Inversion
A chromosomal mutation involving the removal of a chromosome segment, its rotation through 180 degrees, and its reinsertion in the same location. The replacement of a section of a chromosome in the reverse orientation.
2. A chromosomal aberration in which a portion of a chromosome has been reversed relative to the rest of the chromosome.

Isochromosome
A chromosome with two genetically and morphologically identical arms.

Isoenzyme
An enzyme performing the same function as another enzyme but having a different set of amino acids. The two enzymes may function at different speeds.

Isotope
An atom that shares the same atomic number and position as other atoms in an element but has a different number of neutrons and thus a different atomic mass.

IQ
An abbreviation of “intelligence quotient”, usually defined as the mental age an individual (as measured by standardized tests) divided by his or her real age and multiplied by 100. This formulation establishes the average IQ as 100. the usefulness and reliability of IQ as a measure of intelligence has been questioned, inpart because of the difficulty of devising standardized tests that are free of cultural biases.