Chapter 2
Cell reproduction, Mitosis, Meiosis and Crossovers


Lets pauses for just a moment and take a quick review. So far we talked about chromosomes and genes and how they relate to one another. If youíll remember, we said a chromosome was a thread like linear strand of DNA and associated proteins.  That the chromosome strand of DNA was a chain of several genes linked together in a set order or fashion with each gene having its own special location or locus point within the chain.  We discussed how chromosomes occur in sets; and as such, results in the individual gene possibilities being two of the exact same type  (homozygous or pure) or two completely different types (heterozygous or non-pure) yet related to each other through a common locus point.  

Now we will discuss how cells reproduce and how new chromosome/gene combinations can occur.  In short, how genetic traits are inherited.    

Lets begin with a short description of how a cell is constructed.  Each cell is incased within a cell membrane.  This membrane contains the cellís nucleus suspended in a body of cytoplasm.  The nucleus in turn contains all the chromosome sets.  Please refer to   Graphic # 1   It doesnít represent a pigeon cell in particular but is more typical in nature.

New cells are continually required for both growth and or replacement.  In biology this form of cell reproduction is called ďMitosisĒ.  It is a process whereby a single cell divides (replicates) into two new daughter cells each with the same number of chromosomes as the former parent cell.  This provides additional cells for growth or replacement for dead or dying cells.  Either way, there is no change in the DNA or genetic possibilities.  The chromosome gene combinations remain unchanged.   Itís straight DNA replication of the original cell to become two.  Mutation can occur but should it happen it would only effect this individual cell and any future cells it produces.  The remaining cells go unchanged.

The big question for us now is ďIf Mitosis produces two daughter cells each with a complete mix of 80 chromosomes, how does nature produce egg and sperm cells where only half that number or 40 occurĒ?    Since the sire and dam are each responsible for only one half the total number of chromosomes inherited, how is their total cut in half?  Otherwise the number would be twice the amount needed and would multiply with each proceeding generation.  Can you imagine how large that number would be today?

Even though the process is complicated the answer is simple.  Cell replication within the gonads (male testes and or female ovaries) is a different form of replication from that which occurs in all other parts of the body.  Here the biological form of cell reproduction is called meiosis.  It differs from mitosis in that the end result is four daughter cells being formed, each with only 40 chromosomes or one half of the original chromosome set.  Meiosis insures that after fertilization, the recombined number of chromosomes is correct and the Mitosis process for embryo growth can begin.Ē  

Lets go over that again. Both mitosis and meiosis begin with the replication of the chromosomes. The two complete sets of 40 chromosomes each become four complete sets. . The nucleus surrounding the chromosomes breaks up and the homologous chromosomes line up individually on each side of an equatorial plate that is formed.  The cell then divides into two identical daughter cells, each with two complete chromosome sets.  In the case of mitosis the cell nucleuses reform and the process is complete. 

Meiosis replication within the gonads continues after the first cell division with both daughter cells undergoing a second division producing four new cells.  However, in this second division there is no second replication of chromosomes.  Instead, the chromosome sets which are twisted together in a double helix, are separated from one another as the daughter cells undergo their second division.  Each of the four new cells then contains a copy of one complete set of 40 chromosomes.  In the case of a cock this would result in 4 new sperm cells (spermatids) and for a hen 4 new egg cells (ovum).    Please refer to   Graphic # 3

In the process of untangling the double helix, the chromosome gene chains are sometimes broken and end up reshuffled in the mix so that part of the first become part of the second and the reverse being true for the other portion.  This is referred to as a crossover.  It is how genes become reshuffled within that specific chromosome set.  In other words, genes reshuffled from one chromosome will be shifted to the other chromosome of that same set and reconnect at the same locus point as its original chromosomeís.   

A good example of this gene reshuffling or crossing over is the migration of the qualmond gene from the Z chromosome containing the basic color gene for blue to another Z chromosome containing the basic color gene for brown.  The same is true for ash red. When this qualmond gene mutation first occurred we could only produce them in blue.  Today, due to this crossover occurrence we can produce qualmonds in all three combinations.    Since qualmond is a sex linked gene, this particular crossover could only take place within the cockís testes. Had it been a non-sex-linked gene, lets say a pattern gene, then crossover could have occurred in either the cockís testes or henís ovaries. But I digress so letís get back to the subject at hand.

Okay, are chromosomes simply duplicates of their parents or are their differences.   Well the answer gets just a little bit confusing.  Yes some may be duplicates of their parents while others may be a mix of both.  What we find, is that during meiosis replication the formation of a new chromosome for either the egg or sperm is formed from segments of the chromosome pairs being separated. This is done in a totally random fashion taking chunks or segments from each pair for a total equal to one chromosome within the set. In so doing, it may become a complete duplicate of one or a mixture of both.  

The further apart genes are from each other, in the chromosome chain, the greater the possibility that they will become separated as a crossover in the process. 

We know that each pigeon cell contains all the basic building blocks for life as a pigeon.  This is known as the birds DNA.  It varies slightly from one individual to another in their inherited gene combinations.  Only identical twins, which began life as one single cell and or a clone, which is basically the same thing but not occurring naturally, would have identical genes or DNA.   True, many members of a family will share the majority of the same genes but not exactly the same.  Each parent would have a slightly different gene combination.  Under normal conditions there will always be this difference no matter how slight, between all individuals unless they are identical twins or a man made clones.  Therefore, there is no such thing as a truly pure strain or family of pigeons.  When we refer to something as being pure in that sense we are in reference to the vast majority of genetic differences as being reduced to a very small percentage. The less these differences become the more pure the family is. 

