Re: how is a punnett square a model for what happens during a meiosis

Hello Melissa,

The punnett square can be used to figure out the possible outcomes of a mating.  
The single letters (in the top row and left column in the example below) represent 
the possible gametes from each parent, and so represent the possible products of 
meiosis. 

Punnett square for the cross Bb X Bb 

     B       b

B    BB      Bb
     
b    Bb      bb

If this square isn’t lining up right, change to a fixed width font, such as courier or 
Monaco.

In this case, let’s say B/b represents a gene that determines eye color.  “B” is 
dominant and results in brown eyes, while “b” is recessive, and results in blue eyes 
if the individual is bb (homozygous).  Since people are diploid (ie they carry two 
copies of the genome)  everybody carries two copies of this gene.  In the example 
above, both parents carry one recessive and one dominant copy of the gene.  
When they have kids, each parent can only donate one copy to the offspring, either 
B or b.  This is because gametes (sperm or egg) are haploid.  That is, they carry 
only one copy of the genome, so they can only carry one copy of each gene.  How 
does a gamete wind up being haploid?  
     During meiosis, a single diploid cell duplicates its genome once, but goes 
through cell division twice.  So, if you start with two copies of a gene (B and b) and 
go through one round of genome duplication, you have a total of four copies of the 
genome, and so four copies of that gene (B, B, b, b).  If you start with one cell, and 
go through two rounds of cell division, you wind up with four cells.  In other words, 
one cell divides to give two cells, then those two cells divide to give a total of four 
cells.  Altogether then, you have four copies of the genome, and four cells.  Each 
cell has one copy of the genome, and so one copy of the gene in question, either B 
or b.  

Alex