What is Trisomy 21- Down syndrome ?

Down syn­drome is a genet­ic con­di­tion in which an extra copy of chro­mo­some 21 is present in an indi­vid­ual. It was ini­tial­ly observed and is described by a British Doc­tor John Lang­don down in 1866 and third was called down syn­drome after his name.

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Down Syn­drome

How­ev­er the time he could not describe the rea­son of its occur­rence and it was only lat­er in 1959 when tri­somy 21 was iden­ti­fied to be its cause.

Down syn­drome is char­ac­ter­ized by spe­cif­ic facial appear­ance, mus­cu­lar hypo­to­nia, cog­ni­tive delays and intel­lec­tu­al disabilities.Cognitive delays lead to nor­mal behav­iour and slow­er devel­op­ment of speech and lan­guage skills.

Down syn­drome is iden­ti­fied through pre­na­tal screen­ing. par­ents with bal­anced translo­ca­tion involv­ing chro­mo­some 21 are poten­tial car­ri­er of down syn­drome and required genet­ic coun­sel­ing to pre­vent the occur­rence of this disorder.

Mater­nal age is high­ly cor­re­lat­ed with the occur­rence of down syn­drome in child there is increas­es with the increase in age of mother.The chances are 1 in a age of 1000 at the age of 30, 1 in a 100 at the age of 40 ‚while it is 1 in a 30 at the age of 45.

Tri­somy 21 is main­ly caused by abnor­mal­i­ty dur­ing cell divi­sion which can be of three types.Incomplete tri­somy 21 a whole chro­mo­some 21 is present as an addi­tion­al copy that is instead of 23 chro­mo­somes are present in an individual.

Of the total tri­somies 21,90 — 93% cas­es are com­plete tri­somy 21.This Extra copy comes from abnor­mal gamete in which chro­mo­somes 21 has failed to sep­a­rate dur­ing mei­ot­ic division.

Mosa­ic down syn­drome or Mosa­ic Tri­somy 21 is a rare con­di­tion found in <3% in which some cells of the body have an extra copy of chro­mo­some 21,While oth­ers are nor­mal and have only two of its copies. this mosa­ic of cells in the body of organisms.

This mosaicism appears as a result of abnor­mal­i­ty is in cell divi­sion in the zygote lead­ing to nondis­junc­tion of sis­ter chro­matid to sep­a­rate dur­ing Mitosis.

Translo­ca­tion tri­somy 21 is a type of tri­somy which is found in only 2 — 4% cas­es. This con­di­tion aris­es due to translo­ca­tion of some part of chro­mo­some 21 usu­al­ly the long arm, to anoth­er chro­mo­some, most prob­a­bly chro­mo­some for 14.

The par­ent is a car­ri­er in such cas­es and is nor­mal in phys­i­cal and men­tal aspects.In such indi­vid­ual no symp­toms of down syn­drome is appears as the con­sists of all the genes despite hav­ing 45 chro­mo­somes (only one copy of chro­mo­some 21). 

How­ev­er dur­ing for­ma­tion of gametes in such a par­ent the extra part of chro­mo­some 21 is received by a gamete after meio­sis ‚which on fer­til­iza­tion give rise to the off­spring with down syndrome.

The down syn­drome (DS) phenotype:

Some 300 phe­no­type char­ac­ter­is­tics has been asso­ci­at­ed with down syndrome.A few dozen of these traits are present in most chil­dren suf­fer­ing from this dis­or­der and many of them appear at sib­ling at first glance.A child with the ages rapid­ly has an IQ range of 25 to 70 and has a flat facial pro­file and a broad sad­dle nose. The ear are small and round the tongue is large and Flat. The child has a pro­trud­ing low­er lip. A broad short skull thick short­hand with the shab­by fin­gers and thick short feet and body. Down syn­drome chil­dren usu­al­ly have light coloured specks in the iris of their eyes are referred to as brush­field spots. About 30% have an simi­an crease and 30% have heart trouble.

Her­nias and a marked sus­cep­ti­bil­i­ty to infec­tions are asso­ci­at­ed with the syn­drome. They are usu­al­ly retard­ed in phys­i­cal devel­op­ment and are short statured.

Ori­gin of down syndrome:

One of many ways in which the tri­somy con­di­tion orig­i­nate is through nondis­junc­tion of chro­mo­some 21 dur­ing meiosis.

Fail­ure of paired homo­lo­gus to dis­joint dur­ing anaphase first or sec­ond can result in male or female gametes with the n + 1 chro­mo­somes composition.

Fol­low­ing fer­til­iza­tion with a nor­mal gamete the tri­somy con­di­tion results. chro­mo­some band­ing stud­ies have shown that while the dis­tal chro­mo­somes may be derived from either moth­er or father defec­tive ovum is most often the source.

Mater­nal /paternal age and down syndrome :

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Down Syn­drome Family/Parents

Clear­ly down syn­drome more than any oth­er syn­drome shows marked rela­tion­ship to mater­nal age.

A female is born with all the oocyte site will ever pro­duce near­ly 7 mil­lion. These remain in an arrest­ed prophase l of meio­sis from birth until ovu­la­tion, gen­er­al­ly at this rate of one per men­stru­al cycle (about 28 days) after puberty.

A giv­en Oocyte may remain in a state of sus­pend­ed devel­op­ment for some 12 to 45 years or so. It has been observed that the accu­ra­cy of mei­ot­ic divi­sion decreas­es with the age of 30 the fre­quen­cy of nondis­junc­tion asso­ci­at­ed with chro­mo­some 21 increase assis­tant substantially 

For women over the age of 45 years of prob­a­bil­i­ty of pro­duc­ing a down syn­drome child is about one in 40.

High­er than nor­mal fre­quen­cies of chil­dren suf­fer­ing from down syn­drome have also been dis­cov­ered for moth­ers under the age of 17.The exact rea­son how­ev­er is not clear as yet. a con­vinc­ing the­o­ry that make explain the rel­a­tive­ly high inci­dence of down syn­drome for both old­er and younger moth­er relates the fre­quen­cy of nondis­junc­tion to oestro­gen levels.

Estro­gen help to con­trol the rate of meio­sis in pri­ma­ry oocytes. Estro­gen lev­el decreas­es with the age of show­ing of mei­ot­ic process may make nondis­junc­tion more likely.

Oestro­gen lev­els do not sta­bi­lize fol­low­ing puber­ty until about the age of 20 the down syn­drome chil­dren pro­duced by very young moth­ers thus May reflect fluc­tu­a­tion in hor­mone levels.

If age of age-relat­ed fac­tor increas­es the like­li­hood of nondis­junc­tion then tri­somies is involved in chro­mo­somes oth­er than chro­mo­some 21 should also be more com­mon in old­er and moth­er and even in younger than moth­ers. Infact , the inci­dence of tri­somy 13 and 18 is high­er in old­er women. oth­er tri­somies also be com­mon in these women but because they are Lethal they are dif­fi­cult to detect.

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