What doesn't make sense here? Questions, comments, and suggestions are welcome.
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Alleles: The wild type and all variant forms of a single gene. All have the same location (locus) in the genome.
Codominant: Two alleles which manifest one phenotype when one allele is homozygous, a second phenotype when the second allele is homozygous, and a third phenotype when the two alleles are heterozygous. In particular, a mutant allele that is codominant to the wild type allele. See "dominance comparisons".
Dominant: An allele which fully manifests its phenotype when either homozygous or heterozygous. In particular, a mutant allele that is dominant to the wild type allele. See "dominance comparisons".
Heterozygous (slang: het): Having two different alleles in a gene pair. The two alleles may be different mutants. Or the two alleles may be a wild type allele paired with a recessive mutant or with a dominant mutant or with a codominant mutant.
Homozygous: Having two identical alleles in a gene pair (both genes are the same). The genes may be two wild type alleles, two identical dominant alleles, two identical codominant alleles, or two identical recessive alleles.
Mutant: 1. Any phenotype that differs from the wild type phenotype. 2. Any allele that differs from the wild type allele.
Recessive: An allele which manifests its phenotype only when homozygous and does not manifest its phenotype when heterozygous. In particular, a mutant allele that is recessive to the wild type allele. See "dominance comparisons".
Wild type: 1. The most common phenotype in the wild population, AKA the normal phenotype. 2. The standard or normal allele for each gene location (locus) in the genome. When every locus is homozygous wild type, the organism displays the wild type phenotype. 3. The baseline against which mutant alleles are compared to determine whether the mutant is a dominant, codominant, or recessive.
Dominance comparisons: In genetics, traditionally the comparisons have been dominant allele vs. recessive allele and codominant allele vs. codominant allele. These are still customarily used in introductory genetics text books.
Professional geneticists have added the wild type allele as the standard of comparison. This changes the traditional two comparisons to three: dominant mutant allele vs. wild type allele, recessive mutant allele vs. wild type allele, and codominant mutant allele vs. wild type allele.
Gregor Mendel coined the terms "dominant" and "recessive". Their meanings have remained stable.
Researchers have repeatedly discovered mutant genes that do not fit Mendel's dominant vs. recessive pattern. Various people have coined a number of terms for such mutants. While each term may have its own shade of meaning, in all cases the heterozygote (heterozygous individual) can be reliably distinguished from the two homozygotes (homozygous individuals). This makes it advantageous to lump all such cases together under the shortest term, "codominant".
Nature can be sloppy. When a mutant gene does not exactly fit the textbook definitions, we pick the category it best fits and often add that the mutant shows "variable expressivity" or "incomplete penetrance". See a good genetics text for their definitions.
When a list of alleles contains only two alleles, one is always the wild type allele. The mutant allele is either dominant, recessive, or codominant to the wild type allele. But an allele list is not limited to two alleles. A case of multiple alleles (the wild type allele and two or more mutant alleles) turned up in the black rat snake (Elaphe obsoleta obsoleta). In this case, both tyrosinase-positive albino and xanthic are recessive mutants because both are recessive to the wild type allele. However, xanthic is codominant to tyrosinase positive albino (and vice-versa) because the xanthic//tyrosinase positive albino heterozygote is intermediate in color between the two homozygotes. Nevertheless, the two mutants remain recessives because the comparison with wild type is the most important one.