Okay, so why canít we begin a pure homozygous family by breeding identical twins or from two clones?  Well for starters such birds would all be the same sex so that idea is out the window.  The main reason that we canít establish a pure strain from only one outstanding bird is that all pigeons will be both homozygous and heterozygous in there individual gene pair combinations.  It still takes two to tango and every bird has to come from some sort of male and female mating.  Even clones have their origins (one step removed) in that way. 

There is however a way to establish a pure homozygous family through a program of very close inbreeding.  This inbreeding program would be brother and sister followed by brother and sister again for each generation until the gene pool was reduced to an exact number of genes required for one complete single set of totally homozygous chromosomes.  This would take several generations to accomplish and would only occur when you reached the point where all gene possibilities for each chromosome locus were the exact same.   Only then would you have a truly pure family.  Of course Mother Nature is going to throw a monkey wrench at you along the way in the form of new mutations but by continuation of the brother sister matting program it would be returned to pure again. 

Question is, would such a program net you a clone like family of the original outstanding bird.  Unfortunately the answer is "Clone like yes but not like the original outstanding birdĒ.  In fact, it probably would be very different.  Even though all of the genes would be descended from the original bird the total mix would no longer include some of the other original genes that were heterozygous or mixed in their sets.   Of those differing possibilities only one would now remain and it would then be homozygous or pure for itís effect.

 

Okay, lets see if I can say the same things in a different way.   Letís try using another metaphor only this time we visit the local Farmers Market where we find several booths, each only slightly different from the other.    For our metaphor letís compare these booths to being different pigeons.  When we look close we discover that each booth (pigeon) has 80 containers (chromosomes) of stuff on display.   We also discover that these containers are grouped in matched pairs.  Some have their first pair being two baskets of cans (Z chromosomes).  Some however only have one a basket of cans (Z chromosome) and the other (W chromosome) a small thimble of tiny objects.  All the other pairs of containers in each booth are of the same construction such as two wooden baskets, two toolboxes etc., with each containing the same number of like items. Example a pair of cardboard boxes with different kinds of clothes. A set of toolboxes with different types of tools, and so on for a total of 40 sets of containers in each booth.

Next we discover that the farmerís wife placed 50% of these containers or one of each set onto each booth. Among some of the booths she placed a thimble container, while in others she placed a basket of cans.  She placed one of each of the other types of containers into each booth as well.  The farmer on the other hand only provided a basket of cans to each booth in addition to one of each of the other types of containers but no thimble. 

Now the sets that had a can basket and a thimble became a hen (Z / W).  And since the can basket contained a can of blue paint the hen was dressed in blue. Had the farmerís wife placed another can basket and not a thimble the bird would have been a cock (Z/Z) and he would have had two cans of paint. His color would then be the more dominant of the two cans of paint.

We also notice that in each container (chromosome) there are slight differences.   Example: In the cloth boxes we have one box with a short sleeve shirt and the other with a long sleeve.   In this case our bird will be seen to have a long sleeve since it is dominant to the short sleeve.   We also see that there is a transparent grizzled colored coat in one of the other baskets and it will cover the blue long sleeve shirt so now our bird looks blue grizzle and not simple blue.   In the toolboxes we find a chain saw in one and a cross cut in the other.   Well our bird is no dummy so it uses only the chain saw to cut with.   Another bird further down the isle has a cross cut saw in each of itís two tool boxed.   Therefore that other pigeon can only cut wood with a cross cut saw.

None of the pigeons there had their toolboxs filled with only saws since the farmer and his wife are required buy genetic law to only place one cutting device(allele) in each toolbox.   Therefore only two possibilities existed, one in from each toolbox.  Only in the case of the basket of cans and the thimble do we have an exception to this rule and this only happens in the case of hens where no items are to be found in the thimble.  She has no choices for the items in the basket of cans  since she only has one such basket to select from.

Now lets look closely at the contents in some of these other containers which represent the non-sex chromosome.   In the fruit baskets we find that each one has an apple.   However there are differences between the apples.   The first set of fruit baskets has a green cooking apple in one and a red delicious in the other.   The booth down the isle has one yellow delicious and a granny-smith green apple while another further along has only red delicious apples.  These apples would be our various gene type alleles.  All being apples (family of genes) but each being slightly different, a mutation from the original apple.  In these same fruit baskets, (chromosomes) we find other types of fruits (genes) and like the apples, they too come in slightly different variations (alleles). However, like the apples there is only one of each type of fruit found in each basket.  Consequently,  all three of the pigeons represented by these three booths would be genetically different.  

Okay, back to the real world.   How did these chromosomes come into being in the first place.  Are they simply duplicates of their parents or are their differences.   Using the two fruit baskets as our example we might find that the new fruit basket has a handful of fruit taken from one basket with another handful taken from the other basket in that set. However, only one fruit of each type ended up in the new basket.  Therefore the total number of fruits remained the same.  One apple, one banana one watermelon one orange and so on but not all from the same original basket.  Half came from the Farmer's basket and the remainder form his wife's

Since these fruits are always found in the same position within the basket, the handful that is taken often contains the same grouping.  Example, if basket one had a Dixie orange and a Granny Smith apple side by side these two; most likely, would be passed along together.  This is known as being linked. The further apart in the chain these genes are from each other, the greater the possibility they will become separated as a crossover in the process.  

Well I think I had better end here as I may have rambled on too long already. I hope this helps to clear up some of the genetic mumbo jumbo on how genetic traits are inherited.   If you are interested in Meiosis, Mitosis or any other genetic topic you should use your browser to locate additional information on one of the many excellent educational web sites.

 


Back Chapter 1
Next Chapter
Back To Start Page





Copyright 1999 by Ronald Huntley.
Permission is granted to download or copy for non-commercial individual use only.
The author retains all other rights under copyright.


Ronald R. Huntley
Web Page Designer
Duncan SC, 29334