MadSci Network: Genetics

Re: dont understand how meiosis creates..... see below

Date: Sat Feb 2 14:32:55 2002
Posted By: Michael S. Robeson II, Core Nucleic Acid Analysis Manager
Area of science: Genetics
ID: 1006567460.Ge

Well, in most sexually reproducing organisms there are 2 copies of each 
gene. These alternate copies or forms of a gene are called alleles. There 
can be many alleles for a particular gene (say eye color for instance), 
but since we (humans) can only have 2 copies of a gene there has to be a 
way for the variety of other alleles to exist. 

This depends of the genes your parents have, which in turn is affected by 
meiosis - this effects the probability that you (or the offspring) will 
inherit certain alleles. Say that your mother has brown eyes and your dad 
also has brown eyes. Also, lets say that blue eyes are recessive to brown. 
For arguments sake we will assume your mom and dad are both carriers for 
the blue eye gene which we would represent as: Bb (B = Brown; b = Blue).

Another gene must be considered for us to appretiate the role of mieosis. 
We will use hair color. Lets assume both parents are also heterozygous for 
brown hair Hh (H = brown and h = blond) 

 Let's calculate the combinations of alleles that mom can pass 
on. Since, she is a BbHh genotype we will have many possible combinations 
that she can contribute to her offspring (remember she only donate 1/2 of 
her genes (chromosomes copies) to her offspring - the father donates the 
other half). 

So her contribution possibilities are Bb X Hh =
BH, Bh, bH, bh : these are all of the possible combinations that can come 
from these separate genes. Since the father (in this case) has the same 
genotype as the mother he too has the possibility of passing on the same 
four combinations (BH, Bh, bH, bh).

Thus if we cross the possible unions of these combinations together we 
will get the following (shows all possible gene combinations from each 

(Egg)(BH, Bh, bH, bh) X (Sperm)(BH, Bh, bH, bh) = 

BHBh   bHbh
BhBH   bhbH
BhBh   bhbh

Again, we calculate all possible combinations because during meiosis the 
paired chromosomes will somewhat randomly swap segments of DNA between 
homologous chromosomes. Then we have to take into account that half (or 
one copy) of the parents genes that will be passed down - the other half 
being donated from the other parent.

Thus if both your parents had Brown hair and Brown eye (and were carriers 
of other alleles such as blue eyes and blond hair)you could still end up 
looking nothing like your parents (i.e. you may have blond hair and brown 
eyes, blond hair and blue eyes, brown hair and blue eyes or any of the 
combinations that you see above. Note since brown hair and brown eyes are 
domminant (in this case)there is a higher chance that you will look like 
your parents (again, in this case).

So, meiosis creats variability to a very large extent. It allows for 
whole new gene combinations to exist in a short amount of evolutionary 

Genetics: Principles and Analysis, 4th Ed. Daniel L. Hartl & Elizabeth W. 
Jones. Jones and Bartlett Publishers Sudbury, 1998

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