• One allele (designated 1) codes for an active protein. The other allele codes for an inactive protein. Thus, individuals with genotype (1,1) have the greatest amount (or dose) of active protein and the associated phenotype at the organismal level. Individuals who are (2,2) have little or no measurable protein activity, and this is reflected in the phenotype. Heterozygous individuals (1,2) have an intermediate level of protein activity and a phenotype that is also intermediate. For example, in the case of achondroplasia, (1,1) individuals would have two alleles for a growth receptor and a phenotype of normal stature; (2,2) individuals would have few or no functional receptors and suffer from severe growth retardation; and heterozygotes (1,2) would have half as much growth receptor activity as the (1,1) individuals and consequently be short-statured achondroplastic dwarves without the additional health problems of the (2,2) individuals. This example of codominance is admittedly simplified, as students do not study the systemic effects of achondroplasia. However, this model is applied widely in genetics and sometimes referred to as the “dosage” model.

• Both alleles code for active proteins, giving rise to observable phenotypes at the macroscopic level. Heterozygotes display the phenotypes associated with both alleles. For example, in human blood types, individuals carrying alleles for protein A and protein B have both of these proteins on their blood cells. The phenotype is not blended or dosage dependent as in the achondroplasia example above. Instead, both proteins are detected intact in heterozygous individuals.


Cartier, 2000a, 2000b.


White and Frederiksen, 1998, p. 25.


Cartier 2000a, 2000b.


Mayr, 1982, p. 481.


Kitcher, 1993, pp. 20-21.


Richards, 1992, p. 23.


O’Hara, 1988.


Mayr, 1997, p. 64.


Bishop and Anderson, 1990; Demastes et al., 1992, 1995, 1996.


Bishop and Anderson, 1990.


Cartier, 2000a, 2000b; Passmore and Stewart, 2002.


Cartier, 2000b; Passmore and Stewart, 2000.


Cartier, 2000a.


Klayman and Ha, 1987.


Cartier, 2000a.


National Research Council, 1996.


National Research Council, 1996, p. 105.


Bishop, B.A., and Anderson, C.W. (1990). Student conceptions of natural selection and its role in evolution. Journal of Research in Science Teaching, 27(5), 415-427.

The National Academies of Sciences, Engineering, and Medicine
500 Fifth St. N.W. | Washington, D.C. 20001

Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement