The Scheme of Genetics

Recently, I received some questions by e-mail that made me realize how confusing so many of you find genetics. One such question asked if there was a "scheme of genes", and how those genes interacted with each other.

There is no "scheme" per se. I usually group color mutations in three groups: sex-linked recessive, non sex-linked recessive, and dominant. As was mentioned in a previous newsletter, the sex-linked mutations are: cinnamon, lutino, pearl and yellowface. The non sex-linked mutations are: fallow, pied, silver, spangle and whiteface. The dominant mutations are: dominant [to whiteface] pastel, dominant silver and dominant yellowface.

That basically covers all mutations except for normal gray and the recessive pastel. Normal gray can occur a couple of different ways. First, if one parent passes a sex-linked color to a son and the other passes a different sex-linked color to that son (for example, pearl and lutino), that male chick will be a normal gray. Also, there is a gray gene; it is simply a chromosome that has no color-related defects. The other unclassified mutation is recessive pastel mutation. This is non sex-linked, but it is only dominant to whiteface; it is recessive to all other color mutations. A pastel can have the pastel gene on one or both of its chromosomes. If it only possesses one pastel gene, then the other must be whiteface. So, when you look at a pastel’s pedigree, it is either a pastel or a pastel split whiteface (one of the questions after this article pertains to the pastel mutation, to give you an example).

That aside, how are you supposed to know which genes will be expressed and which genes won’t. It’s pretty simple, but with 14 different mutations and all the combinations possible, it can get hectic. In order for any color mutation to be expressed, certain requirements must be met. For sex-linked colors, the female chicks will show the color(s) or a combination thereof that the father carries. For example, a yellowface male will only produce yellowface daughters. A pearl split lutino male will only produce pearl and lutino pearl daughters. A normal male that is split for cinnamon, pearl and lutino can produce anything from a gray to a cinnamon lutino pearl and every combination in between. For sex-linked colors in male chicks, the only colors that will show will be those that have been passed on by both parents. For example, the sons of a cinnamon cock and a lutino pearl hen will be normal gray split cinnamon, lutino and pearl. The sons of a lutino pearl cock and a cinnamon pearl yellowface hen will be a pearl split cinnamon, lutino and yellowface. Why is this last bird only a pearl, because the only common gene he received from both parents is pearl; all the other sex-linked colors came individually. Any color trait, except gray, must come from both parents if it is to be seen.

The non sex-linked colors are a little easier for most people to deal with. For any of these, the bird must receive the same mutation gene (whiteface, pied, etc.) from both parents, the only exception being the recessive pastel which can be produced by receiving a pastel gene from one parent and a whiteface or pastel gene from the other parent, and it does not matter which parent gives which gene to produce pastels. If you have a pair of birds producing pieds (or silvers or whitefaces) then you know that both of those parent birds each carry that gene. Virtually all mutations will co-express, meaning you will be able to see them simultaneously, although some cover others up or change them to the point they’re not recognizable. For example, if you have a pair of normal grays, each split for whiteface, silver and pied, you can see every combination of gray, whiteface, silver and pied. Most of your chicks will be normal grays (based on the statistics involved), but each gene has the potential of being passed to every chick. And when both parents donate one of these mutations to the same chick, you will see it in that chick. The mutations do not have priority over one another; about the only time you see this is with normal grays, and with the pastels.

As mentioned above, when some genes combine, you won’t always recognize the result. For example, you never mix recessive silver, fallow or spangle with each other or with cinnamon or lutino. If you produced a lutino silver or a lutino fallow, it would look pretty much like a lutino with some sort of bizarre cast to its color. And a silver fallow isn’t exactly an attractive bird, either. These mutations are very soft and should be appreciated by themselves. Silver, fallow and spangle do mix nicely with pearl, pied, whiteface, yellowface and pastel, but I wouldn’t recommend mixing them with any other color. Pied isn’t "covered up" by any other mutation, except lutino. Even then, the bird is still a lutino pied; you simply cannot see the pied because there’s nothing for the yellow to show through. Whiteface also shows through every mutation, except for pastel.

I’ve not had a lot of experience with the dominant mutations. It is my understanding they are all non sex-linked. The dominant gene can be on either parent. It may occur on one or both chromosomes, which is referred to as "single factor" and "double factor". A single factor dominant mutation bird will produce single factor offspring roughly half the time (statistics again). A double factor dominant mutation bird will produce single factor offspring all of the time. Note: both of these are based on only one bird in the pair carrying a particular dominant trait.

Theoretically, you could produce a bird carrying all of the mutations listed. It would be a dominant silver pied, but I don’t know what to predict with the dominant pastel and dominant yellowface being together, probably something pastelish. Why would you want to produce such a bird? You wouldn’t. Remember that each color mutation we see in cockatiels is attributed to a defect on one part of the color portion of the X-chromosome. The color portion you think of when discussing genetics only deals with a very few genes out of literally thousands. With each mutation, many of which have appeared due to line breeding (inbreeding), you also potentially introduce other defects into the bird’s chromosomes. Genetics isn’t just about color; it is also about the size and length of a bird. It’s about the bird’s immune system. It’s about the size and shape of the eyes, head and body. It’s about every thing that makes a cockatiel a cockatiel, from the inside out. There are a lot of things to consider when selecting birds to breed beside what color they are. In the next issue I will address those topics.

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