What is the difference between codominance and mosaicism? Can you tell the two apart just by looking at them, or do we need to know their genotypes in order to tell them apart?
Codominance vs. mosaicism
- Thread starter m25
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When I think of a mosaic, I think of little islands scattered about. EDIT (quote from Wikpedia): "In genetics, a mosaic or mosaicism denotes the presence of two or more populations of cells with different genotypes in one individual who has developed from a single fertilized egg"
In Genetics, there are two instances I am familiar with (wikipedia gives a few more): On the one hand you have random X-inactivation in females ("lyonization") which results in one of the two X chromosomes becoming a barr body at random. This process is still poorly understood in humans, but for instance, some females have patches of cells lacking sweat glands due to a mutation on one of the X chromosomes. A more familiar example is the calico cat, which has different patches of fur color.
Another example of mosaicism is mitochondrial inheritance. Mitochondria divide independently of mitosis within each given cell and therefore, all of the mitochondria in an multicellular organism originates from the egg's cytoplasm, prior to fertilization. (The father's contribution of mitochrondria in the sperm is believed to undergo autophagy upon fusion). Individual mitochondria contain its own set of genes on extranuclear chromosomes. Some of these mitochondria might carry a mutation on a given gene. And because these mitochondria are randomly distributed in dividing cells, this too can give rise to some physiological defects due to abnormalties in ATP production.
The way I think of codominance is that on each homologous chromosome, both alleles are expressed and therefore give rise to their own independent phenotype. The classic example of this is human blood type: AB. Both the A antigen and B antigen are both present on the RBC. This is different from incomplete domimance which results in an intermediate phenotype: for instance, a red and white allele for a given flower, results in pink plowers (a color intermediate of the two homozygous phenotypes). It could just as easily refer to activity levels too: one homozygous combination of alleles for an enzyme results in high activity, the other low activity. Both alleles in a heterozygous individual might result in moderate activity, equally in between both levels. The can present this in any way to you, so as long as you realize that on a phenotypic level, incomplete dominance is an intermediate of either homozygous phenotype, you should be okay.
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