Re: odds of having two children exactly the same

	Let me see if I follow how you came up with your answer.  There are 23 
pairs of chromosomes in each parent, and only one of each pair is 
transmitted to each kid.  So the probability of transmitting the same 46 
chromosomes from 46 pairs is 1 in 2^46, assuming equal transmission 
probabilities for each chromosome in each pair.

	Well, technically you're wrong due to biology not statistics.  For each 
pair of chromosomes in a parent, it is rather likely that the transmitted 
chromosome is not identical to either of the parental chromosomes, but 
rather is a hybrid of the two parental chromosomes.  This is due to a 
biological phenomenon called recombination, where genetic information is 
traded between the parental chromosomes during meiosis.  The probability of 
a recombination event is ~1 for each chromosome pair in each meiosis.  The 
likelihood of recombination in "exactly" the same spot (such that the 
recombinant chromatids in 2 kids are the same) is virtually nil.  In order 
for such an event to occur, the crossovers would have to take place within 
about 1500 bp of each other (the average distance between polymorphisms) on 
a 150 million bp chromosome.  Take this likelihood and figure 23 pairs of 
chromosomes crossing over once in each parent, and the probability of 
genetically identical children from a given pair of parents is considerably 
smaller than 1 in 2^46.  

	Chris Carlson
	peterpan@leland.stanford.edu