The sum of a dog’s genetic material can be thought of as a cook book which is split into chapters containing recipes. These recipes are the dog's genes and the letters that make up each recipe is its DNA. Just as a recipe can be used to make a dish of food, a gene can be used to make a protein, a building block of a dog's body.
Read the following information to find out what DNA is, how a gene is made and how these translate into a dog’s body. Learn what happens when genes, or translation, goes wrong and how this can impact on a dog's health.
DNA, genes and chromosomes
What is DNA?
How are a dog’s genetic instructions stored?
The importance of DNA's structure
How many base pairs does a dogs DNA have?
What is a gene?
How many genes does a dog have?
What is a chromosome?
How many chromosomes does a dog have?
Traits and inheritance
Dog breeders carefully choose which dogs to breed from based on a number of different characteristics, such as the way it looks, its general health, its temperament, etc.
A breeder’s aim will be to produce puppies that have similar desirable characteristics to their parents. The process of passing characteristics from parent to offspring is known as inheritance, but how are these traits determined?
What controls characteristics?
What is the function of a gene?
Alleles give variation in characteristics
Homozygous and heterozygous
How are genes passed from parent to offspring?
A dog’s sex cells (sperm or an egg) contain only half of its DNA, with one of each allele being randomly selected. When a sperm and egg come together to form a new set of DNA, the two halves combine, so that each puppy has two copies of every gene, one inherited from its mother and one from its father.
Genotype and phenotype
The combination of alleles a dog has is known as the genotype. The physical characteristics a dog has in known as its phenotype. How the genotype (the dog’s genes) influences the phenotype (the way it looks) is not always straightforward, but some of the mechanisms of gene expression are outlined below.
Dominant and recessive alleles
Alleles can be said to be either recessive or dominant. A recessive allele is only expressed (influences the characteristics of the dog) if both alleles are the same. A dominant allele on the other hand is always expressed, even if it is accompanied by a different allele.
A genetic diagram (or punnett square) can be used to show how dominant and recessive alleles work. Letters are used to symbolise the genotype (the alleles a dog has). A capital letter represents a dominant allele and a small letter represents a recessive allele. The example below shows a made up punnett square for coat colour with the B representing a dominant allele for brown fur and the b representing a recessive allele for blonde (or yellow) fur. In the example below, both parents have a genotype of Bb. Since the B is dominant, then any offspring that has a Bb or BB will be brown, while offspring that has two copies of the recessive b will be yellow.
A blending of phenotypes can sometimes occur when an individual has two different alleles. Using the example in the punnett square, an individual with BB would still have brown fur, an individual with bb would still have yellow fur, but an individual with a B and a b would have a coat colour somewhere between the two.
(light brown fur)
(light brown fur)
For some characteristics, two alleles can both be expressed at the same time. A good example of this is the blood type AB in humans. Individuals with type AB blood produce both type A and type B blood.
Multiple allele series
These are traits that have more than two possible alleles. A dog will still only have two copies of each gene, one from each parent, but there will be a variety of possible alleles within the population. A good example of this is once again blood type in humans, where there are three possible alleles, iA, iB or i. An individual can therefore be iAiB, iAi, iBi or ii. Having more than two alleles increases the possibilities of the phenotypic characteristics in a population.
These genes influence the degree to which other genes control their characteristics, e.g. the coat colour pattern of piebald spotting (pigmented spots on an unpigmented white background) in dogs can be more colour and less white, or more white and less colour, depending on whether a plus modifier or minus modifier is present.
Sometimes the effect of one gene can mask the expression of another unrelated gene. Coat colour in Labrador Retrievers is a good example of this. A black coat colour allele (B) in Labradors is dominant, while a brown coat (chocolate) allele (b) is recessive. Despite this, a second gene found in a different area of DNA can override these and create a yellow coat. A yellow coat is produced a Labrador is homozygous recessive, i.e. has two copies of a recessive allele.
|Black||BBEE, BbEe, BbEE, BBEe|
|Brown (chocolate)||bbEE, bbEe|
|Yellow||BBee, Bbee, bbee|
These genes can either switch on, or switch off the expression of other genes. These regulator genes are commonly used during development, soon after conception, and are used to ensure that certain proteins (and therefore parts of the body) are made at the correct times. These genes are also used as a dog develops and changes throughout its lifetime.
Some genes do not have an impact on the individual unless certain environmental factors occur, e.g. the genes that cause multiple sclerosis in humans can be triggered by the Epstein-Barr virus.
These are genes inherited by both men and women, but are usually expressed by only one of the sexes. A good example of this would be the genes that control the amount of milk a female dog can produce, which will be found in males, but will not be expressed.
