Biology majors: Question about identical twins

[Qadgop_the_Mercotan]

*Originally posted by Anomalocaris *
**I fail to understand why “polar twins,” ie. an egg splitting prior to being fertilized by seperate sperm, would be any different from fraternal twins, where two seperate eggs are fertilized by two seperate sperm. All eggs in the mother are geneticly identical, so a ‘split’ unfertilized egg (assuming each egg contains haploid DNA) would be no different than two seperately released eggs. The genetic recombination occurs only after fertilization. **

lemur corrects your misconceptions very nicely. I just want to add that the process of making the gametes (eggs and sperm) in the first place is called meiosis, & it splits and shuffles the chromosmes quite well. Mitosis is the process in which somatic cells divide.

[Anomalocaris]

Thanks for the clarification, lemur. I forgot about linkage and crossing over during the diploid phases Perhaps that is why my cloning industry has yet to adequately produce-
…oops, I said too much. Gotta go…

[shellity]

My two cents… My sister and I look identical, had one placenta present at birth, and have had a gazillion blood tests that suggest we are identical, however my sister is colour-blind and I am not. It has been suggested that one of us went through some kind of mutation whilst still a cluster of cells, hence the difference. Indeed, my mother and brother are colour-blind, which indicates that my father carries the gene (since it’s recessive on the X chromosome, right?), which all suggests that I should have turned out colour-blind. Does that all make sense?

[yo_han_go]

My nieces looks like good candidates for polar twins. They are 8 now and belived to be fraternal twins. Although I can’t stand them apart. Both looks the same to me, except one is left handed and other one is right handed.

[Smeghead]

*Originally posted by shellity *
**My two cents… My sister and I look identical, had one placenta present at birth, and have had a gazillion blood tests that suggest we are identical, however my sister is colour-blind and I am not. It has been suggested that one of us went through some kind of mutation whilst still a cluster of cells, hence the difference. Indeed, my mother and brother are colour-blind, which indicates that my father carries the gene (since it’s recessive on the X chromosome, right?), which all suggests that I should have turned out colour-blind. Does that all make sense? **

Um…sorta. Your brother would have gotten his Y chromosome from his father, and his X chromosome from his mother. So the fact that he’s color-blind indicates only that your mother is at least a carrier. Since she is, in fact, colorblind, that means she has two copies of the gene (assuming we’re talking about X-linked red/green colorblindness, and not some other type).

Now, if your sister is colorblind, that means she has two copies of the gene as well, which means that she got a bad X from your father. However, since you’re not colorblind, that would indicate that you got a good X from your father. Since your father only has one X to give, you’re right - there probably was a new mutation somewhere along the line. Tell your parents to have another girl, so you can figure out which one is the mutant.