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diversity is reduced, the likelihood that a puppy will receive two recessive alleles (copies of the gene) is in- creased. It is especially difficult to breed recessive traits out of a population, because they are not always visible in the parents (unless or until genetic testing is avail- able). As a result, carriers continue to be used for breed- ing, transmitting the TMT cardiomyopathy allele to approximately half of their offspring, who will in turn be carriers.


An important strategy for avoiding the accumulation of deleterious recessive alleles in a population is out cross- ing and maintaining as much diversity as possible in the population. This can be accomplished by using a variety of sires rather than only the most popular ones and breeding more bitches for fewer litters each. If dogs with affected relatives are bred to unrelated dogs, the odds that two recessive alleles will come together in a puppy will be reduced. With more inbreeding, the alleles are more likely to be identical by descent and, depending on the family of Toy Manchesters you are working in, they could be the genes related to cardiomyopathy.


Many breeders wonder how to use inbreeding coeffi- cients to make decisions about which parents to use for the next generation, and how far back in the pedigree to use data when making the calculations. Most modern software can process as many generations of data as you are able to get, and you should use as many generations as possible. There are many common ancestors in the TMT pedigree further back than we were able to show in Figure 1. If you eliminate that data from your calcula- tions, you are underestimating the inbreeding coeffi- cient. There is no fixed number describing an optimal inbreeding coefficient. The most important thing to keep in mind is that as the inbreeding coefficient increases, so does the risk of producing puppies with a problem. The best way to reduce the risk is to choose breedings that result in puppies with the lowest possible inbreeding coefficients while keeping in mind your other goals for your breeding program.


Another frequent question concerns whether relatives of affected dogs should be used for breeding. They can be used with the understanding that their offspring will be at increased risk of developing or carrying TMT cardio- myopathy. So, for example, if this disease is, as we be-


Genetics 101


Simple Inheritance Patterns for a Trait Caused by a Gene at One Locus


When a trait is caused by a gene at a single locus, there are a limited number of ways inheritance can occur. Since TMT breeders are already famil- iar with von Willebrand's Disease (vWD), we’ll use that as an example. Keeping in mind that one copy of each gene is inherited at random from each parent, we can think about ways in which the two copies of a gene can interact.


Dominant: A dominant trait will be expressed every time it is inherited, whether the individual receives one or two copies of the mutant or dis- ease-causing allele (alleles are the different forms of a gene; usually we are talking about a normal version and a mutant, disease-causing version).


Recessive: A recessive trait will only be expressed if the individual inherits two copies of the mutant disease-causing allele, one from each parent. Each parent must have at least one copy of the disease causing allele (one copy would make them a car- rier while two copies would make them affected with the disease). This is how vWD is inherited in TMTs. Each carrier has a 50% chance of passing the disease-causing allele on to his offspring. If two carriers are mated, about 25% of the puppies will be affected, 50% will be carriers, and 25% will be clear. (For more about eliminating reces- sive traits, see http://doggenetics.com/ popgenforweb.html).


Sex-Linked: The inheritance of sex-linked traits depends on the sex of the individual. Each dog has 78 chromosomes in 39 pairs. 38 of these pairs contain the same genes in every individual, but the two sex chromosomes (X and Y) differ. Each male has an X and a Y chromosome, and each female has two X chromosomes. Males only get X chromosomes from their mothers, and since they only have one, they are affected more frequently by X-linked recessive traits than females, who would need two copies of a disease allele to be affected.


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