How does the Y chromosome crossover during meiosis?

[musicgold]

Homework Statement

This is not a homework problem.

Recently I finished reading two books: The Selfish Gene and She Has Her Mothers Laugh, and one of the things I am confused about is this: How does the Y chromosome crossover during meiosis?

Relevant Equations

I understand how the 22 pairs of chromosome in a human male crossover during meiosis, but the 23rd pair is made up of two different types of chromosomes (X and Y). Not only are they different in size but are also in terms of the genes they carry. For example, the sex gene SRY is only on the Y chromosome.

So, I am trying to understand how the 23rd chromosome carried by a sperm is created.

Thanks

 

[jim mcnamara]

Short answer is the Y chromosome does not crossover. There are haplotypes (genes) that are passed directly from father to son. These genes along with mitochondrial DNA are the least affected by meiosis. So they are important in determining family lineages - ancestry. com or 23and me.com

For more on haplotype:
https://www.nature.com/scitable/definition/haplotype-haplotypes-142/

 

[Ygggdrasil]

For homologous chromosomes to pair correctly during meiosis I, the homologous chromosome need to crossover with each other. This obviously presents a problem in males because the X and Y chromosomes need to pair up and crossover, yet they have very different sequences.

The solution to this problem is that the X and Y chromosomes contain regions called pseudoautosomal regions at the ends of the X and Y chromosomes that allow the X and Y chromosomes to cross over and pair only at these regions. Crossing over does not exchange much genetic information between the X and Y chromosomes (since the PAR regions are at the ends), but they do allow pairing so that meiosis can go forward.

 

[musicgold]

but they do allow pairing so that meiosis can go forward.

That's what I thought. I read somewhere that fertilization is not possible even if one pair doesn't crossover.

Okay. So, if X and Y do crossover somehow, what makes the resultant chromosome X or Y?
I am assuming the absence of the SRY gene on the amalgamated chromosome makes it a X chromosome. Is that a correct assumption?

 

[Ygggdrasil]

Crossovers between the X and Y chromosomes can occur only within the PAR regions and the PAR regions are at the ends of the X and Y chromosomes:

(source: https://en.wikipedia.org/wiki/Pseudoautosomal_region)

Because crossover cannot occur within the larger bodies of the X and Y chromosomes, most of the important genes on the X chromosome cannot move to the Y chromosome and vice versa. The X and Y chromosomes can recombine genes within the PAR regions (listed in the figure above), but recombination is limited just to those genes. Because the PAR regions on the X and Y chromosomes encode the same genes, recombination of these genes proceeds like recombination would in the other 22 autosomal chromosomes (hence why these regions are called pseudoautosomal regions. Despite being on sex chromosomes, genes on the pseudoautosomal regions can recombine and their inheritance patterns resemble those of normal autosomal genes).

 

[musicgold]

... Because the PAR regions on the X and Y chromosomes encode the same genes, recombination of these genes proceeds like recombination would in the other 22 autosomal chromosomes (hence why these regions are called pseudoautosomal regions. ...

Thanks.

Do you have a picture of a recombined 23rd gene?
What makes a recombined gene either X or Y, once parts of the PAR regions are swapped?