We use population genetics to study complex economic traits that are controlled by many genes. This is sometimes called “multifactorial genetics” as there are many genes involved. So improving “money making” traits in farm animals like growth and fertility is what population genetics is all about.
- The first thing we notice about these traits is that they are not expressed as being present or absent, like coat colour or polledness.
- They show “variation” and in a flock, herd or population you will see the full spread from worst to best.
- Variation is the raw material we need to change (improve) a population, and the tool we use to do it is selection.
- Selection is where the breeder decides which animals are going to be the parents of the next generation, and which are not.
- We use the bell curve or normal distribution to describe this variation.
The normal distribution
- If you weighed a group of animals and recorded them in weight ranges, you could draw a histogram like the Figure below.
- You could then smooth out the blocks with a curve – the normal distribution.
The features of this normal distribution are these:
- Drop a line from the shoulder of the curve where it changes direction.
- Two thirds of the observations will be in that area.
- There will be one sixth in the poor end and one sixth in the good end.
So this precisely describes the variation in your herd or flock. The majority of the individuals are around the average, there are not too many poor ones but neither are there many really top animals which is always frustrating.
- The distribution can be tall with a narrow base showing there is little variation, or flat and with a wide base showing a lot of variation.
- Our job as breeders is simple. We must improve the average of the population, i.e. move it to the right and we do this by:
- Culling the animals in the lower end –e.g. castrating surplus males.
- Breeding from the very best animals in the good end.
- Remember the next generation will also show a normal distribution – but hopefully the mean will have improved.
- The real practical problem is to decide which the “good” animals are and which are the “poor” ones, and what criteria to use in defining these terms.
- It’s here where we risk being confused by the old question of whether it’s nature or nurture that is important.