A gene is a segment of DNA that codes for a functional protein or an RNA molecule. Based on the functional activity, genes are classified into

a) House-keeping genes or Constitutive genes
b) Luxury genes or Non-constitutive genes or Specialist genes

House-keeping genes are constitutive genes that are expressed in all cells of a multi-celled organism and the protein products coded by these genes are involved in essential cellular activities.

Luxury genes are genes that are expressed only in some type of cells where its encoded proteins are needed.
luxary genes

House-Keeping Genes  vs Luxury genes

House-Keeping Genes  or
Constitutive genes
Luxury genes or Non-constitutive genes
or Specialist genes
Are functional in all types of cells.

Are functional in only certain types of cells.
Are expressed all the time or constitutively. Are expressed only when needed and remain inactive most of the time.

Needed for essential basic cellular activities. Needed only for specific cellular activities.

Expression is not regulated as it is required all the time. Expression is regulated by regulatory proteins, hormones etc as it is required only under certain conditions.

Genes coded for enzymes of glycolysis are active in all cell types 24X7through out the life as the reaction is essential for cell survival. Often tissue specific or express only in certain types of cells. Ovabumin gene in oviduct epithelial cells, gene for antibodies only in plasma cells, Hb gene in RBCs.
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