This page applies to: Real Animals

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Genetics

There are many resources available if you want to learn about genetics in detail; a current Biology textbook is a good starting place. This website was created to explain the basic concepts of genetics to someone who needs a easy-to-follow introduction. It has lots of graphics, so the big text was used to make everything easy to read. And yes, there are this many cartoons drawn in my "real" AP Biology notes. :)

Lesson 1: Dominant and Recessive Genes and Breeding

Each trait that an animal has --- hair color, eye color, etc., is determined by its genes. Every animal has two of every gene, one from Mom and one from Dad. These genes live on certain spots on a chromosome, made up of DNA, which is like their neighborhood. A copy of these "neighborhoods" is in nearly every cell of an animal's body, and parts of the cell will knock on doors and ask the genes what it should be doing.

It is up to the genes to tell the cell what it should look like and what it should do, and these decisions can be seen in the way that the whole animal looks. Genes are like members of a committee: all of the genes (the genotype) sit down at the table and decide what the animal is going to look like (the phenotype). Each gene could be one of many flavors (called alleles) that make different things happen. Like people on a committee, genes have personalities!

Dominant genes are loud and bossy. They're always convinced that their ideas are the best, and they have to express them to everyone!

Recessive genes are very shy. They stare at their feet and just mumble something whenever they're asked a question. They might have some cool ideas, but they are afraid that they'll be laughed at if they tell the dominant genes about them, so they keep quiet.

When the committee is deciding on what the animal will look like, the genes split up into little sub-committees for each trait. Some traits are controlled by many genes (polygenic). This means that a big group of genes gets together to decide how the trait will come out. (Note: polygenic traits are harder to simulate and unusual in artificial life games.)

But for many traits, the subcommittee consists of only the copy of the gene from Mom and the copy of the gene from Dad. These two genes might discover that they're identical and they both agree that the exact same thing should be done (making the animal homozygous or pure-breeding). Then the genes don't have anything to debate, and they probably leave early to go get a pizza.

What is interesting --- and what you have to worry about when breeding --- is when the two genes are different flavors (alleles). (When the alleles differ, the animal is called heterozygous.) This means that each one has a different idea about how the trait should turn out, and they have to sit down and discuss it.

So, what happens when genes pair up?

When a dominant and a recessive gene get together, the animal is said to be heterozygous for the trait, and the recessive gene usually can't get a word in edgewise! The dominant gene won't listen to anything that he has to say. So when the decisions are made about what the animal will look like, you see only the dominant gene's ideas.

When two dominant genes get together, they may be identical --- in which case they agree on what the animal should look like. If they aren't the same, they might both talk at once (codominance) and both affect how the animal looks. Or, one of them might be the loudest and bossiest and manage to be dominant over the other gene.

An example of this can be found in tabby markings on cats. The mackeral tabby gene is dominant over the classic tabby gene, but the ticked tabby gene is dominant over both.

When a pair of recessive genes get together, they finally have a chance to put their ideas into use! Recessive genes might come up with something really cool, like beautiful satin fur on cats. But you have to be careful because their ideas don't always work, and some recessive genes will kill an animal if they occur in a pair.

Why is all of this so important?

People who breed animals (real or virtual) work hard to figure out which traits are dominant (the ideas suggested by dominant genes) and which are recessive (the ideas suggested by recessive genes) so that they can predict what they will get when they breed two animals together, and plan breedings that will result in the animals that they want --- and avoid negative traits!

Punnett's Squares

We can predict the outcome of any mating by drawing a Punnett's square. We know that the baby will get one copy of a gene from Mom and another copy of the gene from Dad and that these genes will do something together. A Punnett's square is used to calculate how many different combinations are possible and approximately what % of the babies will have a trait.

Keep in mind that the squares below are drawn for only one pair of genes; planning a breeding is going to involve looking at all of the genes. And most genes are very sociable and don't care which other genes they hang out with, so if you predict that 25% of the babies are going to show one trait and 25% are going to show another, that doesn't mean that 25% will have both traits!

Also, basic rules of probability apply. It's like flipping a coin --- you know that you have a 50% chance of getting heads and a 50% chance of getting tails, but the coin doesn't know what it came up as last time so you may get heads ten times in a row.

Back to Punnett's squares: to draw a Punnett's square, you need to know (or guess) which genes each parent has. Take the genes from one parent and write each one (by itself) along the top of two by two square:

	___	___
	___	___

Next, take the pair of genes from the other parent and write them along the rows of the square:

	___	___
	___	___

Now, notice that each empty box is the intersection of a row (with a gene) and a column (with another gene). Each empty box is one way that genes can pair up in the babies. Go through and write down the results of pairing up the genes:

		___
	___	___

You may also want to write in each box the result of that pair of genes meeting up (for example, "longhair" or "shorthair").

I am using pictures to represent genes, but most people use letters. Dominant alleles are represented with capital letters and recessive alleles are represented by lowercase letters.

Each box represents one-fourth (25%) of the two parent's offspring. If boxes are the same, then you can add them together (here, 25% + 25% + 25% + 25% = 100%).

It is possible to construct really complicated Punnett's squares, but for online games, I like to simply use this small one for each trait. After you have looked at a few, you will be able to do it in your head. There are example cases below for different combinations of dominant and recessive genes.

1. If one parent has two dominant genes and the other parent has two recessive genes, then all of their babies will have one dominant gene and one recessive gene.

Every single one of the babies are heterozygous - - and you cannot see the recessive gene in any of them. The babies can be described as carriers of that recessive gene. ->If it's rare, it might ride along from generation to generation for a very long time before it finds another copy of itself and only then will you see its effect in the babies!

2. But what if one parent is heterozygous instead of homogynous ? You won't be able to tell by looking at it - that recessive gene is hidden by the dominant gene. These animals will look identical to the pair above, so you may breed them and expect the same results - and you'll be surprised when you see:

Half of these babies show the recessive trait! This demonstrates how much offspring can tell you about the parent's genes. Until we saw the recessive trait, we might not have known that one parent is instead of .

3. Look what happens when you breed two heterozygous - - animals together:

You get a little of everything! Each parent has a hidden copy of the recessive gene, so 1/4 of the babies will express it. The other 3/4 of the babies will look the same, but some of them will be carriers of the recessive gene and some won't be.

Look for more articles on genetics in the future! :)

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