Just one cell needs to meet with just one egg to make a baby

The 23 chromosomes form the sperms nucleus merge with the 23 chromosomes in the eggs nucleus and now you have 23 pairs of chromosomes. This new cell is called a zygote.

So a haploid sperm meets a haploid egg and they make a zygote.

The zygote becomes a embryo, the embryo becomes a fetus, a fetus becomes a baby

So which sperm cell meets which egg cell? 

That is completely random

So Fertilization is Random

So we call it ..... Random Fertilization

When we add up the 8 million (2^23) possible Chromosome combinations that are possible from Independent assortment. Thus 8 million different gametes per person

Then because sexual reproduction involves two different people, we take that 7 million and times it by the other persons 8 million possible different gametes and we get... 64 trillion possible offspring... but that is just with independent assortment... we then need to add in cross over back during the formation of those gametes and we start getting numbers that are like the number of stars in the sky. So each gamete is unique. 

Then we add in that males produce 300 million sperm cells and woman make 300,000 oocytes

Then we add in that the oocyte released is random. Then we add in that the sperm cell that wins the race is random. So that we get Random Fertilization

Best way to remember it is that the winner of the race of 300 million is random, so the sperm that fertilizes the egg is random, so it is Random Fertilization.

But these numbers are just with a fixed male and female - like a deserted island scenario. 

We now need to chuck in mate selection. Then we add in mate selection!! There are 3 billion females that could randomly mate with 3 billion males, that gives us 9000000000000000000 possible couples, then that times the 64 trillion possible gametes from each and you get 576000000000000000000000000000000 possible unique offspring. Then chuck in that cross-over back when the gametes were made and chuck in a few random mutations and the number becomes effectively infinite.

So, in the grand scheme of things, you are an infinitely unique individual.

Yes, you are like your parents, and like your siblings. You inherit this from mum and that from dad. But you are still not them, and you are not your siblings. You are unique

A fully unique set of alleles for the genes. A individual that is like no other. An individual that has never existed before or will again. An individual that is a result of their entire ancestry;  Their 2 parents, their 4 grandparents, their 8 great grand parents, their 16 great great grand parents. All had to meet at a very specific moment in earths history, for that individual to exist. 

You are completely unique. And you carry the heritage and of your ancestors. 

It took millions of 'fortunate' events for you to exist. You are special and you have your own journey. You have your own story to write, you are special, you impact people, you matter, you exist.

Well this is because, of the 23 chromosomes, only the 23rd pair determine the sex of the individual

These Chromosomes are the sex chromosomes. They are designated X or Y and can be identified by size - the X is much bigger. 

If a person has 2 X chromosomes then they are a female

If they have an X and a Y then they are male

During Meiosis the gamete will only receive one sex chromosome.

This means for Females that one option is X and the other is also X

For Males, one option through independent assortment is a gamete that has X. The other option is a gamete that has a Y

So all egg cells (oocytes) are X

But sperm cells are either X or Y

The sperm that arrives first is random, so the fertilization of the egg is random

If the Egg is fertilized with a sperm carrying a X chromosome then the zygote will be XX and the individual will be female

If the egg is fertilized with a sperm carrying a Y chromosome then the zygote will be XY and the baby will be Male

So either way, it is your father that determined your biological sex!!