Question

why does a third copy of chromosome 21 result in the complex pattern of developmental and physiological abnormalities associated this this syndrome. How does this extra copy - or dose - cause these defects/differences?

Answer 1

The third copy of chromosome cause 21 trisomy causes downs syndrome.it is charecterized by presence of third copy of 21 chromosome.the occurence generally increase s when mothers age is more than 35 years of age.The children generally have mental impairment, stunted growth,umbilical hernia ,narrow roof mouth,cleft palate,flat head , abnormal hairs,slanted eyes,poor muscle tone,flat nasal bridge,large tongue and single crease of palm .growth is slower, glaucoma.Humans generally have 46 chromosome s into cell which can be divided into 23 pairs.Two copy of chromosome 21 one copy from each parent form one of the pair.trisomy increase s the 21 number chromosome which affects the chromosomal harmony so many complex pattern of developmental and physiological abnormalitie s are present.in many downs syndrome this 21st trisomy is due to 1)translocation where part oh chromosomes s here total full copy number of chromosome remains 46 but an additional full partial copy of chromosome 21 attatches to one of the chromosome.

2)Moscaicism-when there is two type of cell.some contain 46 and some contain 47.47 contains extra chromosome.

3)non disjunction.cosists of three copies of chromosome instead of usual 2 copies of chromosome.as embryo developd the extra chromosome is replicated in each cell .

All this brings changes within nitrogenous base which cause alterations in chromosome s