By Dr Clive Dalton
The normal distribution
• This is what you are really dealing with every day.
• Realise that you cannot have all top animals. Your flock, no matter how long you’ll work at it will always show the classical “normal distribution” or bell curve.
A normal distribution or bell-shaped curve.
- If you weigh all your sheep and plot their weights in groups along the bottom from lowest to highest, the finished shape will usually come out as a bell-shaped curve with most animals around the middle where the average is.
- If you draw a line down from the shoulders of the curve to the base where it changes direction, you’ll see that two thirds of the individuals will be in that area around the average, one sixth will be in the bottom end on the left, and sadly only one sixth will be in the “top” animals on the right.
- So to make progress and move the average to the right, you need to breed only from the top ones and keep as far away from the average as possible. If you breed from average animals you’ll generally get average animals!
- If all you ever do is to cull off the bottom of the flock each generation, you won’t get very far. Genetic progress comes from exploiting the top animals.
- Once you have improved the average, you’ll still have the same number of above and below the average stock in your flock.
- Of course the fastest way to improve the average of the flock would be to feed them better but this would not affect their genetics as it improves the environment and not their genes.
- However, the main thing about genetic gain is that although it may be slow, it’s permanent unlike changes brought about by improving the environment.
- Environment has a massive effect on the animal’s genetics, as many an animal’s “genetic potential” has been masked by poor rearing which is environment (E).
- We can never know how much of an animal’s performance is G and how much E.
- Many people in the past have paid big money for a ram and have mistakenly bought E when they thought they were buying G.
- Their error was not apparent until five years later when the ram’s genes were spread through at least three generations of offspring and the animals bred from them!
- When you look at an animal you see its “phenotype”.
- Realise that this doesn’t always reflect how well it will perform, or how well it will pass on its genes or “genetic potential” to its offspring.
- An animal’s genetic potential is called its “genotype”.
- So phenotype may not always be an accurate indication of genotype because of the environment.
- This is put into a neat formula that says an animal’s Phenotype equals its Genotype plus the Environment or P=G+E.
- We also talk about how strongly traits are inherited or passed on from parent to offspring. Some traits like growth are regularly passed on to the next generation – i.e. are highly heritable or have high heritability.
- Other traits like fertility and survival are not regularly passed on to future generations so have low heritability.
- It all depends on how strong the effects of the environment are – if environment effects are large the result is low heritability.
- So don’t be surprised if you see poor-looking animals that produce good offspring, and some good looking prize winners being a disappointment in the offspring they produce. The good-lookers had mediocre genetics but had been well reared, well fed and prepared for sale by a skilled stockperson.
- The basic rule is to buy on genotype and not just phenotype, and to do this you need to understand about performance records (see later)..
You can forget everything else about genetics as long as you understand this next bit. By this time in most animal breeding lectures students are sound asleep when they should leap up and say – “Ah is that all there is too it?”. To make some real genetic gain in the flock in your human lifetime you have three things to get right:
- Heritability – i.e. how strongly or weakly inherited the trait is. See below.
- Selection Differential. This is the amount by which the animals selected as sires and those selected as dams are above the average of the flock. You calculate this for sires and dams and then add them together and divide the sum by two to get a mean value.
- Generation Interval. This is the average age of the parents when the offspring are born and is about 3 years in sheep. There’s a limit to this as in practice you can’t breed from animals before puberty.
Genetic gain/year = (Heritability x Selection Differential) / Generation Interval
Think about the maths in this formula. When you multiply things together on the top line, the result gets bigger. When you divide by something on the bottom line the result gets less. So to really make rapid progress in your human lifetime, you need to:
- Select for clearly-defined traits that are strongly inherited – and restrict their number to no more than three. Go for high heritability.
- Select parents as far above the average of the flock as possible. Go for a high Selection Differential.
- Keep the generation interval short by breeding from young animals.
