Appendix C: 

 Glossary 



Linkage map: A map of the relative positions of genetic loci on a chromosome, 

 determined on the basis of how often the loci are inherited together. Distance is 

 measured in centimorgans. 



Locus (plural: loci): The position on a chromosome of a gene or other chromosome 

 marker; also, the DNA at that position. Some restrict use of locus to regions of 

 DNA that are expressed. See iicne expiTssion. 



Mapping: See liene nuippiiii;. Ilnkas^c map. physical map. 



Marker: An identifiable physical location on a chromosome (e.g.. restriction 



enzyme cutting site, gene) whose inheritance can be monitored. Markers can be 

 expressed regions of DNA (genes) or some segment of DNA with no known coding 

 function but whose pattern of inheritance can be determined. See RFLP, restriction 

 frai>mi'ni length polymorphism. 



Meiosis: The process of two consecutive cell divisions in the diploid progenitors of sex 

 cells. Meiosis results in four rather than two daughter cells, each with a haplold set 

 of chromosomes. 



Messenger RNA, mRNA: RNA that serves as a template for protein synthesis. See 

 genetic code. 



Multifactorial or multigenic disorders: See polygenic disorders. 



Mutation: Any heritable change in DNA sequence. Compare polymorphism. 



Nucleotide: A subunit of DNA or RNA consisting of a nitrogenous base (adenine, 

 guanine, thymine, or cytosine in DNA: adenine, guanine, uracil, or cytosine in 

 RNA), a phosphate molecule, and a sugar molecule (deoxyribose in DNA and 

 ribose in RNA). Thousands of nucleotides are linked to form a DNA or RNA 

 molecule. See DNA. base pair, RNA. 



Nucleus: The cellular organelle in eukaryotes that contains the genetic material. 



Oncogene: A gene, one or more forms of which is associated with cancer. Many 



oncogenes are involved, directly or indirectly, in controlling the rate of cell growth. 



Phage: A virus for which the natural host is a bacterial cell. 



Physical map: A map of the locations of identifiable landmarks on DNA (e.g., 



restriction enzyme cutting sites, genes), regardless of inheritance. Distance is 

 measured in base pairs. For the human genome, the lowest-resolution physical 

 map is the banding pattems on the 24 different chromosomes: the highest- 

 resolution map would be the complete nucleotide sequence of the chromosomes. 



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