Autosomal Recessive Inheritance

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This second article in the CA Information Center is used with permission from the Scottish Terrier Club of America. One of the tools to control genetic problems is an understanding of how they are inherited.

Many canine health issues can be attributed to a genetic condition or disease that is inherited from the parents. Genes, or unique sequences of DNA material, are the way that these genetic conditions are passed from parent to offspring.

The canine species has a total of 78 chromosomes and all dogs, regardless of the breed, have similar chromosomes. Chromosomes are large enough that they can actually be observed under a microscope. We also know that every cell in a dog contains a copy of the same 78 chromosomes and that they are organized within a cell as 39 pairs. Figure 1 below is an illustration of these chromosome pairs. Note that the chromosomes are labeled 1 to 38 and the last one is labeled as the X-Y chromosome pair.

Figure 1.  Canine Chromosomes (39 pairs)

Genes are specific sequences of DNA material that actually control the characteristics of living beings. These genes are carried at specific locations on each chromosome. Each chromosome contains hundreds (or thousands) of gene sequences that actually control inheritance. In order to work, the DNA material on each chromosome pair must come together and create a complete gene. This means that every gene receives half of the DNA material from the sire and the other half from the dam.

Some inherited traits are simple traits. These traits are controlled by a single gene that is ultimately located on a single chromosome pair. As mentioned above, this means that one gene (with half of its DNA material coming from the sire and the other half from the dam) controls the trait.

Other inherited traits can be polygenic.  This means that more than one pair of genes control the inherited trait.

Conditions that are controlled by a single gene fall into one of four inheritance patterns. Through scientific observation it can be determined if the inheritance is a) autosomal recessive, b) autosomal dominant, c) X-linked dominant or d) X-linked recessive. Note that the term “X linked trait” (both dominant and recessive modes) is used to refer to a condition that is linked to a gene found on the X chromosome. All other single gene traits are collectively referred to as “autosomal” or non-sex linked.

If scientific observation shows that a genetic condition follows an autosomal recessive inheritance pattern, this determination implies that the manifestation of the condition is controlled by the presence of one recessive gene pair. For a dog to be affected with the trait (e.g. actually express the condition) both chromosomes must carry the same recessive gene. Conversely, if either parent passes on a dominant copy of the gene, then the dog will not display the condition (but the dog may still be a carrier of the condition).

Not all autosomal recessive conditions are necessarily bad.  Some may actually be desirable.  Therefore it would be incorrect to label all recessive gene as “defective.”

The following table shows all of the genetic possibilities for inheritance of a simple autosomal recessive condition. The genetic possibilities are 1) BB, 2) Bb or bB and 3) bb. These representations follow a standard format where the large B stands for the dominant gene; while the small b represents the recessive gene. The only dogs that will display the autosomal recessive condition are the third group, because these dogs have a copy of the recessive gene on both chromosomes. The dogs with Bb or bB will actually be normal in phenotype but can produce the recessive trait if they are bred to another dog that carries the recessive gene. Dogs that have a little b gene on either chromosome are called carriers because they don’t actually exhibit the genetic condition, but they are capable of producing it in their offspring.

 Table 1.  Inheritance of Autosomal Recessive Genes.

Notice that Table 1 does not label either parent as the Sire or the Dam because autosomal characteristics are not dependent upon the sex of either parent. The only factor is whether or not each parent is either an actual affected dog or a non-affected, carrier of the recessive gene.

Another way to describe this inheritance pattern is to use several color coded simple inheritance charts as illustrated in Figure 2. This figure uses Green, Yellow and Red Scotties to depict the genetic characteristic of each OES.

Conclusions

Simple autosomal recessive genetic conditions are inherited only from parents that were themselves affected by the condition or were a carrier for the recessive gene. Both affected and carrier parents can pass the recessive gene on to their offspring

• Both parents of an affected animal are "obligate" carriers of the recessive gene.
• Breeding carriers will not always produce an affected offspring.
• Clear bred to Clear will only produce Clear offspring.