromosomes which are then separated into pairs, of which there are 23. Each pair of chromosomes usually has two copies of a gene, one from each parent. The chromosomes are responsible for carrying information and instructions that make up an organism. Chromosome 21 is the smallest human chromosome and it represents about 1.5% to 2% of the total DNA found in cells. Down’s syndrome usually occurs when there is an extra chromosome 21, resulting in 47 chromosomes. There are mainly three types of Down’s syndrome, consisting of regular trisomy 21, translocation, and mosaic. Trisomy 21 occurs in 95 percent of those who have Down’s syndrome, and is caused by nondisjunction during meiosis I. Nondisjunction is when a pair of homologous chromosomes do not separate during cell division. This will eventually lead to one gamete receiving two of the same type of chromosome and another gamete with no copy. If the gamete with the extra chromosome is fertilized with a normal gamete, then the baby will have three copies of the chromosome 21 in each cell. An extra copy means an extra thousands of genes. With extra genes, there will also be extra proteins which may result in the over-expression of some proteins which interferes with normal cellular function. In translocation Down’s syndrome, there are still 3 copies of chromosome 21 but one of the chromosomes attach or translocate to another chromosome. Unlike regular and translocation Down’s syndrome, mosaic Down’s syndrome is when only a certain amount of the individual’s body cells have an extra chromosome 21, not all of them. At the cellular level, individuals with Down’s syndrome may have anemia in which their red blood cells are not able to carry enough oxygen throughout the body. Their red blood cells may also have an iron deficiency in which their red blood cells lack iron. There are many phenotypic symptoms for Down’s syndrome at the bodily scale level. Each individual is different, but typically, those with the condition have characteristic facial features. These include: small heads, ears, and mouths, a flattened facial profile and nose, and upward slanting eyes. In addition to characteristic facial features, other symptoms include: delayed development, short stature, heart defects, susceptibility to respiratory infection, and mental retardation. Individuals will also have a small and hypocellular brain and be cognitively impaired, with the severity varying in different individuals. Most will be sexually underdeveloped and sterile and many newborns have hypotonia, which means decreased muscle tone. Individuals with Down’s syndrome are at a higher risk for Alzheimer’s disease and leukemia. At the cellular level, A cure is currently unknown for Down’s syndrome, however, treating it early can help people live a better life. There are various treatment options for sufferers. One option is through self-care, where the patient does physical exercise which can improve cardiovascular function. In addition, there is special education that addresses the special needs of the affected individuals. Therapy can also help people with Down’s syndrome. There are several different kinds of therapy that may help, including physical, which help build motor skills, speech-language, which can help improve communication and language use, occupational, which helps with everyday tasks, and emotional and behavioral therapies which help the person cope with their behavior. Furthermore, there are many support groups who allows individuals to interact with those who have similar conditions. Although some people with Down’s take drugs or supplements, many of them have proven to be ineffective and unsafe and the newer ones have yet to prove if they are any better. Assistive devices are also used to help with tasks and learning. For example, a person with Down’s may use hearing aids, special pencils which make writing easier, touchscreen computers, and keyboards with larger letters.