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For those interested in reading and learning more about what Brittany was referring to I found this article on the internet:


BASIC GENETICS

To understand the fundamentals of breeding of dogs and the transmission of inherited characteristics it is important to have a basic knowledge of the science of genetics. Inherited characteristics include quite superficial factors, which are easy to see, such as coat colour, but they also include basic conformation factors, such as bone structure and musculature, and defects such as hereditary diseases. Note also that environmental effects may influence some factors, for example the musculature of a dog will be predetermined by its genes but nutrition and exercise will have significant roles in determining the result.

The inheritance of characteristics may range from simple to extremely complicated. The old adage "Like begets like" is a generalisation that cannot be relied upon. Various inherited factors, good and bad, may be carried in a hidden state in a dog and only revealed in its descendants. Inbreeding (including line-breeding) has been used to concentrate desirable genes, but beware as it also tends to concentrate undesirable genes

The Genetic Material

The body of each animal is composed of cells that are so small that to be seen they must be viewed under a microscope. Each cell has a central nucleus that contains the genetic material inherited from its parents, the chromosomes, which are made of long strands of protein in coils (DNA). The chromosomes are in pairs and one of each pair is inherited from each parent. The dog has 39 pairs of chromosomes, with 39 single chromosomes coming from each parent via the sperm and the ovum to regain the 39 pairs in the resulting offspring. A gene is a particular section of a chromosome at a certain location (locus) on the chromosome and it has a specific role in determining the development of a certain feature of that animal. Different forms of the one gene are called alleles, and may produce different appearances, conditions or behaviour in the animals inheriting them. The total complement of the particular forms of genes that an animal has is termed its genotype. Some genes will be masked or modified by others with the result that the appearance of the dog, its phenotype, does not indicate its full genetic make-up.

Dominant/Recessive Mode of Inheritance

To illustrate a frequently-seen mode in which inherited characteristics are passed on from parents to their offspring it is useful to take the relatively simple example of the transmission of coat colour in Labrador Retrievers, firstly looking at the inheritance of the basic pigment colours black and the rarer chocolate, and then how yellow is obtained.

Each Labrador dog and bitch has two genes relating to the black/chocolate colour series, one inherited from each parent. These genes are situated at a specific locus on each of a particular chromosome pair. There are two different alleles, one for black pigment and one for chocolate pigment, and, depending on the specific combination, producing black or chocolate coat colour. Black is said to be dominant over chocolate because only one allele for black needs to be present for the animal to be black. For an animal to have a chocolate coat then there must be no gene for black, i.e. both alleles must be for chocolate. Chocolate is said to be recessive to the dominant black. If the dominant allele for black is represented by "B" and the recessive allele for chocolate by "b", then in a Labrador there are three possible genetic combinations:

Possible……………………Phenotype
Genotypes………………….Appearance)

BB………………………….Black
Bb…………………………..Black
bb…………………………..Chocolate.

Where the black allele is present, i.e. BB or Bb, then the animal will be black. Where both alleles are chocolate, i.e. bb, then the animal will be chocolate. Note that while Bb is black in appearance because of the presence of a dominant B, it will pass on either B or b to each offspring depending on chance alone. An animal with the genotype Bb is known as a carrier - one that carries a hidden allele but is able to pass it on to its progeny. This is also termed the heterozygous condition (Bb), whereas if both genes are the same (BB or bb) the term used is homozygous. If two black-coated carriers (genotype Bb) are mated, each parent is able to pass on either B or b to each offspring. To determine the potential progeny genotypes it is useful to develop a table of the possible combinations of ova (eggs) and sperm as follows:

Possible Offspring Genotypes from a Bb x Bb Mating



Ova

Sperm
B
B
b

BB
Bb

b
Bb
bb


It can be seen from this table that the expected ratios of genotypes and phenotypes in the progeny are: -

Genotypes: 1 BB : 2 Bb : 1bb
Phenotypes: 3 black : 1 chocolate

So, if the two black carriers were mated, then according to probability any one offspring would have a 75% chance of being black and a 25% chance of being chocolate. Approximately 75% of the litter would be expected to have the chocolate allele. Also note carefully that a chocolate animal must have received a chocolate allele from each parent. Within any litter these expected ratios may not result, just as the result of tossing a coin 10 times may not result in 5 heads and 5 tails.

Yellow is a common coat colour in Labradors. This colour is produced by the action of a different pair of genes, the Extension or E series, which governs extension of the pigment into the hairs of the coat. For coat colour to be black or chocolate, a dominant allele for extension, represented as E, must be present in the genotype. The recessive e if present in duplicate produces yellow coat, regardless of the black or chocolate genes for pigment, however the skin colour of the nose and lips will indicate the pigment status. This mode of inheritance is said to be epistatic, where one pair of genes over-rides another pair. Try working out the expected progeny phenotype ratio for the mating of two black parents with the same heterozygous genotype BbEe (answer at the end of the chapter). The potential genotypes and phenotypes resulting from the mating are: -

Phenotypes……………………Genotypes
Black ………………………….BBEE, BBEe, BbEE, BbEe
Chocolate………………………bbEE, bbEe
Yellow with black nose………..BBee, Bbee
Yellow with chocolate nose……bbee

This exercise is a good example of the variation in potential phenotypes and genotypes in a litter when considering just two pairs of genes. In addition to the basic coat colours above, there are many variations in pigment particle size and shape which produce different shades of colour in chocolates and yellows, and there may be patterns of light and dark over the body, more obvious in the yellows. These are due to modifier genes. Environmental factors such as nutrition and sunlight may produce further minor variations in colour within the three basic coat colours. This illustrates the point that inheritance of what may appear to be a single factor may in fact be the result of the interaction of many genes and environment.

Other Modes of Inheritance

There are various modes of inheritance other than the above, and the more frequently encountered are:

(a) X-linked

Sex is determined via a pair of chromosomes called the sex chromosomes. All other chromosomes are known as autosomes, and each one of an autosomal pair has the same complement of genes. The sex chromosomes are of two different types known as X, the female chromosome, and Y, the male chromosome. The Y chromosome is much smaller than the X and contains little genetic material. A male has one Y and one X, whereas a female has two X chromosomes.

Sex of an offspring is determined by the sire passing on either an X or a Y chromosome: the dam passes on either one of her X chromosomes to each offspring. As well as the obvious sex characteristics inherited via the sex chromosomes there are other factors inherited on the X chromosome, including certain hereditary diseases, and these are said to be sex-linked or more correctly X-linked.

(b) Incomplete Dominant

This is where the inheritance often appears due to a single dominant allele passed on from one parent, but sometimes not. This is usually due to incomplete knowledge of the precise mode of inheritance.

(c) Polygenic

This is where a number of different genes are involved in inheritance of a particular characteristic. The classic example is canine hip dysplasia.

Genetic Testing and DNA

One of the exciting scientific breakthroughs of more recent times is in the field of molecular biology, where tests have been developed to find the locus of specific genes and identification of their various alleles. For example, related to the above, there are tests for colour genotype in Labradors. This enables a breeder to determine the genetic status of a Labrador for colour and the expected colour ratio from a particular mating. In some instances it may be important enough for a breeder to pay for such genetic testing to know whether a black dog is a carrier of chocolate or yellow, or a yellow dog is a carrier of chocolate. Of far greater significance is the use of genetic testing in determining hereditary disease status.

Answer to the puzzle The phenotype ratio is :

9 black : 3 Chocolate : 3 yellow with black nose : 1 yellow with chocolate nose