|MadSci Network: Genetics|
Dear Matt: Despite a good bit of research, I was unable to determine the exact mechanism for color determination in polar bears. However, I would suspect that, because the vast majority of polar bears are white, white coats are a dominant trait in polar bears. I believe your question about recessive genes probably stemmed from learning about albinism, which is indeed a recessive trait. However, polar bears, while white, are not albinos. True albinos lack or have very little of a pigment called melanin. Melanin is what colors hair, eyes and skin. True albinos, because they lack melanin, typically have red or blue eyes and very pale hair and skin. If you look at a polar bear, they have dark eyes and a dark nose indicating that they do indeed produce melanin. Of course, it is possible for a polar bear to be albino. An albino polar bear would have pink or blue eyes and a pink nose. Now, just because a polar bear is an albino, it does not mean that ALL its genes are recessive. Organisms, like bears, are composed of many, many genes which are located on chromosomes. This collection of genes is termed the “genome.” Each individual gene in the genome has several variants. These variants are called alleles. Some alleles are dominant and some are recessive. Every organism’s genome contains a mixture of both dominant and recessive alleles, depending on the gene in question. Remember that there are two copies of each gene in a cell because there are two copies of each chromosome in every cell. An albino has 2 copies of the recessive allele for the gene that controls melanin production and, thus, determines the color of the bear. So, while an albino polar bear has 2 recessive alleles for the melanin gene, it still has a bunch of other genes that may have 2 dominant alleles (this is called homozygous dominant), 2 recessive alleles (homozygous recessive) or 1 dominant and 1 recessive allele (heterozygous), once again, depending on the gene in question. I hope that this has answered your question. Good luck in your genetic studies! Gail Admin note: As an aside, polar bears have white fur but black skin. In fact, polar bear skin has a remarkably high melanin content to protect it from the light scattered by its white fur.
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