Breeders regularly get it wrong and end up being very frustrated at having made no genetic progress in their lifetime. They select far too many confused traits that are weakly inherited, they select only average animals as parents when they are short on numbers of top stock caused by environmental problems, and they only keep offspring from their older proven animals. This formula is the core of animal breeding so think about it a lot.
- We talk about how strongly traits are inherited or passed on from parent to offspring. Some are passed on regularly or have high heritability. Some are low or are weakly inherited (see Table).
- Notice how any traits related to fertility or survival are low because the environment has such a large impact on them, and how wool and growth traits generally have high heritability.
Key points from the table
- Think of these values as percentages.
- Anything above 30% is high.
- Traits involved with fertility are low.
- Growth traits are high.
- Wool traits are high.
In the old days breeders didn’t worry about performance records. They said they didn’t need them because their knowledge of the pedigree of the animal and its offspring, and their skills at predicting performance from what traits they could see on the animal were more than enough. So by eye they could tell you which were the best sheep. The definition of “best” was where all the fun began!
The trouble here is, and you’ll still see it today, the human memory often recalls the good ancestors and forgets the bad ones. Go to a thoroughbred horse auction to see this where races lost are never listed in the catalogue or given out by the auctioneer – only the races won.
So from this ancient culture of “eye appraisal” came the whole business of “stock judging” and exhibiting stock which is still alive and well today, and is taught to young folk around the world who aspire to be animal breeders. Part of this is the belief that good stock judges are born and not made as the skills are not easy to learn.
Correlations between traits
Understanding correlations is an important part of population genetics as they are used widely in the calculation of Breeding Values and Indexes. A correlation is where one trait is related to another trait, so you can use one trait you can see or measure easily, to predict another that you can’t see or measure. There are two kinds of correlations – genetic and environmental.
Genetic correlations are where the genes for one trait also control the genes of another. Environmental correlations (non-genetic) correlations are where the environment affecting one trait also affects the other trait.
Environmental correlations are also called phenotypic correlations. Correlations can be positive (where both traits improve together) or negative (where as one goes up the other goes down).
- There is a high positive genetic correlation between body weight and fleece weight, so if you select for heavy sheep you will genetically increase fleece weights. Also, if you select for high weaning weights, you’ll genetically improve all later live weights fairly rapidly.
- There is a small negative genetic correlation between number of lambs born (NLB) and hogget fleece weight. So when you select for NLB you’ll genetically reduce hogget fleece weight (HFW). It may take a long time though.
- Examples of an environmental correlation is in a poorly-fed flock where all ewes rearing twins have superfine wool. This has nothing to do with genetics but is simply because the sheep have been starved and produce “hunger-fine” wool.
Here are some historic examples of beliefs that grew up before people started to check what was fact and what fiction:
- Wrinkles were introduced in the Merino by American breeders who believed that they increased surface area and hence fleece weight. Wrinkles became fashionable and spread to Merinos in Australia where breeders never checked the facts before they almost ruined the breed. It was found to be nonsense but wrinkles and their problems still remain.
- In NZ Romneys as an indicator of high wool production, breeders selected sheep with wool over their eyes and face and on their legs right down to their hooves. This didn’t increase fleece weight at all, but added enormously to shearing costs and frustrated shepherds trying to muster wool-blind sheep.
- Deep bodies and good “spring of rib” were supposed to indicate plenty of room for the heart and lungs to function, so deep bodied sheep were said to be more efficient.
- Short-legged “dumpy” sheep were wrongly believed to have more meat content than “leggy” sheep. So for 30 years sheep got smaller with short legs which increased lambing problems and made then unsuitable for hill country. Lambs born and lamb survival also got worse. Dumpy sheep have now gone.
Combined eye appraisal and records
Some breed organisations and sheep classes at livestock shows have recently tried to incorporate performance records with eye appraisal. It’s a very difficult if not impossible challenge for a judge and the practice has not gone very far as yet. It’s all a bit too difficult.
This material is provided in good faith for information purposes only, and the author does not accept any liability to any person for actions taken as a result of the information or advice (or the use of such information or advice) provided in these pages.