These are genes that are expressed in both sexes, but in a slightly different way. An example of sex-controlled genes is gout in humans. Both men and women can have the genes, but 80% of men who have the gene develop gout, while only 12% of women are affected.
Some genes can have a different impact depending on the sex of the parent that they were inherited from. If an allele from the father is imprinted, then is silenced, or doesn’t work, and only the allele from the mother is expressed and visa versa.
One gene can sometimes be responsible for two or more characteristics, e.g. the gene for a merle coat colour can increase the risks of deafness and eye defects when a dog has two copies of the merle allele.
Some inherited diseases become more severe with each generation that inherits them. Segments of these defective genes are doubled with each generation and so worsen the effect.
Many characteristics are controlled by more than one set of genes and are known as polygenic traits. A good example of this will be your dog’s size, which will be controlled by the large number of genes which produce their legs, paws, back, head, etc.
Coat colour and eye colour can also be controlled by a number of different genes and may not be inherited in a simple way.
From DNA to protein
What is a protein?
How are proteins made?
Why are two steps required to make a protein?
What is transcription?
This is the first step in decoding DNA’s code. In the cell’s nucleus, a copy of the code is made in order to transport it out of the nucleus and in to the cytoplasm. To initiate this process, the DNA molecule unwinds and separates. An enzyme (RNA polymerases) travels along the unwound DNA and builds a new complementary version of the code, called RNA (ribonucleic acid). RNA is similar to DNA apart from
The particular type of RNA that is made is called messenger RNA (or mRNA) because it carries the information, or message, from the DNA in the nucleus into the cytoplasm.
What is translation?
How is protein production regulated?
Mutations and disease
Why do inherited conditions occur?
The impact of a mutation
What type of error can occur?
The DNA sequence of a gene can be changed in several different ways:
- Missense mutation: one base pair is changed and the type of amino acid that is produced is different
- Nonsense mutation: one base pair is changed which causes the cell to stop building a protein where the error has occurred. This results in a shortened protein that may not function correctly, if at all
- Insertion: when one or more bases are added into a region of DNA
- Deletion: one or more bases may be removed from a sequence of DNA
- Frameshift: each strand of DNA is made of sequences of bases which are “read” in groups of threes, called codons. Each codon produces one amino acid. The deletion or addition of one or more DNA bases can change the way in which a gene is “read”, shifting the reading frame along and resulting in a faulty protein
- Point: just one base is changed in a DNA sequence – this may be silent, missense or nonsense
- Silent: a change that occurs still produces the same amino acid as before and has no impact on the protein produced
- Splice site: a change to a number of bases that causes a gene to be incorrectly copied into mRNA during transcription
- Chromosomal translocation: part of a chromosome that reattaches in the wrong place
Is a mutation always bad?
When can mutant genes cause health problems?
A health condition that can occur when a dog has only one copy of a faulty gene (either inherited from its mother or its father). Many of the more severe autosomal-dominant conditions are generally not passed on to any further offspring because the dog is often too ill to reproduce, or dies before it reaches sexual maturity. For this reason autosomal-dominant conditions are usually quite rare.
A health condition that can only occur when a dog has two copies of a faulty gene (inherited from both its mother and father) is known as an autosomal-recessive condition.
Dogs with only one copy of the mutant gene are said to be carriers and are unlikely to show any sign of the disease, but can pass the gene on to their offspring. The mutant genes for autosomal-recessive conditions can be the most difficult to predict, because they can be passed on from generation to generation without being noticed or identified.
As long as the dog also has a healthy copy of the gene to do its normal job, then the mutant gene may never be noticed. Often, there is no way to know that these mutant genes exist, or what they cause, until they are expressed in a dog with two copies. Every organism, including dogs and humans, are carriers for many autosomal-recessive conditions which have been passed from generation to generation without ever being noticed.
Complex inherited disorders
Complex inherited disorders are often caused by a number of different genes and are also influenced by environmental factors, such as diet and exercise. The way in which these conditions are inherited is not straightforward; hence the name complex inherited disorders.
One allele may increase or decrease the chance of a condition developing, but the impact actually be very slight. Lots of genes may contribute to the risk of a dog developing a condition and have an additive effect.
Each individual has two sex chromosomes. Men have an X and a Y chromosome and women have two X chromosomes. Some conditions result from a mutation on the X chromosome. These conditions don’t usually significantly affect females because they usually have one normal copy of the X chromosome which can counteract the mutated chromosome. Although women may not be affected by X-linked conditions they can still be a carrier. If a male inherits a mutation on the X-chromosome, he will develop the condition because he only has one X-chromosome.
