Predicting the next generation
I can go on for ages about various genes (and no doubt will over time) but I thought I'd divert slightly for a couple of posts and talk about genetic prediction, because people often ask 'if I cross this bird with that, what will I get?'. The answer is relatively simple, whilst at the same time not particularly intuitive. As such people who don't understand genetics usually get it wrong, but once you have the right tools it's not particularly difficult. Hopefully I can explain it properly so that everyone can do it.
Gregor Mendel was a scientist and and priest who lived in the 1800s and had a deep interest in the inheritance of characteristics. He spent an unhealthy amount of time cross-breeding pea plants, and divised his rules of inheritance, which are still used today. He is considered the father of modern genetics.
Unfortunately the importance of Mendel's work wasn't understood until after his death. Like many ground-breaking scientific discoveries, his work was rejected at the time. The generally held theory at the time was that of 'blending inheritance' whereby the offspring receives all the traits of the parent, but at half strength. Many observations seemed to uphold this theory, and even after it was debunked scientifically it stayed in the popular consciousness. A prime example is that dreadful piece of genocide-by-dilution that was promoted as part of the White Australia Policy, where the idea was that if Australian Aborigines had children by Europeans, then eventually everyone would be white. It was supported by things like this:
As you can see, each generation becomes more European-looking; people thought that each generation had a dilutional effect. What's actually happening is that traits such as skin colour and the shape of facial features are not governed by one gene, but by many. Each generation only inherits one copy of each gene from the mother, and one from the father, so each time there is a smaller and smaller chance of inheriting an allele that gives a more Aboriginal phenotype.
Another piece of nomenclature: 'Genotype' is what the instructions in your genes say you should look like and how you should function, 'Phenotype' is what you actually look like and function. They are not always the same! A simple example is a sister and brother with the genes for blonde hair. By the time they are middle aged their genes still say 'have blonde hair' but you might find that, due to some other, unrelated genes, the brother is actually bald. This is also a nice example where you can tell the hair colour genes carried by the female, but not the male.
Things like height, weight, hair colour, eye colour, skin colour, shape of facial features etc are all governed by multiple, complex genes which makes them difficult to predict. Then again the Red Jungle fowl looks very different to the Green Jungle Fowl, when one meets them for the first time, due to the large number of genes that are different between them.
I would just like to say that I want a Green Jungle Fowl. Those combs look awesome.
Now if we were to try to guess the genetics of these birds above, we'd all probably be as lost as each other. Fortunately Humanity has a long and enthusiastic heritage of chicken breeding and the experience and genetic heritage that goes with it. We have many breeds of particular colours and patterns which have been standardised and whose inheritance is well understood, and we have researchers who have dedicated a lot of time and work to sussing out the genetic basis of various traits. This then makes it relatively easy to determine a single genetic trait and predict it's inheritance.
If you're still with me after that giant ramble, here's the pointy end of the post.
The tool used for predicting inheritance is something that Gregor Mendel invented, and it's called the Punnet Square. It looks like this:
Yep, that's right, four empty boxes. That's all it is.
Cool story, how do we use them?
First of all I need to explain a little more on chromosomes, genes and inheritance.
As I've mentioned before, Humans have 23 chromosome pairs, for 46 in total (and I'm too lazy to look up how many chickens have). Every cell in your body (exception rule) has these chromosomes, and they tell your body how to be what it is. One of the exceptions is your sex cells. Sperm and egg cells have only one copy of each of the 23 chromosomes, they don't have a pair. This is because the sperm and the egg will join together and their respective chromosomes will pair up, giving the resulting baby it's own set of 23 pairs. In each pair one chromosome came from mum and one from dad. Which one of mum or dad's two chromosomes gets incorporated into the sperm/egg and inherited by baby is random
. This is important.
So whilst Dad has two copies of a particular gene, baby only inherits one of his. Same goes for mum. Same goes for chickens. And whilst your daughter or son might look like a clone of one of their parents, they are also half the other one! In there, somewhere...
One final note: It is important not to confuse inheritance with expression. If an allele is Dominant that means that it expresses over a recessive allele. It does not
mean that all the offspring will inherit the dominant allele, only that those that do inherit it will show it. Predicting single gene inheritance
Most of the time when we're asking what the offspring will look like, we're asking about the results of a single gene, with the birds being identical for all other genes. Lets make up a gene here, and I'm going to call it 'A'. A has two alleles, they are A1 and A2. Now remember that ever bird has two copies of each gene. This means that a bird can be homozygous A1 (A1,A1), heterozygous (A1,A2) or homozygous A2 (A2,A2).
Lets start with a Rooster who is homozygous A1. So his genotype is A1,A1.
Lets put him with a Hen who is homozygous A2. So her genotype is A2,A2.
Note the two alleles in each bird's phenotype. Each bird will pass one of those alleles onto their offspring.
Lets take our punnet square:
What we do is we grid out the parent's genes. So lets start with Dad and what he will contribute to the offspring. We will put him at the top of the square and each allele above each column.
Then we will take Mum and do the same at the side of the square.
Now each box gives you the potential offspring combinations, and has a probability of 25%. What we do next is combine the alleles in the squares.
As you can see, every box says 'A1/A2'. This means that 100% of the offspring will be heterozygous A1/A2. This is called the F1 or First Cross.
But what happens if we then cross the offspring together, brother to sister? This is called the F2. It is rather more interesting.
Note that this time each parent can contribute either an A1 or an A2, as they carry both. When we fill in the squares we get some interesting combinations.
The results are as follows:
1 x A1/A1
2 x A1/A2
1 x A2/A2
Since each square has a probability of 25%, that gives us the following ratios:
These are called the Mendelian ratios, and are universal for single-gene inheritance. What that means for the resulting bird is dependant on the nature of the alleles' expression.
I shall let folks digest that, and then give some practical examples in the next post.