Re: Are allele combinations determined by chance?

Hi Maria

Thank you for your question.  This is a tough topic and a lot of 
students have a hard time understanding genetic 
recombination.  Your question relates to two events that occur in 
meiosis:  independent assortment and crossing over.  By the way, 
diagrams can really help with this concept, so I encourage you to 
consult your Human Genetics text and confirm what you're reading.  
Here are two websites, which might also help:
 http://www.hysa.org/~jkimball/BiologyPages/M/
Meiosis.html
(particularly the section on genetic recombination)
 http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/
main.html

Basically, the combination of alleles one inherits is determined by 
the meiotic events that occurred when the particular sperm and egg 
that combined to make an individual were being created during 
meiosis.  Gametes are haploid--they contain only one set of 
chromosomes (n=23 in the case of humans).  To become haploid, 
the diploid number of chromosomes is reduced by half in the first 
meiotic division.  In this division, homologous, replicated 
chromosomes pair up and are separated from one another as the 
new cells (secondary oocytes or spermatocytes) form.  Each of these 
new cells now contains one set of replicated chromosomes, but the 
combination of these chromosomes is random, determined by the 
way the pairs happened to line up on the metaphase plate in 
Meiosis I.   The number of possible combinations resulting from this 
division can be calculated by 2n where n = the haploid number.  For 
instance in the following example, n = 2, and there are 22 = 4 
combinations. I have used “M” and “D” to indicate the origin of the 
homologous chromosomes (from Mom or Dad), and the 
chromosomes are represented in replicated form, e.g. M1M1= 
replicated chromosome 1 of maternal origin, etc.:

  1.  M1M1:D1D1     2. M1M1:D1D1    3. D1D1:M1M1   4. D1D1:M1M1          
      M2M2:D2D2	       D2D2:M2M2      M2M2:D2D2       D2D2:M2M2

As humans have a haploid number of 23, there are 223 possible 
combinations of chromosomes!

The second way in which alleles are uniquely combined also occurs 
during meiosis I, when the homologous chromosomes pair up.  This 
pairing presents them with an opportunity to exchange small bits of 
DNA between homologous chromosomes, so alleles  that were 
originally on the maternally derived chromosome change places with 
the corresponding alleles at the same loci on the paternally derived 
chromosome.  Remember that the chromosomes are replicated at 
this point, and since crossing over only occurs between homologous 
chromatids (not both of the replicated chromosomes), this results in 
4 chromatids which are homologous to one another but have slight 
variances in their genetic sequence—depicted below by M & D again, 
this time representing the origin of each chromosomal segment.  X 
marks the spot where crossing over occurred:

M	M		D	D
M	M		D	D
M	M		D	D 
M	DX	M	D
M	M		D	D
M	M		D	D 



So, by the time meiosis has completed, and the haploid gametes 
have formed, each one is genetically unique, and when one of those 
combines with another genetically unique gamete to form a zygote 
the result is a novel combination of alleles, ensuring that no two 
humans (or other organisms that follow this developmental program) 
are alike.   The only exception to this is monozygotic twins, which 
develop from the same fertilized egg and are thus genetically 
identical.  

You ask in your question “how  can we claim that our allelic 
combinations are random since there are still genes that have not 
been discovered and tested yet?”  I hope that you will be able to 
derive the answer from the information I’ve given, and the websites 
referenced above.  Please don’t hesitate to write again if you need 
further clarification on this or any other topic.  

Warm regards,
Jen

phillips@uoneuro.uoregon.edu