Chromosomes are individual, large DNA molecules in a cell nucleus.
They contain the genes along with structures that hold the DNA and aid it in expressing heredity by orchestrating cellular operations. Down's syndrome is the most familiar of the genetic birth defects known as chromosomal abnormalities - those that involve an abnormality in the number or form of the chromosomes. Because each chromosome contains many individual genes, any such abnormality is likely to have widespread effects on the individual who inherits it. Many chromosome duplications or mistakes in development are fatal either before birth or shortly after. Down's syndrome has relatively mild effects when compared with other chromosomal abnormalities.