genetic terms

Here is a list of genetic terms you may have forgotten but may find useful in discussing certain diseases or conditions:

Term	Definition	Example/Explanation
autosome	a gene located on the 46 chromosomes that are not the sex chromosomes	Blood type
gene	that portion of the genetic code that specifies a certain trait (in actuality, a protein or an enzyme)	Blood type
dominant gene	a gene that is always expressed, that is directly detectable in the individual	Blood types A and B
recessive gene	a gene that is only expressed when two identical copies coding for the same trait are present, one from the mother, one from the father	Blood type O or Rh-negative factor
sex-linked gene	a gene that is located on one of the sex chromosomes	Blood clotting factor VIII resides on the X chromosome (see also more detailed discussion)
expression	whether a gene is detectable by examination of the individual	Blood types A and B are always expressed - if either or both genes are present, they are detected by routine blood typing
chromosome	the DNA containing structure in the cell where the genes are located	chromosomes are numbered 1-23 plus the two sex chromosomes X and Y
sex chromosome	the X and Y chromosomes that determine the sex of an individual	XX female XY male
genotype	the actual genes present at a given site on the chromosome
phenotype	how the genotype is expressed	Blood type A gene is always expressed as either type A (or AB if the B gene is also present
mutation	a change in the genetic code for a given trait - a new genotype which did not exist in either parent	25% of cases of neurofibromatosis represent new mutations
genetic disease	a disease caused by a defect in the genetic code for a given trait - a disease state caused by inheritance of a defective gene (genotype)	examples of genetic diseases include sickle cell disease and Marfan syndrome
trisomy	abnormal presence of an extra chromosome - three instead of two copies of the respective chromosome	Trisomy 21 - Down syndrome

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