You probably weren’t expecting that title, but let us explain!
Did you know that in real life, you can have a stud’s semen “sorted” for X- or Y-chromosome-bearing sperm to increase your chances of getting a filly or colt?
The Science Behind It
There’s a machine called a flow cytometer that can take in a pool of cells in liquid and sort them based on various properties using fluorescence and lasers. Without getting too deep into the weeds, you can use antibodies or dyes to target specific proteins or structures to cause cells to glow (or not glow). The machine then sorts them based on whether they glow and how brightly.
So how does this work for sperm? It comes down to a small but significant difference between X- and Y-bearing sperm.
Sperm are haploid cells, meaning each sperm cell has one set of unpaired chromosomes—half of a whole genome. Each one carries either an X or a Y chromosome (excluding rare cases), which determines whether the embryo becomes a filly or a colt.
While the X and Y chromosomes are a pair, they’re not equal. The X-chromosome is larger than the Y and contains about 3-4% more DNA. DNA can be stained with a fluorescent dye called Hoechst, which makes it glow blue when hit with the right wavelength of laser. Because X-chromosomes have more DNA, they absorb more dye and glow slightly brighter than Y-bearing sperm. That small difference is usually enough for a flow cytometer to distinguish them! This example figure isn’t specifically for sperm, but you can get an idea of how this works:
This technology is commonly used in the dairy and beef cattle industries, where female or male offspring (respectively) are more desirable. It’s also used for horses—and it will be available in Legacy V2!
How It Works In-Game
When collecting from stallions, you’ll now see an option to sort the semen. If you choose this, your vet will charge you $200 per collection to sort the straws and will probably give you sorted straws. The ratio of X- vs Y-bearing straws will vary, so you won’t always end up with a perfect 50/50 split.
In your semen storage, sorted straws will display a gender icon in the lower right corner, indicating what gender the foals will likely be if you use those straws.
The Catch
You probably noticed all the italicized probably and likely qualifiers. This technology isn’t guaranteed, so there are some chances of failure:
- Collection failure: Sometimes the machine won’t be able to distinguish X- vs Y-sperm, and you’ll end up with unsorted semen and be out the $200 collection fee. This isn’t common, but it will happen occasionally.
- Sorting errors: Sometimes the machine makes mistakes while sorting, and some X-sperm will end up in the Y-straws or vice versa. Because of this, you’ll have a 90% chance of getting the correctly gendered foal when using sorted semen.
So there are some risks, but this technology will be quite useful if you want a specific gender from a cross!