How does sex work? In particular, how do sexually reproducing organisms give rise to offspring that have genetic material from each parent? Before we can answer that question, we need to clarify a few things about how genes are encoded on chromosomes. So recall that a chromosome is a structure that contains DNA and protein. And generally, we draw a chromosome like this. And now, of course, we know that this particular drawing of a chromosome represents a chromosome after prophase, right, or in prophase after it's condensed. And what's important to know about chromosomes is that most of them have partners. So I'll draw a second chromosome like this. And these partners, we call these partners homologues, these partner chromosomes carry the same genes. And so, for example, if this chromosome has a gene for eye color, and I'll draw it like right there, this chromosome, it's partner chromosome, it's homologous chromosome, also carries a gene for eye color. I'll draw it there. And as indicated by the fact that I use different colors for those two genes for eye color, genes can have different versions, right? And we call these different versions different alleles of a gene. And so, and so because there are like different visible aspects, say for eye color, right, I have blue eyes, but some people have brown eyes, right? That means that I have alleles, I have particular versions of the gene for eye color, that encode for blue eyes, that mean that I have blue eyes, but a person with brown eyes has different versions, has different alleles for the genes for eye color. Humans have 46 chromosomes. 44 of those chromosomes come in pairs like this. So there are 22 homologous pairs of chromosomes in most of the cells in your body, and the two remaining chromosomes, right? Chromosomes 45 and 46 are actually the sex chromosomes. And so in humans, we call those sex chromosomes X and Y, Y kind of looks like that. The key though is that while most of the cells in your body have 22 homologous pairs of chromosomes like this, some of the cells in your body only have one set, right? They don't have pairs, they'll just have one chromosome from each homologous pair. And those are the cells that are the gametes. Those are the cells that recombine with the gametes from another individual, another parent to make offspring. And so, for example, if these are the chromosomes that are present in in a non-gamete cell, then perhaps a gamete cell only has, let's say, that chromosome plus this chromosome. And let's say that the gamete cell carries this orange allele instead of the pink allele. And so, if a gamete has n chromosomes, right, where n is a variable in this case n equals 2, then non-gamete cells have twice that number, right? They have two times n chromosomes. And the idea, of course, is that if this is a gamete from one individual and another gamete from another individual, let's say looks kind of like this, right? If this is gamete from another individual and this gamete also has n chromosomes, then when those two gametes recombine, when those two gametes combine, right, this is sexual reproduction, then they produce a zygote, a single cell that has two n chromosomes again, right? And the single cell that is produced by these two gametes coming together is going to eventually develop into a full organism. And so, meiosis is the process of making cells that have n chromosomes from cells that have two n chromosomes. So, these gamete, or gamete producing cells that have just one set of chromosomes, we call haploid and the cells that have two full sets of chromosomes, we call diploid and meiosis is the process of producing haploid cells from diploid cells. And so next, we're actually going to dig into the details of that process.