Sentidos de métrion y métron: a) En H.G Liddell y R Scott

Down’s syndrome is a genetic disorder caused by the presence of all or part of chromosome 21 (Hu, Feng, Lazar & Kumin, 2013). Also, it is the most frequent genetic cause of learning disability and is associated with particular difficulties with language and communication. It is not a disease (Burgoyne, Duff, Clarke, Buckley, Snowling & Hulme, 2012). Antonarakis and Epstein (2006) point out that DS is associated with learning disabilities and congenital heart defects. The condition was initially labelled “Mongolian” by John Langdon Down, the British doctor who first described this syndrome in 1862. He thought that the children born with the syndrome had facial features especially in the angle of the eye similar to the Mongolian race and therefore named them in this way. Beliefs that DS was linked to race continued. In 1959, Down discovered that this condition was due to the additional version of chromosome 21. The term Mongolian was gradually replaced by Down’s syndrome but the former term is still used from time to time. The change in appellation was brought about because of official claims from a distinguished group of genetic experts at the “Sipa Symposium” in 1967, who stressed the importance of not using phrases with racist connotations. They emphasised the importance of switching to the term DS (Conor, 1999). Roizen and Patterson (2003) mentioned that the term DS became more widely used in the USA and the UK. Children with DS have physical features and characteristics that can be distinguished from other learning disabilities. Some common physical symptoms include smaller-than-average head and facial features, flat face and nose, eyes that are slanted up, short fingers and hands, weak muscles (Dey, Bhowmik, Chatterjee, Sinha, & Mukhopadhyay, 2013; Roizen, Magyar, Kuschner, Sulkes, Druschel, van-Wijngaarden & Hyman, 2014). There are also some medical and health issues that affect such individuals, for example, congenital heart defects, gastrointestinal issues, coeliac disease, thyroid conditions, leukemia, upper respiratory tract infections, ear problems, eye defects, atlantoaxial instability, dental issues, podiatric issues, skin and hair issues, sleep apnea and Alzheimer's

disease (Bull, 2011; Dierssen & Torre, 2012; Raut, Sriram, Yeoh, Hee, Lim, & Daniel, 2011; Steingass, Chicoine, McGuire & Roizen, 2011; Vis, De Bruin-Bon, Bouma, Huisman, Imschoot, Vaxn den Brink & Mulder, 2012). Furthermore, children with DS also suffer from delayed muscle development due to stress and poor muscle growth (Hernandez‐Reif, Field, Largie, Mora, Bornstein & Waldman, 2007). Also, they show several orthopaedic anomalies including bone deformity of the forefoot, flat foot, isolated calcaneal valgus, knee valgus and pronated flat foot (Concolino, Pasquzzi, Capalbo, Sinopoli & Strisciuglio, 2006). Children with DS are at risk of thyroid disorders and other body glands disorders, which appear in the prevalence of obesity, which is one of the most important problems facing parents, especially in the child’s late childhood (Magge,O’Neill, Shults, Stallings & Stettler, 2008; Melville, Cooper, McGrother, Thorp & Collacott, 2005). Figure 3.1 summarises the main physical and health characteristics of people with DS.

Source: http://www.arabnews.com/saudi-arabia/news/827381

Fidler (2005) points out that

the wealth of research on early development in Down’s syndrome may be attributed to the higher incidence of Down’s syndrome than that of other genetic syndromes, as well as technological advances enabling early identification of Down’s syndrome. Since the late 1960s, it has been possible to screen pregnant women for Down’s syndrome via amniocentesis and karyotyping of fetal cells. This stands in contrast with the delay of diagnosis often seen in other genetic disorders, for example, Williams syndrome. (p. 87) Sherman, Freeman, Allen and Lamb (2005) argue that around 6% to 10% of DS cases are caused by an error in spermatogenesis. Maternal non-disjunction can be due to age, alcohol consumption, maternal irradiation, fertility drugs, oral contraceptives, spermicides, parity, and low socio-economic status, although the effect of some of these factors have not been proven.

Bull (2011) mentions that when evaluating a newborn with DS, the first step is usually a careful review of the family history, as well as prenatal information, if studies are carried out on the fetal chromosome. Therefore, Bull (2011) states that “previous children born with trisomy 21 or developmental differences or pregnancies that ended in miscarriage may be significant clues that a family may carry a balanced translocation that predisposes them to have children with trisomy 21” (p. 395). Bull also notes that DS may be inherited although this is rarely the case. Rodríguez-Hernández and Montoya (2011) argue that DS is caused by an extra copy of chromosome 21 in each of the body’s cells. It is a chromosomal accident, dissociated from anything the parents may have done before or during the pregnancy and it is only very rarely inherited. Heywood, Mills, Wang, Hogg, Madgett, Avent, & Chitty (2012) agree that the cause of DS is a genetic accident, occurring at the time of conception. It therefore cannot be attributed to parental factors; thus, the exclusion of hereditary factors is emphasised.

According to Rodríguez-Hernández and Montoya (2011), there are three types of DS and these are all caused by different factors,as shown in Figure 3.2. The first type is Trisomy 21, which is due to one of the parents, through non-disjunction, giving two of chromosome 21 to the sperm or the egg instead of one. It gives the child an extra chromosome 21 in every cell. The reasons behind non-disjunction are as yet unknown and this type of DS is not inherited,which accounts for 95% of cases, so there is very little chance of another child in the family being affected. The second type is Translocation, which accounts for about 4% of cases of DS. where the additional chromosome 21 is joined with chromosome 14; meaning that there are 46 chromosomes instead of 47. The difference here is that one of the chromosomes is larger since it carries an additional chromosome. This type of DS is in fact inherited and there is the chance of it reoccurring in a sibling. This would contradict Heywood et al.’s (2012) view that DS is not inherited. The third type of DS is Mosaicism, which happens when there is a mixture of two types of cells, some containing the usual 46 chromosomes and some containing 47. Those cells with 47 chromosomes contain an extra chromosome 21. This type is the least common of DS and accounts for only about 1% of all cases.

Figure 3.2 Types of Down’s Syndrome