Principles of Biochemistry, Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transfer
DNA: Structure of Chromosomes and Genes
Some eukaryotic genes are present in multiple copies

Some genes are present in The Cell in A large number of copies, at least during certain Stages of the lifecycle. The most striking example is the set of genes encoding the four rRNAs, which occurs in multiple copies in a normal somatic cell. Amphibian oocytes are capable of further amplifying the number of genes for three of the four rRNAs. This is a vital necessity, as these Cells undergo very rapid growth and division following Fertilization, during which they require a vast number of Ribosomes to synthesize all cellular Proteins. Genes encoding Histones are also present in large quantities—up to 1,000 copies in several eukaryotic species. Early-stage embryos must synthesize histones very rapidly during their accelerated growth. Similarly, genes encoding Keratins, the structural proteins of feathers, are found in multiple copies in the chicken genome.

One might expect that genes encoding other proteins abundantly expressed in various eukaryotic Cells and Tissues (such as Hemoglobin, serum albumin, Collagen, or Ovalbumin) would also be present in multiple copies. However, this has been shown not to be universally true. The majority of eukaryotic genes exist as single copies or in very low copy numbers.