MadSci Network: General Biology Query:

### Re: odds of having two children exactly the same

Date: Thu Mar 26 12:21:19 1998
Posted By: Christopher Carlson, Grad student Genetics
Area of science: General Biology
ID: 888292472.Gb
Message:
```
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

```

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