Can crossing over occur between the X and Y chromosome?

Short answer: yes, there is! But only in specific spots where the X and Y chromosomes are similar enough. These special regions are called pseudoautosomal regions, or PARs for short. During sperm production, crossing over occurs at PARs to produce recombinant sex chromosomes - and, in fact, this process is so important that reproduction would not be possible without it!

More on that later - for now, though, let’s dive into what crossing over actually is anyways. 

Mixing It Up: Crossing Over Explained

Crossing over is when two chromosomes swap pieces of DNA during something called meiosis, which is a special kind of cell division that makes sperm and eggs. You might remember that mitosis is the type of cell division which makes identical body cells - like when your skin heals after a cut or you grow taller. In meiosis, on the other hand, we don’t end up with identical cells after cell division; rather, we get cells that have half the usual number of chromosomes. So although human cells generally have 46 chromosomes (two copies of each chromosome from mom and dad), the sperm and egg cells that come out of meiosis have 23 chromosomes each - one copy each instead of two! 

During the first phase of meiosis, called prophase I, chromosomes line up in pairs. Each chromosome finds its other copy – called a homologous chromosome – and they saddle up next to each other near the middle of the cell. While they’re paired up, the chromosomes swap matching pieces of DNA in a process called crossing over, also known as recombination.

Think of it as a kind of exchange - each chromosome keeps most of its own DNA, but small sections are swapped with its partner. This creates new combinations of genes, which is why siblings can share traits but still look different. Recombination is crucial to maintaining genetic diversity!

Normally, crossing over happens between the same-numbered chromosomes, like chromosome 1 from mom and chromosome 1 from dad. But in people with XY chromosomes, things get trickier. The X and Y aren’t exactly a matching pair - for one, the X chromosome is much bigger than the Y chromosome. In fact, there are around 1000 genes on the X chromosome while the Y chromosome only has approximately 45!¹ What’s more is that most of these genes are not shared between the X and Y chromosomes. So you might be asking - how could the X and Y chromosomes possibly exchange genetic information?

Meet the PARs

Here’s the twist - notice how I said most genes are not shared between the X and Y chromosomes. It turns out that there actually are two special regions near the ends of the X and Y chromosomes which match: PAR1 and PAR2. These are called pseudoautosomal regions (PARs) because they are inherited just like any autosomal (non-sex chromosome) despite being physically located on the sex chromosomes! PAR1 is slightly larger than PAR2, containing 24 genes versus 5 genes.² These genes contribute to diverse biological processes, such as bone growth and the synthesis of melatonin (the hormone which makes you sleepy!).³

So although crossing over can’t happen along most of the length of the sex chromosomes, it does in fact happen at these special locations! But you might be asking yourself: cool, but why do these PARs matter?

PARs: Key Players in Meiosis

Without PARs, the X and Y chromosomes wouldn’t properly pair up during sperm production. These tiny regions function kind of like magnets, letting the X and Y chromosomes find each other during prophase I. If the X and Y chromosomes don’t line up right, they might not separate evenly - and that could lead to sperm with the wrong number of sex chromosomes.  

Beyond helping the X and Y chromosomes pair up, PARs are actually required for meiosis to occur in XY individuals. In fact, scientists have found that cells in which XY recombination does not occur fail to complete meiosis!⁴

When Crossing Over Goes Off Track 

Although recombination between the X and Y chromosomes normally happens in PARs, it actually can occur outside these regions, although this is very rare. In situations like these, pieces of DNA from the X and Y chromosomes become translocated – a phenomenon where a segment of one chromosome becomes attached to the other in an abnormal spot. 

These X-Y translocation events can have significant effects. For instance, if a portion of the Y chromosome carrying the SRY gene (which triggers male development) is translocated onto the X chromosome, an individual with two X chromosomes may develop male characteristics.⁵ Conversely, loss of the SRY gene from the Y chromosome can result in XY individuals developing female traits. 

Unlike normal crossing over in the PARs, which is tightly regulated and necessary for meiosis, these translocation events are accidental and can be highly disruptive. They can lead to infertility or differences in sexual development because the chromosomes can no longer properly separate during meiosis.

Does XY crossing over affect traits?

A little bit! The PAR regions contain some genes that aren’t strictly “male” or “female.” Since these genes exist on both the X and Y chromosomes, they can influence traits in everyone. 

For example, a gene called SHOX, found in PAR1, affects bone growth. People with only one copy of SHOX instead of two often have shorter stature, which is one reason people with Turner syndrome (who only have one X chromosome) may be shorter than average.⁶

So, even though crossing over between X and Y happens only in small regions, it’s still critical to human health. Small but mighty, PARs prove that sometimes the tiniest pieces of DNA make the biggest difference!