I have green eyes. What you see in my eyes is not the same thing that scientists see in them. The current thought behind green eyes is that a thin layer of yellow pigmentation overlays the "blue" color of an iris, and the result is a yellow color.
The gene responsible for this yellow pigmentation is completely separate from the brown/blue gene; thus, to have green eyes, one must have a low concentration of melanin in the iris and produce the yellow pigment.
Here is a chart that demonstrates how melanin in the front and back epithelia of the iris and the structure of the stroma create eye color. You will notice that the specific combination which is required for green eyes is more complicated.
Of course, we're talking about gene networks here so in reality this isn't cut and dry as I made it sound. If it was, there were be a higher percentage of the population with green eyes.
Eye color is complicated than many may think and the combination required to be born with green eyes is rare. And it has been an area of study for scientist for decades. A lot of people tend to tell me it is because of X or Y, but they are just using layman's terms to try and explain something that they are ignorant about. In other words, anyone who says they know definitively isn't being completely insightful or in other cases honest.
Eye color is highly polygenic and not entirely understood.
A lot of genetics is not as simple as Mendel's pea experiments might lead you to think. Mendel figured the basics out mostly because he picked traits that have distinct, qualitative phenotypes that were only controlled by the expression of a single gene.
This tends to be the exception rather than the rule, but it's still taught in schools because it provides a clear simplified way of explaining the mechanism of why traits are passed on or not.
It is more likely that many different genes play a role in determining eye color (which is a spectrum of colors really, not just "green" or "blue"). While some of the Brown vs. Blue in European populations genes have been worked out (OCA2), the mixtures that make up intermediaries are not entirely clear. What would start off as a binary state becomes very messy when you start adding in 11 other SNPs with incomplete penetrance and variable dominance terms.
As for the rarity, that's just simple population genetics.
The alleles for green eyes, whatever their nature, are simply less common in European and Middle Eastern populations than brown or blue. Hence, they are less likely to be observed.
Since eye color is usually not selected for or against when people choose whom they wish to mate with, the frequency of the "green allele" remains more or less the same. Though I did have a boyfriend tell me once that he was only together with me because of my eye color. That sort of creeped me out and I dumped him a week or so later.
SucculentsFor succulents, water, light, and temperature are pretty well understood. For example, we all know a succulent without enough light will stretch (called etiolation). Obviously, it’s seeking more light. But what we might not know is that behind the scenes, the plant is putting extra energy into stretching. This could lower the plant’s immune system, making it more prone to pests and infection. So where do we start when trying to provide our succulents with a better environment where they will not only survive, but thrive?
It Is In The DirtA plant’s roots play a role in receiving all but one of the six elements: light. One particularly important but often neglected element, here, is air. Roots need oxygen and airflow as much as they need water. This is why root rot happens. It’s not because of too much water, but rather because water can cause a lack of oxygen to the roots. Think about this for a moment; if water were the issue, how could plants and even succulents grow hydroponically? They could not. They water needs to have oxygen in it for them to develop a healthy root system. Air is the reason why a gritty mix is so good for plants. It is a potting medium that, when done correctly, completely eliminates the issue of “overwatering” by eliminating the layer of water at the bottom of the pot that suffocates the finer roots – the ones that do the most work in getting your plants water and nutrients.
DrainageThe most important aspect of a container is a drainage hole. This is non-negotiable if a plant is to survive for than a few months. A pot without a drainage hole nullifies all the advice above. Even with a gritty mix, it is impossible to keep a plant healthy without a drainage hole. A plant in a container with no drainage hole will either be under watered due and experience fertilizer burn due to watering in sips, or conversely over watered with its roots sitting in water. If you would like to have your plant in a container that is not perfectly suitable for plants, such as a cute ceramic planter with no holes, an old shoe, or a glass terrarium, use a cachepot. That is, use a normal pot with drainage holes and simply take out the plant to water it. When finished watering, put the cachepot back into your other container.
Fear of Watering/OverwateringA common piece of advice you’ll here with succulents is to not overwater them. Unfortunately, this advice usually leads to people watering their plant in sips, which has its own set of issues, which I will explain in a later section. Instead of soil, people are often taught that better drainage is the solution, especially for succulents. The natural reaction then, is to use a sandy mix, to add rocks, or to add a drainage layer. Unfortunately, these are not good solutions. Sand is very fine and will cause water to perch even more. Rocks added to a soil won’t do anything if the soil still contains over 60% its original medium. But perhaps the most interesting one is the drainage layer. Beginner’s guide to watering:
- Prepare the water by adding a 3-1-2 fertilizer at half the recommended dosage from the container’s instructions.
- Water until excess comes out the bottom of the pot’s hole(s).
- Discard the excess water immediately. Avoid letting the plant sit in the water to reabsorb.