Well, their sex cells have to meet

So that means that one sperm cell will be able to get its nucleus into a egg cell and the 2 nuclei (protonuclei) will become one

Once they become one nuclei, the chromosomes will now have their partners. As each chromosome is now a pair of chromosomes, it is a unique cell with a allele combination for its thousands of genes that has never occurred before and will never occur again. It is a unique moment, and unique set of instructions that will give rise to a unique individual

To get the unique pairing of chromosomes to make a unique individual, we need to step back. Those egg cells and sperm cells where they also unique? Well yes.

Firstly, those sex cells need to have just one of each chromosome so that when they meet they can make a pair

To have one of each chromosome, they must halve the number of chromosomes - as half of a pair is an individual

This process of halving the chromosomes from a stem cell to make a sex cell is called Meiosis

The product of Meiosis will either be egg cells (oocytes) or sperm cells

In woman the creation of the eggs happens while she herself is still in the uterus

In men the creation of sperm cells occurs after puberty starts

Women will make about 300,000 eggs, most of which will not be released

Men will make billions of sperm cells. Over 100 Million in each ejaculate 

All you need is one egg and one sperm to make a baby

So how are these cells made

What are the steps of Meiosis?

And how do these steps affect our uniqueness and the variation seen within the human population?

Meiosis is how Sex cells are made. So how does it work

Well, surprisingly it starts with DNA duplication

So the chromosome number starts with 2n and then becomes 4n

The cell then splits like it normally does, and we get two cells that are 2n

But then...... the cell splits again!!!!!!

No duplication or anything like that, it just moves the pairs or chromosomes to either side of the cell. Like the 2n becomes n space n. And then it splits! Each new cell is just n. That n will randomly be part of the pair, which one of the pair it is is completely random.

This makes a grand total of 4 cells that are all n

We start with one 2n, diploid, cell. 

We end with four n, haploid, cells. 

So if we follow the number of chromosomes it goes 2n, 4n, 2n, n

If we follow the number of cells it goes 1, 1, 2, 4

So 1 cell with 2n 

Then that one cell does DNA duplication during Interphase and we get 4n

Then that cell does PMATC and becomes 2 cells that are both 2n

Then those two 2n cells do PMAT again and become 4 cells that are all just n.

If we play through the cell cycle for Meiosis it goes:

• Interphase - DNA Replication

• Prophase 1 - DNA condenses into visible chromosomes, nucleus goes poof - crossing over occurs

• Metaphase 1 - Chromosomes line up in the middle independently

• Anaphase 1 - Chromosomes are pulled apart

• Telophase - New nucleus, cell starts to split 

• Cytokinesis - Cell has fully split

Then......!!!!

• Prophase 2 - DNA condenses into visible chromosomes, nucleus goes poof

• Metaphase 2  - Chromosomes line up in the middle independently

• Anaphase 2 - Chromosomes are pulled apart

• Telophase 2 - New nucleus, cell starts to split 

• Cytokinesis 2 - Cell has fully split

Now you have 4 sex cells that each only have 23 individual chromosomes

The spot where they cross over is called the  chiasma

Multiple Chiasma are called Chiasmata as the plural for Chiasma

If you look at these homologous chromosomes above, you will see that they all actually cross over, the red and green on at just one location and the orange and yellow at multiple locations

The point of all this crossing over is to make the 4 homologous chromosomes each as unique as is possible

Thus increasing the variation of traits in the offspring 

Something unusual happens during Meiosis Prophase 1

The non-sister homologous chromosomes have a cuddle

For instance Chromosome 7 from mum will cuddle with ch7 from dad

During this, they swap parts - like a hug where people switch their arms

Because these homologous non-sister chromosomes do this swapping, you can end up with different alleles on the chromatids as shown in the clip below

There are multiple places where these chiasma can occur, thus there is multiple possibilities for the allele combinations on the new recombined  (recombinant) chromosomes. 

The cuddles of cross-over occurs during prophase 1, they are then ripped apart during Anaphase. Each daughter cell at the end of Meiosis will have unique allele combinations

When the chromosomes line up side by side during Metaphase, which one of the chromosomes will be on the right, and which one on the left? 

If we take Chromosome 7, then is it:

•  Ch 7 maternal, then Ch 7 paternal

Or is it

• Ch 7 paternal, then Ch 7 maternal

The same then applies to the other chromosomes, all 23 of them must line up and for each of them the it will be completely random as to which one, maternal or paternal, is one the right or the left

So for the first 3 chromosomes it might be:

• ch1 M ch1 P

• ch2 M ch2 P

• ch3 P ch3 M

Or it might be

• ch1 P ch1 M

• ch2 M ch2 P

• ch3 P ch3 M

Or it might be

• Ch1 P Ch1 M

• Ch2 P Ch2 M

• Ch3 P Ch3 M

So there are multiple possible combinations

Carry this on for all 23 chromosomes you will then see that there are 2^23 possible combinations (8 million possibilities)

The assortment of the maternal and paternal chromosomes along the middle of the cell is independent

Thus this is called, Independent Assortment

If Independent Assortment gives rise to 8 million possible combinations, then you chuck in the random nature of cross-over and then each gamete produced by meiosis gives an un-calculable level of uniqueness

Once combined with the random nature of mate selection, and then the random sperm that wins the race to fertilize the random egg that was released then the individual created is infinitely unique