Understanding genetics in pigs

The first step in improving your pig herd using genetics is to understand some of the concepts about how genes influence the traits and characteristics of your pigs.

How genes influence the traits of your pigs

All cells of living organisms, including pigs, contain genes. These genes drive all the biochemical processes that make up life and determine the characteristics that make up individuals. In other words, individuals vary because they have different genes and because these genes have adapted to different environmental conditions.

Genes are distributed along thread-like structures called chromosomes, which are arranged in pairs. A pig has 19 pairs of chromosomes. One member of each pair comes from the sire and the other from the dam.

The action of genes on pigs can be either simple or complex.

Simple inheritance

A trait that is passed onto offspring by simple inheritance is usually under the control of a single pair of genes. If a dominant and recessive gene are both present, the dominant gene will determine the trait.

An example of this is how pigs inherit either white or coloured coats. Most genetic diseases in pigs are also inherited this way. Genes that cause abnormalities are usually recessive to the normal (dominant) genes with which they are paired, so an animal carrying only 1 defective (recessive) gene is normal.

Not many economically important traits in pigs are passed on by simple inheritance. One exception is stress susceptibility, under the control of the halothane gene. Under high summer temperatures and long transport distances to market, a pig with this gene has an increased chance of stress death, resulting in an inferior quality of meat (pale, soft pork).

Complex inheritance

Most economically important traits are more complex. Each animal in a population will exhibit a trait somewhere along a smooth variation from one extreme to the other.

Economically important traits that are inherited in a more complex manner can be divided into 2 groups:

• performance traits
  • growth rate
  • food conversion ratio
  • carcass leanness.
• maternal traits
  • conception rate
  • litter size
  • piglet survival
  • piglet growth.

Growth rate example

Each pig has some positive genes on its chromosomes that increase growth and some negative genes that decrease growth.

In a pig of average growth, positive and negative genes balance. Fast growers have more positive genes and slow growers have more negative genes.

Taking 1 pair of chromosomes from each, we can depict the 3 growth types as faster growth, average growth and slow growth. All these types of animals exist in any unimproved pig herd.

A selection program for growth aims to increase the number of pigs with a high proportion of positive genes by making sure that animals picked as parents have a high proportion of positive genes.