A gene that codes for a certain trait or characteristic can come in a number of different variants, called alleles. Genetic diversity is the range of different alleles in a population, or breed.
For example eye colour- there may be an allele for brown, blue, green and grey eye colour. If some of the colour alleles are lost, diversity for eye colour is reduced in that population.
This applies to all genes in a population, not just those coding for traits such as colour, coat, size and so on, but also to those required for health and reproduction.
An individual will inherit one allele from each parent. If the inherited alleles are identical, the individual in homozygous for that allele. If two different alleles are inherited, the individual is heterozygous. Ideally, we want a high level of heterozygosity.
The greater the diversity within the genetic pool of the breed, the better for the health of individual dogs and the entire breed- there is much published good quality research to show that this is the case.
In breeds where diversity has been lost through close breeding over many generations, there is an increase in health problems associated with inbreeding. Recent research looking at diversity in a number of breeds has identified that particularly immune disorders become more prevalent- these may be skin disease such as atopic dermatitis or sebaceous adenitis, thyroid disease, adrenal gland disorders such as Addison's disease, but also conditions that are much more widespread such as allergies and abnormal responses to vaccines.
As the gene pool is depleted, other inherited disorders become more common, too. A number of high profile breeds have been featured in the media recently, including the English Bulldog, because of major health issues in the breed associated with the loss of genetic diversity in the shrinking gene pool.
The Genetic Diversity Project at UC Davis genetic labs has looked at a number if breeds, including Havanese. Whilst the number of samples submitted by Havanese owners is relatively small, the results so far are very encouraging- we are VERY lucky to have a breed with good genetic diversity. Of course we need many more samples to get accurate information on the breed worldwide, but we are encouraged that the breed is still in good shape!
There is no place for complacency, though, as Havanese could lose diversity if breeding practices are not adjusted to accommodate new scientific findings in genetics.
This is a genetic diversity result from a Havanese with a lower than average level of inbreeding.
Selection and breeding practices have a major impact on genetic diversity. Breeders have always tried to do the best they could for their breeds, and they are experts in many aspects of breeding. There are a number of criteria responsible breeders use to select appropriate mates for their litters. These include breed type, structure and conformation, temperament and health.
Breeders also try and balance maintaining breed type with bringing in 'new blood'.
One way to assess the level of inbreeding is to study pedigrees and search for common ancestors in prior generations.There is pedigree software that help breeders estimate how closely inbred litters would be when using a specific pair of parents, and those estimates are based on pedigree data. You may have heard of the Coefficient of Inbreeding, which is what most of these programs calculate. The Coefficient of Inbreeding is a statistical estimate of how likely the offspring are to inherit the same allele from both parents, how high the level of homozygosity will be- so it gives us an idea of the level of diversity in the individual. So if the COI is 30%, that dog may have up to 30% of its genetic material lacking diversity.
However, now we don't have to rely on estimating the degree of inbreeding from pedigrees alone- whilst that is still important, we now have DNA testing that can tell us how closely related two dogs actually are. This means that from a shortlist of possible mates, we can select the pairing that would result in the most diverse offspring.
This is a genetic diversity result from a Havanese with a higher than average level of inbreeding.
The genetic pool of any breed is limited by the fact that dogs within the breed are only bred to each other, there are no new alleles coming in. This makes it very important that we try and preserve existing diversity, by avoiding inbreeding and utilising many different Havanese breeding lines in our breeding programs. Use it or lose it!