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Name: Robyn
Status: Other
Grade:  Other
Location: CA
Country: United States
Date: February 2006

In studying Mendelian inheritance, I understand dominance and recessiveness by figuring out gametes and using the Punnett square. I was curious,though,what exactly happens at the chromosome that gives the dominance in a heterozygous. Simply put, how does the body know to phenotypically express the dominant allele (beyond statistics)?

Good question! It really depends on the trait. We really should change our definition of dominance to the allele whose phenotype is "seen". Remember that genes code for proteins. It's really about the way the proteins coded for by each allele and the way they interact. Also remember that you inherit a version from your mom and one from your dad. Usually, both are made. If we are talking about sickle cell trait for instance, both versions of the hemoglobin are made and your blood will have a combination of the two. If we are talking about flower color in snapdragons (classic case of incomplete dominance), in order to get red snapdragons both chromosomes have to make the pigment.

The recessive allele is really one that is making no color because the directions for making the protein are too flawed by mutation. So when one makes color and the other doesn't, you get pink, which is really half red. If neither chromosome produces color, the flower is white. Sometimes one version of the protein is defective and interferes with the functioning of the normal version. This is what happens in Huntington disease, therefore we say it is a dominant disorder. So to make a long story short, the old definition of dominant is quite simplified.


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