Individuals with down syndrome have an extra chromosome 21. this is an example of which type of chromosome disorder?

Down syndrome is an example of NON DISJUNCTION CHROMOSOME DISORDER. Non disjunction disorder occur when the homologous chromosomes or sister chromatids fail to separate properly during cell division. This failure results in daughter cells with abnormal chromosomes. Another example of non disjunction disorder is Turner syndrome.

They include lobsters and crabs

It would be the equator