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Okay gentle scholars, a double-dip today in two separate posts. First, a final word about diversity in gametes.
If it seems that I’m going back and adding details on in layers, go to the head of the class!!
One of the great difficulties students encounter in studying meiosis is that there are a blizzard of details and new vocabulary that cause one to give up hope half-way through. It’s best to spread this material out in bite-sized pieces.
Building on all that we have seen, let’s go back to the beginning of meiosis where the homologous pair of chromosomes undergoes DNA synthesis. And let’s recall that what makes a pair of chromosomes homologous is that they have the same genes at the same location from top to bottom along the chromosome. Here comes the diversity.
A gene, recall, is a stretch of DNA whose nucleotide sequence is a code for building a certain protein. Now, let’s say that the first gene on a chromosome is the gene coding for hair color. We’ll call that gene the hair color gene. However, there are blondes, brunettes, raven black hair, and red heads. So clearly, not all hair color genes are the same. There are alternative forms of the hair color gene, as there are alternative forms of a great many genes. We call these alternative forms of genes alleles.
Some alleles are mainifest or what we call expressed in a dominant manner. That means if a dominant allele and a recessive allele are inherited for hair color, the dominant allele gets expressed.
Let’s consider hair color. My mother was a strawberry blonde. Dad had brown hair. That means mom gave me a recessive allele and dad gave me a dominant allele. Therefore, what’s left of your professor’s (rapidly) graying hair is brown. When the homologous pair of chromosomes contains a mixed pairing of alleles, the dominant allele gets expressed. Brown and Black are Dominant. Blonde and Red are recessive. (If the gentle scholars want a class on those pesky Punnett Squares for predicting offspring traits, let me know in the com boxes.)
Dominant alleles are designated with an upper case letter.
Recessive alleles are designated with a lower case latter.
So, as things stand we get one chromosome in a pair from mom and the other from dad. That mens that half our gametes will contain one, and half will contain the other after meiosis. But nature has a way of shuffling the genetic deck even further. It turns out that during the first phase of meiosis the chromosomes from mom and dad in a homologous pair overlap or what we call cross over and exchange pieces of DNA. Such chromosomes where recombination of alleles has occurred are called recombinant DNA.
Here are two videos showing this process. The first video is shorter and more generalized. The second is a little longer with more specifics.
Still with me here?
Now for the payoff.
First, imagine meiosis without crossing over and consider the possibility for different gametes.
Half of the gametes could contain all 23 chromosomes from my mother, the other half all 23 from my father.
Some could contain 22 chromosomes from my mother, 1 from my father.
Some could contain 21 chromosomes from my mother 2 from my father.
These can occur in any of a mind-numbing series of combinations.
NOW add to that the crossing over and exchange of alleles in each of the chromosomes.
Add to that the fact that crossing over and exchange of alleles on any given chromosome pair occurs at many different loci means almost infinite possibilities for genetically unique gametes in any given parent. Then, the offspring are the result of two gametes from such wildly different genetic backgrounds.
The result is a genetic uniqueness never duplicated in nature, save for identical twins. Even among identical twins, there are differences in appearance, personality and longevity.
So human individuality is not the result of one’s collected neurological experiences, but is written in our genome. It is this unique genetic identity that controls neurological development and function. To the extent that behaviors have a genetic etiology, these instructions are present from the moment of conception.
Therefore individuality is ultimately, at the biological level, a function of genetic inheritance.
That begins at conception. It is never repeated again.
Photo via johnlarroquetteproject.com