Rural Gardening: Genetic for Dummies, 1 by DrDoolotz

DrDoolotz wrote:
trying again...

BASIC GENETICS!

Ok, I’m going to try to make this simple, simple, simple. If anyone wants to argue the finer points of genetics, please d-mail me! I realize there are exceptions to much of what I am about to say. I have an MS in genetics so I am not clueless! There is no “easy” way to explain genetics but I’m doing my best here to make something folks can understand. This is my attempt at giving a basic genetics lesson without overloading anyone. If it’s too simple, let me know, and we’ll get complicated!

Inside a cell are structures called chromosomes. A chromosome is a big long “chunk” of DNA that is tightly twisted into a specific structure. All the DNA that makes up a single chromosome could contain hundreds or thousands of genes. It also contains a lot of DNA that we don’t know the purpose for (sometimes called “junk DNA” but it’s not really junk, we just don’t know enough about it yet). Inside a chicken cell, there are 78 chromosomes, while a human cell only has 46 chromosomes. They come in pairs. So, a chicken has a pair of chromosome 1, a pair of chromosome 2, etc all the way up to chromosome 39.

Because you (or a chicken) have 2 copies of every chromosome, you have 2 copies of every gene. One copy of each gene came from your mom, and one copy came from your dad. Down to basics, every egg cell and every sperm cell only has 1 copy of each chromosome. This is called a haploid cell. When the two come together, they make a diploid cell. A cell with 2 copies of every chromosome is a diploid cell. Most of our cells are diploid.

So, with chickens, the rooster gives 1 complete set of chicken genes (on chromosomes) to the baby chick, and the mother gives 1 complete set. Each gene can have multiple forms, and this can get really complicated, but for this simple outline, let’s say there are two forms: dominant and recessive. Dominant means that the dominant gene will “mask” the recessive gene. So for colour, let’s just say, that purple feathers is dominant over green feathers.

A chicken could have 2 copies of the purple dominant gene like this: PP. That chicken is purple.

A different chicken could have 1 copy of the purple dominant gene, P, and one copy of the recessive green gene, g. So they would have Pg, but the purple is dominant over the green, so they are purple too, but they carry the green gene.

A 3rd chicken could have 2 copies of the green gene, gg. That gene is recessive, but there is no purple gene to mask it. Therefore, that chicken is green.

When you have two copies of a gene that are the same, like PP or gg, it’s called “homozygous” for that trait. When you have two different copies, like Pg, it’s “heterozygous”

So, let’s do some chicken sex!
PP rooster (purple) meets gg hen (green) and they have babies. All babies get one gene from mom and one gene from dad. They each get a P gene from dad, because that’s all he has to give. They also each get a g gene from mom, because that’s all she has to give. All babies are therefore Pg. So, the parents were homozygotes, the babies are heterozygotes. They are all purple though, so you might think they were PP, but they are all carrying the green gene.

What happens if two of the babies mate?
Pg x Pg (we use x symbol to show mating crosses)

Each baby gets a P or a g from mom, and another P or g from Dad. Soooo….how do we figure this out? Easiest way is with something called a ‘punnett square.’

You put the one parent’s genes along the top, and the other’s along the side, like this:

------P------g
P
g

Then you fill in the squares for the possible combinations, by using the letter from the row and column that you’re in, like this:

-------P-------g
P-----PP-----Pg
g-----Pg------gg

Ah-hah! So, we have 25% of the babies will be PP (purple homozygous), we have 50% of the babies will be Pg (purple, heterozygous) and we will have 25% of the babies being gg (green, homozygous.

This 25/50/25 ratio is very common in offspring.

Another common ratio is 50/50, which you get from breeding a homozygote to a heterozygote, like this:
-------P-------g
g-----Pg-----gg
g-----Pg-----gg

So there, you would have 50% Pg (purple) and 50% gg (green).

So, the trouble is, that very few traits are controlled by only one gene. For example, last I heard, I think there are 20-something genes involved in human eye color, which is why occasionally brown eyed people have a blue eyed baby, etc. Same with hair colour, and of course, same with feather colour. There are MANY genes involved.

So, to start a breeding project, you should ensure that you are starting with parents that are homozygous for the trait of interest. That means they are not carrying some recessive genes that will introduce bizarre results into your breeding. How do you know if the parents are homozygous? The only way is to breed them, and to ensure that you see 100% of the offspring having the trait, and THEN breed the offspring, and ensure you see 100% of the trait again. Only that way will you be able to determine which genes are being carried by which birds. Let’s say black is dominant over white, and you start with a pair of birds (one black, one white, but the black carries the white trait but it’s masked because black is dominant), you will get completely different offspring than if you start with a black homozygote bird and a white homozygote bird. In the first case, you won’t know which babies carry which genes, but in the second case you will. In order to ensure good record keeping and knowledge of which bird has which gene, the latter is highly preferable.

I hope some of this helps, just trying to teach some basics!
Claire
p.s. Usually the dominant and recessive copies of a given gene are represented by the same letter, P and p. I used P and g for simplicity, but if you are reading other materials, you will see Pp or Gg for example.

This message was edited Mar 1, 2009 5:01 PM