Genetics 103 - Using Sex-linked Punnett Squares for Birds
This page presumes you have read and understand Genetics 101 and Genetics 102. If you have not read them or do not understand them, this page will be more difficult to understand.
Info on Sex-linked Mutations
Sex-linked mutations travel and express differently than autosomal mutations. They travel ONLY on the sex chromosomes, which are different in birds than in humans. Where in humans the males are XY and the females are XX,* birds have a Z chromosome and a W chromosome, and the males are ZZ and the females are ZW.**
- *Obviously sex and gender in humans is a lot more complicated and there are other variations, but we're here for peafowl genetics, and we're keeping it simple to learn the basics.
- **While scientific notation is ZW with both capitalized, I use Zw in this guide to differentiate between white (W) and the second sex chromosome (w), as using "W" to refer to white is common practice in peafowl.
- You only need to track the sex chromosomes if the bird has a sex-linked gene. In autosomal mutations, you can assume 50% of males and 50% of females will be any given possibility.
Sex-linked Punnett Squares
Below is a Punnett square filled out with the sex chromosomes. It's important to note that these are NOT genes- this is the entire chromosome. It will carry different genes, but it will always be represented by Z and W.
Normally you do not note "WT" on the Z chromosome, it would just be a Z with no notation (as seen above) and the "wild type" is assumed because there's no other notation. If it helps you to add a WT placeholder, go for it! The w chromosome is ALWAYS wild type, as no color or pattern mutations exist yet that are hen-only.
To fill in the offspring cells, we still take the cock row and combine it with the hen column where they meet, just like autosomal genes. However, with sex-linked genes, any cell with two Z's is a COCK. Any cell with one Z and one w is a HEN.
To fill in the offspring cells, we still take the cock row and combine it with the hen column where they meet, just like autosomal genes. However, with sex-linked genes, any cell with two Z's is a COCK. Any cell with one Z and one w is a HEN.
This time you have four results, but the results are separated by sex. All boxes with ZZ are males, all boxes with Zw are hens. Unlike with autosomal mutations, each box doesn't represent 25% of all offspring, but rather 50% of one sex of the offspring only. So, the upper right box represents 50% of the male offspring, and the lower right box represents 50% of the female offspring. The top row of boxes represents 100% of all male offspring, and the bottom row represents 100% of female offspring.
This kind of tracking will work for any single-gene sex-linked gene by itself. It gets more complicated for peafowl, specifically, because of the Peach gene.
If you have questions about how to use Punnett squares to track sex-linked genes, please feel free to join us on the Peafowl Discord Server!
Otherwise, feel free to head on to the next lesson, multi-gene tracking using Punnett squares!
If you have questions about how to use Punnett squares to track sex-linked genes, please feel free to join us on the Peafowl Discord Server!
Otherwise, feel free to head on to the next lesson, multi-gene tracking using Punnett squares!