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Terms of Heredity - Genetics Definitions

These "terms of heredity" are provided to help people use the Advanced Genetics Wizard.
Since there seems to be some confusion about codominance, I've provided references
to published, scientific material to back up my use of the term.

Basic definitions of some common genetics terms are at the bottom of this page.


Published References About Codominance

     "In his law of dominance, Mendel did not accommodate different degrees of dominance. As such examples were discovered (Bateson 1913), various new terms were introduced. Within and between textbooks of genetics, definitions are inconsistent. Various names have been used: partial dominance, incomplete dominance, codominance, lack or absence of dominance, intermediate dominance, imperfect dominance, egalitarian dominance, and transdominance. The definitions vary from text to text and depend on interpretation of allelic function, although an alleleís function is seldom known and often must be assumed. In all these usages there is one consistent aspect; each genotype has a distinguishable phenotype, and the genotype may be inferred from the phenotype. Perhaps none of the terms that have been used are all-inclusive, but some such term is desirable for teaching purposes. We have chosen the term codominance as simplest, shortest, and adequately inclusive. One can still use specialized sub-definitions for well-analyzed cases. In our more that 20 years of teaching this method has worked well."
BioScience Vol. 45 No. 2 Feb. 1995 pp. 98-104
Three neglected advances in classical genetics.
By Wilmer J. Miller and Willard F. Hollander



     "The definitions given or inferred for the intermediate form of dominance usually require knowing the exact function of the gene. This is very seldom known for most characters at present. All the "codominant" usages actually show or imply 3 phenotypes for 2 codominant alleles for the monohybrid genotypes in diploids. Why not use this consistency among names and their usages as the single criterion and use only one name? I prefer the shortest, codominance."
     "So, for beginning students, as well as teachers and researchers, all genes fit one of  3 categories: dominant, codominant, or recessive. When more sophisticated nuances implying different functions are desired, then other names can be used with a definition (please) for this next level of usage."
Brazilian Journal of Genetics
20, 4, 663-665 (1997)
By Wilmer J. Miller

Both of the quotes above were reprinted here with the permission of
Dr. Wilmer J. Miller, Professor of Genetics, Emeritus.



Basic Genetics Definitions

Allele - either of the two paired genes affecting an inherited trait (one from the father, one from the mother).
Codominant - an allele that causes the homozygous form to look different than wild type and the heterozygous form to have traits of both. (All three look different from each other.)
Dominant - an allele that causes the homozygous form and the heterozygous form to look the same as each other, but different than wild type.
Recessive - an allele that affects a animal's appearance if it's present in the homozygous state only.† An animal that's heterozygous for a mutant, recessive gene looks wild type, but that gene can be passed on to offspring.
Heterozygous - having two different alleles for a genetic trait.
Homozygous - having identical alleles for a genetic trait.
Wild Type - the way an animal looks with the greatest frequency in a wild population ("normal").


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