Rather than a condition being caused by a mutation of a specific gene, chromosomal conditions occur when an individual has too many or too few chromosomes. These conditions are not usually inherited but can occur randomly before or soon after an egg is fertilised.
Gene pools and the impact of selection
What is a gene pool?
A gene pool is a hypothetical collection of all the variations of genes in a population. This could be a population of rabbits in a field, fish in a pond, or dogs in a breed. In a closed population, such as pedigree dogs, the numbers of gene variants is unlikely to increase, unless new dogs are brought into the breed, or mutations occur (which is rare and usually harmful). A gene pool can, and most likely will, get smaller when genes are lost through complete chance (i.e. not passed on to any descendants), or when dogs do not reproduce.
Sometimes an animal having a certain trait can influence how likely it is to survive and/or reproduce, this could be a faster rabbit evading a fox, a better camouflaged fish not being seen by its predators, or a pet dog having a good temperament and being chosen for breeding. All of these selection pressures can, over time, shape a population, making some genes associated with these benefits more common, while others become rarer or are lost from the gene pool.
How does selection impact a gene pool?
Dog breeders will choose carefully and select dogs that possess specific desirable traits, such as an excellent level of health and good temperament. By applying a selection pressure, (or a breeding criteria), to a breed, it makes some traits, and the genes that control them, more common, while others which control less desirable traits become rarer.
Dogs with desirable traits are likely to be bred from more frequently, while others that do not possess these traits may not be used for breeding at all. Over time, the gene variants associated with these popular dogs become common in the breed, while those associated with the less desirable dogs may be lost and disappear forever. These lost genes may include those that controlled the less desirable traits, but may also include other genes that just happened to be found in the less desirable dogs.
e.g. if a longer coat is desirable, then dogs with a long coat are more likely to be bred from and pass on their genes. Dogs with a short coat may not be bred from at all and so will not pass on any of their genes. These lost genes may include those that produce a shorter coat, but also includes all of the other genes that contributed to the rest of the dog, i.e. its eye colour, leg length, quality of hips, temperament etc.
What impact can a shrinking gene pool have on a population?
Understanding inbreeding and the importance of genetic diversity
What is inbreeding?
The pros and cons of mating related dogs
Mating two relatives that share similar genetic material means that their children are expected to be more alike and therefore have more predictable traits, e.g. mating two Labradors together will produce offspring that are Labrador shaped, while mating a Labrador to a Poodle can produce a range of different offspring. Although producing puppies with more predictable shapes may be beneficial, close inbreeding can come at a cost.
High degrees of inbreeding can lead to inbreeding depression (reduced litter size, increased puppy mortality, reduced fertility, a shorter lifespan, etc.) and an increased risk of developing both known and unknown inherited disorders.
What is the relationship between inbreeding and simple inherited disorders?
Dogs that are related to one another are likely to share similar genetic material. The more closely related dogs are, the more similar their genetic material is likely to be – this is known as Identical by Descent. This similar genetic material could be genes associated with positive traits, but it could also include faulty genes too.
The more closely related dogs are, the higher the risk is that they are both carriers for the same autosomal-recessive conditions (a health condition that can only occur when a dog has two copies of a faulty gene - inherited from both its mother and father). If these two dogs mate, then there is a risk that the puppies will inherit a copy of the faulty genes from both parents and will therefore be affected. This risk of producing dogs affected by inherited health conditions therefore increases with the degree of inbreeding.
What is the relationship between inbreeding and complex inherited disorders?
Some autosomal-recessive conditions can have a large and noticeable impact on a dog's health and welfare (e.g. forms of blindness, epilepsy, etc.), while others may only have a very small, and mostly unnoticeable effect.
As the degree of inbreeding increases, so too does the chance of a dog inheriting more than one autosomal-recessive condition. As the number of these smaller conditions increase, they can have an accumulative effect, leading to a decrease in the general health of the dog, otherwise known as inbreeding depression. This can lead to reduced litter sizes, increased puppy mortality, reduced fertility and a shorter lifespan.
Can DNA testing reduce the risk of inbred dogs inheriting autosomal-recessive conditions?
Yes, but only for the condition tested for.
Remember that every dog is most likely already a carrier for many autosomal-recessive conditions. DNA tests are available for only a small number of the known mutations in dogs, but there are likely to be many more recessive mutations that we currently know nothing about.
It is important that breeders DNA test their dogs they are intending to breed from in order to guard against producing puppies affected by conditions that are known about. It is also just as important to take steps to guard against conditions that cannot be known about. The best way to do this is by considering the impact of inbreeding prior to mating.