<TABLE WIDTH="100%" BORDER="1" CELLSPACING="2" CELLPADDING="2">
<TR>
<TD colspan="4" ALIGN="center">PHENOTYPES OF<BR>DOMINANT, RECESSIVE, AND CODOMINANT MUTANTS</TD>
</TR>
<TR>
<TD WIDTH="25%"></TD>
<TD WIDTH="25%" ALIGN="center">Homozygous Mutant<BR>(Two Identical Mutant Genes)</TD>
<TD WIDTH="25%" ALIGN="center">Heterozygous Mutant<BR>(One Mutant and One Wild Type Gene)</TD>
<TD WIDTH="25%" ALIGN="center">Homozygous Normal<BR>(Two Identical Wild Type Genes)</TD>
</TR>
<TR>
<TD WIDTH="25%" ALIGN="center"> Dominant Mutant</TD>
<TD colspan="2" ALIGN="center"> Full Expression of Mutant Phenotype</TD>
<TD WIDTH="25%" ALIGN="center"> Wild Type or Normal Phenotype</TD>
</TR>
<TR>
<TD WIDTH="25%" ALIGN="center"> Recessive Mutant</TD>
<TD WIDTH="25%" ALIGN="center"> Full Expression of Mutant Phenotype</TD>
<TD colspan="2" ALIGN="center"> Wild Type or Normal Phenotype</TD>
</TR>
<TR>
<TD WIDTH="25%" ALIGN="center"> Codominant Mutant</TD>
<TD WIDTH="25%" ALIGN="center"> Full Expression of Mutant Phenotype</TD>
<TD WIDTH="25%" ALIGN="center"> Like Neither Homozygote*</TD>
<TD WIDTH="25%" ALIGN="center"> Wild Type or Normal Phenotype</TD>
</TR>
</TABLE>
*Depending on the test, the codominant heterozygote's phenotype can quite variable. It may be approximately intermediate between the phenotypes of the two homozygotes. For example, The heterozygous tiger or tiger reticulated python is roughly intermediate in appearance between the normal reticulated python and the homozygous tiger or super tiger reticulated python. Or if the test is sensitive enough, the heterozygote may manifest the phenotypes of both homozygotes. For example, type A human blood cells (homozygous for type A, I<SUP>A</SUP>//I<SUP>A</SUP> genotype) form large, visible clumps only when exposed to antibody A. Type B human blood cells (homozygous for type B, I<SUP>B</SUP>//I<SUP>B</SUP> genotype) clump only when exposed to antibody B. And type AB human blood cells (heterozygous for A and B, I<SUP>A</SUP>//I<SUP>B</SUP> genotype) clump when exposed to either antibody A or antibody B.
---------------------------------------------------------
Alleles: The wild type and all variant forms of a single gene. All have the same location (locus) in the genome.
Codominant: Two alleles which manifest one phenotype when one allele is homozygous, a second phenotype when the second allele is homozygous, and a third phenotype when the two alleles are heterozygous. In particular, a mutant allele that is codominant to the wild type allele. See "dominance comparisons".
Dominant: An allele which fully manifests its phenotype when either homozygous or heterozygous. In particular, a mutant allele that is dominant to the wild type allele. See "dominance comparisons".
Heterozygous (slang: het): Having two different alleles in a gene pair. The two alleles may be different mutants. Or the two alleles may be a wild type allele paired with a recessive mutant or with a dominant mutant or with a codominant mutant.
Homozygous: Having two identical alleles in a gene pair (both genes are the same). The genes may be two wild type alleles, two identical dominant alleles, two identical codominant alleles, or two identical recessive alleles.
Mutant: 1. Any phenotype that differs from the wild type phenotype. 2. Any allele that differs from the wild type allele.
Recessive: An allele which manifests its phenotype only when homozygous and does not manifest its phenotype when heterozygous. In particular, a mutant allele that is recessive to the wild type allele. See "dominance comparisons".
Wild type: 1. The most common phenotype in the wild population, AKA the normal phenotype. 2. The standard or normal allele for each gene location (locus) in the genome. When every locus is homozygous wild type, the organism displays the wild type phenotype. 3. The baseline against which mutant alleles are compared to determine whether the mutant is a dominant, codominant, or recessive.
Dominance comparisons: In genetics, traditionally the comparisons have been dominant allele vs. recessive allele and codominant allele vs. codominant allele. These are still customarily used in introductory genetics text books.
Professional geneticists have added the wild type allele as the standard of comparison. This changes the traditional two comparisons to three: dominant mutant allele vs. wild type allele, recessive mutant allele vs. wild type allele, and codominant mutant allele vs. wild type allele.
