so maybe I am cheating since I have had elementary statistic which includes probability theory. This genetics problem set is not too dissimilar to probablity theory ( tossing coins and dice and stuff.)
On the other hand, there are a lot more difficult problems to solve in this arena that are not in the problem set --- like traits that are on the same chromosome near each other and therefore are not completely independently selected.
DOMINANCE OF GENES ON the X chromosone ???!!!
I do have a question on sex-linked attributes on the X chromosome. Take an individual who is heterozygous for a trait that is on a autosomal chromosome. If the phenotype of that individual is an expression of one gene on one of the chromosomes, we say that gene is dominate. In this case every cell in the body has two chromosome Dd, the chromosome D is expressed. This is our definiton of dominance.
OK. But take a sex linked trait like orange vs. black cat. Our problem set says that yellow is dominate over black. I do NOT think it is technically correct to say one color is dominant over the other when the color is on the X chromosome. A male cat only has 1 X. Therefore there is no question of which trait (which chromosome ) will be expressed in the phenotype. If the X has the yellow (orange) gene, the male will be orange. It it has the black gene, it will be black. There is no dominance in this relationship. Whatever gene is on that X is going to be expressed.
So what of the female of the species. She has two X's. This is similar to the autosomal complement. If she is heterozygous for orange/black and if you say that orange is dominant, then you would expect an orange cat. This is not the case. In order for females to not have more active genetic material than males of the species, one of the X's is permanently inactivated in a Barr body sometime in the developmental stage of the organism. Thus for every cell in the female cat one of the X's is permanently disabled. Therefore, there is still only 1 functional X chromosome per cell just like male cat cells. That is where you get the calico coloring in cats. Some patches of cells on the cat have an active orange gene X and some have an active black gene X. Therefore there is really no chance for dominance to occur because there is only 1 active orange/black gene per cell.
The red/green color-blind X-based seems like the same problem. some of the cones in a heterozygous woman would be color-blind and some of the cones would not be color-blind, depending on which X is disabled. On the other-hand, it may be ok in this instance to say that non-color blind attribute is dominate over color blind. It is not true at the celluar level, because one X is still permanently disabled. However, according to this research, some of the heterozygous women with a mixture of color-blind vs non-color blind cones actually have better color recognition than other people. (http://aris.ss.uci.edu/cogsci/personnel/kjameson/assess.pdf ). This one surprised me, because I thought I had heard that women that were heterozygous for red/green color blindness had less color sensitive due to the mixture of color-blind and non-color blind cones.
No comments:
Post a Comment