Genes are what we continually hear people talk about. What is a gene? A gene itself is made up of DNA (deoxyribonucleic acid) which determines the genotype and therefore the phenotype (outward appearance) of the animal in question. The DNA itself is very long but is folded to make it take up less space. Thousands of genes combined together form what is known as a chromosome.
Inside all the cells in the body of the dog we find that each cell contains 39 chromosomes that are paired together (meaning that there are two copies joined together). This is not the case for the sperm and the egg cell. These cells only contain one copy necessary for fertilization. When the single 39 chromosomes of the sperm cell combine and join with the single 39 chromosomes of the egg cell, a new cell (individual) is created containing 39 paired chromosomes.
Dog breeding is like playing cards. You have two decks of cards, one deck of cards represents the chromosomes of the sire and the other deck of cards represents the chromosomes of the dam. Each dog has 39 chromosomes, thus each deck contains 39 cards (each card represents one chromosome of which thousands of genes are attached).
When the two decks of card are combined and shuffled we can compare this to the sperm cell fertilizing the egg cell. We remove ½ of these cards and this represents our new deck of cards. This can be compared to the new genotype of the new individual created during conception. If this sounds simple and logic it should be taken into consideration that each chromosome (each card) contains thousands of genes that each act separately becoming active or inactive when combined with the genes of the chromosome received from the other parent. This is a crude explanation bit it should give you a better insight as to how genes combine and interact during conception.
The above example illustrates that during conception there are millions of different genetic possible combinations that can be created. It is therefore impossible for two dogs in a litter to be 100 % identical (they may be similar in phenotype but not with regards to their genotype). This means that on the outside appearance (phenotype) a dog can never be 100 % “identical” to one of its parents or littermates. They may resemble each other quite closely bit this is dependent as to which genes are inherited from which parent and in which sequence.
Yet in the world of dog breeding we are continually encountered with some dogs and bitches whose offspring have inherited a lot of their features (working ability and conformation). The question quickly arises as to how this is possible? Coming back to our deck of cards we take one card from the stack and examine it. Remember that this card represents one chromosome on which thousands of genes are attached. These genes each possess two alleles which determine as to how the gene is expressed. There are three possible options which are as follows:
- One allele is dominant (A) and the other allele is also dominant (A) resulting in an (AA) genotype. This gene is represented in the dominant form. This gene is referred to as being homozygote dominant.
- One allele is dominant (A) and the other allele is recessive (a) resulting in an (Aa) genotype. This gene is also represented in the dominant form (A) prevents (a) from displaying itself. This gene is heterozygote. On the outside (phenotype) the trait for this gene is identical to that of the gene that is homozygote (AA). For example a black Labrador that is dominant black (homozygote) cannot be distinguished by a black Labrador that carries yellow or chocolate (heterozygote) on the outside.
- One allele is recessive (a) and the other allele is also recessive (a). This gene is represented in the recessive form (aa) (homozygote recessive).
In the case of example 1, this gene (trait) will always be passed on to the next generation and expressed in the dominant form regardless of what the allele of the other parent is. This is referred to as homozygote dominant (AA). The dominant black Labrador can only produce black offspring regardless of which colour the bitch is.
In the case of example 2, this gene (trait) is not always passed on to its offspring. This is dependant as to what the allele of the other parent is. Should the parent have a gene code of (Aa) and the other parent has the same gene code (Aa) then 75 % of the offspring will express the dominant trait (AA or Aa) and 25 % will not have this trait (aa). An example is the black Labrador that carries yellow who is bred to another black Labrador that also carries yellow. In the litter 75 % of the puppies will be black and 25 % will be yellow.
In the case of example 3, this gene (trait) is recessive and does not occur frequently in the population. It could be a desired trait or an undesired trait. Should one parent be (aa) and the other also be (aa) then all the offspring will be (aa) and exhibit this trait. An example is of two yellow Labradors bred together producing only yellow puppies. Some readers may object since it has occurred in the past that on very rare occasions that two yellow Labradors did produce a black Labrador. This is referred to as reverse epistasis (when two recessive genes work against each other, but this is a VERY rare incident).
Unfortunately things still become more complicated because many of the desired traits breeders strive to breed for such as correct tail set, correct front and rear angulation and head type are regulated by many genes in long sequences (for example AABBCCDDEEFFGGHHIIJJKK). Most of the traits that are of interest to breeders are influenced and regulated by numerous genes. These are referred to as polygenic traits.
To ensure that all (or most) of the offspring look like one parent or have the desired attributes that were selected for to produce the desired traits in the offspring means that at least one of the parents must have the dominant genes (AA) for the desired trait and no (aa) genes. Unfortunately it is rare to find a breeding animal that contains such a dominant genotype for a certain trait (AABBCCDDEEFFGGHHIIJJKK). Usually such traits are exhibited by the parent(s) but in the heterozygote form (AaBbCcDdEeFfGgHhIiJjKk). The parents when bred to each other result in offspring having different genetic compositions for the desired trait resulting in not all or sometimes none of the offspring exhibiting the desired trait.
When a dog/bitch has the genotype of AABBCCDDEEFFGGHHIIJJKK for a specific trait this dog is homozygous dominant for that trait. This is often referred to as the dog being prepotent for such a trait or numerous traits. People will say that stud dog X can be bred to a goat and still produce Labradors! The ultimate test to find out as to how prepotent your dog is (confirmation lines) would be to breed him/her to a field trial dog (two totally different types). Should the dog/bitch in question be prepotent, than all of the offspring born would strongly resemble the dog or bitch in question. The offspring would have inherited the dominant gene from their sire or dam causing them to look like them and not like the other parent.
This is where line breeding becomes a useful tool in upgrading canine families. The principle of line breeding implies that the dog and bitch that are bred to each other are related with the intention of making sure that the genotype of the offspring contains the dominant genes required for the desirable traits (example front angulations). This increases the chance that the offspring will possess the homozygote dominant genotype for a particular trait(s). The intention is to ensure that the genotype is represented in a homozygote form (example AABBCCDDEEFFGGHHIIJJKK) thereby fixing this trait in the genotype of the next generation.
When an animal form such a combination is bred to an unrelated animal lacking the desired features, the underlying theory is that the offspring will inherit the dominant gene/allele from the line bred parent allowing the offspring to have a genotype containing at least one allele that is dominant (A_B_C_D_E_F_G_H_I_J_K_). This allows the desired trait to be expressed again in the next generation.
This is not to say that all line bred dogs are prepotent. It is also possible for a dog with an outcrossed pedigree to be prepotent (for example having no common ancestors in the first 5 generations). The chance is greatly lessened, but is possible when one breeds type to type with the hope that the desired dominant genes are on the correct chromosomes and passed on in the correct sequence to the next generation.
The above examples and explanations give a crude explanation as to how genes interact with each other. It should only been seen as a “blue print” with regards to understanding as to how genes interact with each other. The point made is that when conception occurs all of the millions of genes of both the sire and dam combine in this way resulting in numerous possible combinations. It may result in offspring displaying traits that their parents displayed and selected for but the contrary may also occur with undesirable genes and traits coming to the surface. People will tell you that “dog breeding is like gambling, you never know what you can expect.” There is a lot of truth to this point but understanding genetics and knowing how certain traits are inherited results in the breeder being able to manipulate this genetic lottery to their advantage.
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