Glossary 
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Gene: A functional unit of heredity that is a segment of DNA located 
in a specific site on a chromosome. A gene directs the formation of an 
enzyme or other protein. (NIH) 
Genome: The total gene complement of a set of chromosomes. (SMD) 
Germ cells (or primordial germ cells): A gamete, that is, a sperm or 
egg, OR a primordial cell that can mature into a sperm or egg. (NRG) 
Germ layers: The three initial tissue layers arising in the embryo — 
endoderm, mesoderm, and ectoderm — firom which all other somatic 
tissue-types develop. (NRG) 
Gonad: An organ that produces sex cells (testes or ovaries). (SMD) 
Gonadal ridges: Embryonic structures arising in humans at about 
five weeks, eventually developing into gonads (either testes or ova- 
ries). 
Green Fluorescent Protein: A protein naturally occurring in some 
animals including jelly fish that spontaneously fluoresces. It can be 
used as a noninvasive marker in living cells by attaching it to differ- 
ent proteins and then letting it fluoresce so as to track the cell. 
Haploid human cell: A cell such as an egg or sperm that contains 
only twenty-three chromosomes. (OR) 
Hematopoietic stem cell: A stem cell from which all red and white 
blood cells develop. (NIH) 
Hepatocyte: Liver cell. 
Histocompatible: The immunological characteristic of cells or tissue 
that causes them to be tolerated by another cell or tissue; that allows 
some tissues to be grafted effectively to others. (NRG) 
Hurler syndrome: A heritable condition involving deficiency of an 
enzyme (alpha-L-iduronidase), leading to abnormal accumulations of 
materials inside cells, then resulting in abnormal development of car- 
tilage and bone and other systems. (SMD) 
ICM cells: Cells fi:om the inner cell mass, a population of cells inside 
the blastula that give rise to the body of the new organism rather 
than to the chorion or other supporting structures. 
PRE -PUBLICATION VERSION 
