In humans,which disorder is caused due to the absence of one of the $X$ chromosomes,i.e.,$45$ with $XO$?

Philadelphia chromosome is

What is Down's syndrome? Give its symptoms and cause. Why is it that the chances of having a child with Down's syndrome increase if the age of the mother exceeds forty years?

Gynaecomastia is the symptom of

Down syndrome is caused by an extra copy of chromosome $21$. If an affected mother and a normal father produce offspring,what percentage of the offspring will be affected by this disorder (in $\%$)?

When the chromosome number of a given organism has one additional chromosome in one of the homologous pairs,the addition is known as