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?

How a dog looks and behaves is determined by a combination of the environment it lives in, the environment it has grown up in and its genetics.  Environmental factors could include a dog’s diet, how much exercise it gets or the levels of hormones in the uterus it was raised in when it was an embryo. A dog’s genetics are determined before its birth and are the only way in which characteristics can be passed from parent to child. For example, a dog’s coat can be influenced by what it eats, sunlight, time of year, how short it is trimmed etc., but none of these factors will impact the coat of the puppies it has in the future, while its genes on the other hand will.

What is the function of a gene?

A dog's genome (the sum of its 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 makes up each recipe is the DNA. Just like a recipe can be used to make a dish of food, a gene can be used to make a protein. Proteins are the building blocks for ever organism and make up bones, teeth, hair, muscle, etc. Genes are therefore vital in producing proteins which impact on a dogs characteristics.

Alleles give variation in characteristics

Each dog has two copies of every gene, one of which it inherited from its mother and one from its father. These two genes may be the same or they may be slightly different. Different versions of the same gene are called alleles and can cause variation in the protein that is produced, or where, when and how much of the protein is produced.  These differences in how the protein is produced contribute to each dog’s unique physical features and account for the differences between each dog and each breed.

Homozygous and heterozygous

When a dog has two copies of the same allele they are said to be homozygous.  When the two alleles they have are different, they are known as heterozygous.

How genes are 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.

 

 

B

 

b

 

B

 

BB

(Brown fur)

 

Bb

(Brown fur)

 

b

 

Bb

(Brown fur)

 

bb

(Yellow fur)

Intermediate expression

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. 

 

 

B

 

b

 

B

 

BB

(Brown fur)

 

Bb

(light brown fur)

 

b

 

Bb

(light brown fur)

 

bb

(Yellow fur)

Codominance

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.

Modifying genes

These genes influence the degree to which other genes control their characteristics. For example, 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.

Epistatic alleles

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.

Coat colour                 Genotyoe

Black                          BBEE, BbEE, BBEe

Brown (chocolate)         bbEE, bbEe

Yellow                         BBee, Bbee, bbee

Regulator genes

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 change throughout its lifetime. 

Incomplete penetrance

Some genes do not have an impact on the individual unless certain environmental factors occur, for example the genes that cause multiple sclerosis in humans can be triggered by the Epstein-Barr virus.

Sex-limited genes

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.

Sex-controlled character

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.

Genome imprinting

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.  

Pleiotropy

One gene can sometimes be responsible for two or more characteristics, for example 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.

Stuttering alleles

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.

Complex inheritance

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.

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