Heterozygosity

Heterozygosity as an indicator of genetic diversity

Genetic diversity is a key prerequisite for animals, humans and plants to adapt to ever-changing environmental conditions and diseases. It is essential for the health and longevity of canine populations, as limited genetic variation over generations can lead to increased risk of inherited diseases, reduced fertility and overall population decline. Therefore, as a part of responsible breeding, it is important to monitor the level of inbreeding and take measures to minimise it with changes.

One of the newest tools in canine genetic evaluation, recommended as a new standard by the International Society of Animal Genetics (ISAG) from 2020, is SNP (Single Nucleotide Polymorphism) DNA profiling, based on the analysis of single nucleotide changes in DNA. This method allows not only the unambiguous identification of an individual and the determination of paternity, but also the analysis of genetic variability through the calculation of heterozygosity. This indicator plays a key role in ensuring the long-term health of the breeds bred.

The importance of heterozygosity

Heterozygosity refers to the percentage of genetic markers that have two distinct variants - one inherited from the mother and one from the father. In other words, heterozygosity indicates the extent to which an individual's DNA is genetically diverse.

The higher the heterozygosity, the greater the genetic variability of the individual, which means a lower risk of inbreeding depression. Conversely, the lower the heterozygosity, the higher the likelihood that the dog comes from a narrow breeding pool, which can mean a higher risk of genetic disease and reduced viability.

Consequences of low heterozygosity

Low genetic diversity can lead to so-called inbreeding depression, which can manifest itself in various health and reproductive problems:

Reduced fertility - fewer puppies in a litter, more frequent abortions
Higher puppy mortality - underdeveloped immune system, higher risk of infections
Susceptibility to inherited diseases - more likely to develop recessive genetic defects
Reduced resistance to disease - weakened immunity, poorer recovery from illness or injury
Reduced life expectancy - genetically burdened breeds may have a shorter life expectancy

In some breeds the genetic variability is very low, even between unrelated individuals. This means that even if two dogs appear to be unrelated by pedigree, their genetic makeup may be very similar, leading to a further reduction in heterozygosity in the population.

Measurement of heterozygosity

Heterozygosity is measured by SNP DNA profiling, where several hundred genetic markers are analysed (e.g. 230 SNPs in the new ISAG 2020 standard). The test results in a heterozygosity score, which is expressed as a percentage of the population that would be heterozygous for each randomly selected locus.

Higher heterozygosity (e.g. 45-55%) - the dog has a sufficiently diverse genetic make-up, low risk of inbreeding.
Medium heterozygosity (30-45%) - common in most breeds, but other genetic factors should be monitored.
Low heterozygosity (<30%) - high risk of inbreeding depression, crossbreeding with genetically more distant individuals is recommended.

The range for diversity varies from breed to breed and a sufficient number of samples in the database is needed to determine it. In addition, if the pool of tested dogs is large enough, the heterozygosity of a particular individual can be compared to the breed average. This data allows breeders to make better decisions when planning matings and avoid undesirable genetic combinations.

Use of genetic tests in breeding