Gregor Mendel coined the terms "dominant" and "recessive". Their meanings have remained stable.
Researchers have repeatedly discovered mutant genes that do not fit Mendel's dominant vs. recessive pattern. Various people have coined a number of terms for such mutants. While each term may have its own shade of meaning, in all cases the heterozygote (heterozygous individual) can be reliably distinguished from the two homozygotes (homozygous individuals). This makes it advantageous to lump all such cases together under the shortest term, "codominant".
Nature can be sloppy. When a mutant gene does not exactly fit the textbook definitions, we pick the category it best fits and often add that the mutant shows "variable expressivity" or "incomplete penetrance". See a good genetics text for their definitions.
When a list of alleles contains only two alleles, one is always the wild type allele. The mutant allele is either dominant, recessive, or codominant to the wild type allele. But an allele list is not limited to two alleles. A case of multiple alleles (the wild type allele and two or more mutant alleles) turned up in the black rat snake (Elaphe obsoleta obsoleta). In this case, both tyrosinase-positive albino and xanthic are recessive mutants because both are recessive to the wild type allele. However, xanthic is codominant to tyrosinase positive albino (and vice-versa) because the xanthic//tyrosinase positive albino heterozygote is intermediate in color between the two homozygotes. Nevertheless, the two mutants remain recessives because the comparison with wild type is the most important one.
<TABLE WIDTH="100%" BORDER="1" CELLSPACING="2" CELLPADDING="2">
<TR>
<TD colspan="4" ALIGN="center">PHENOTYPES OF<BR>DOMINANT, RECESSIVE, AND CODOMINANT MUTANTS</TD>
</TR>
<TR>
<TD WIDTH="25%"></TD>
<TD WIDTH="25%" ALIGN="center">Homozygous Mutant<BR>(Two Identical Mutant Genes)</TD>
<TD WIDTH="25%" ALIGN="center">Heterozygous Mutant<BR>(One Mutant and One Wild Type Gene)</TD>
<TD WIDTH="25%" ALIGN="center">Homozygous Normal<BR>(Two Identical Wild Type Genes)</TD>
</TR>
<TR>
<TD WIDTH="25%" ALIGN="center"> Dominant Mutant</TD>
<TD colspan="2" ALIGN="center"> Full Expression of Mutant Phenotype</TD>
<TD WIDTH="25%" ALIGN="center"> Wild Type or Normal Phenotype</TD>
</TR>
<TR>
<TD WIDTH="25%" ALIGN="center"> Recessive Mutant</TD>
<TD WIDTH="25%" ALIGN="center"> Full Expression of Mutant Phenotype</TD>
<TD colspan="2" ALIGN="center"> Wild Type or Normal Phenotype</TD>
</TR>
<TR>
<TD WIDTH="25%" ALIGN="center"> Codominant Mutant</TD>
<TD WIDTH="25%" ALIGN="center"> Full Expression of Mutant Phenotype</TD>
<TD WIDTH="25%" ALIGN="center"> Like Neither Homozygote*</TD>
<TD WIDTH="25%" ALIGN="center"> Wild Type or Normal Phenotype</TD>
</TR>
</TABLE>
*Depending on the test, the codominant heterozygote's phenotype can quite variable. It may be approximately intermediate between the phenotypes of the two homozygotes. For example, The heterozygous tiger or tiger reticulated python is roughly intermediate in appearance between the normal reticulated python and the homozygous tiger or super tiger reticulated python. Or if the test is sensitive enough, the heterozygote may manifest the phenotypes of both homozygotes. For example, type A human blood cells (homozygous for type A, I<SUP>A</SUP>//I<SUP>A</SUP> genotype) form large, visible clumps only when exposed to antibody A. Type B human blood cells (homozygous for type B, I<SUP>B</SUP>//I<SUP>B</SUP> genotype) clump only when exposed to antibody B. And type AB human blood cells (heterozygous for A and B, I<SUP>A</SUP>//I<SUP>B</SUP> genotype) clump when exposed to either antibody A or antibody B.
