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Population normal distribution

Within any population of animals there is what is known as a normal distribution around the population average. This is illustrated in Diagram 1 and is commonly called a ’bell curve’.











A normal distribution tells us that the majority of animals in the population lie reasonably close to average and that there are decreasing numbers of animals at the extremes, high or low, in expression (or value) of each trait. By knowing which animals differ significantly from average we can select from this group to move’ the population average for traits of economic importance in the desired direction (whether it be up or down).

While the shape of the bell curve may differ, the principle of normal distribution of animals’ performance applies to almost all traits that sheep breeders are interested in. Faecal egg count distribution is a noted exception.

Genetic improvement

Genetic improvement occurs in a population when the average genetic merit for a trait is moved in a desired direction, e.g. weight gain and ribeye depth increase, or fat depth decreases. That is, when the average of the population is moved, as illustrated by the bell curve in Diagram 2.












It should be noted that for some traits the breeding objective might be to maintain the current breed average and endeavour to minimise variation (eg birth weight, where moderate birth weights are desirable as opposed to lambs that are small or heavy).


The complete Lamplan Breeders Guide is available here.


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Diagram 1: Population normal distribution or ‘bell curve’

Diagram 2: Genetic improvement is ‘shifting the bell curve’

Average

values

Lower

Higher

Average

values

Lower

Higher

No. of Animals

No. of Animals

Genetic improvement

e.g. fat decreases

Genetic improvement

e.g. weight gain increases