Understanding General Canine Genetics

That is to say, even though each embryonic cell contains the same DNA, they activate, or “express”, different genes and the genes they express determine the enzymes they produce and that, in turn, determines the kinds of cells they become: brain cells, liver cells, nerve cells, skin or hair cells, etc. Once differentiation begins, that cell will forevermore produce the same type of cell when it divides. Chromosomes are made up of extremely long, tightly packaged DNA molecules in combination with chromosomal proteins. Because chromosomes are made up of DNA, chromosomes contain the necessary information for building more cells.

When examined under an extremely powerful microscope, chromosomes exhibit an "X" or a "Y" shape while DNA looks like two threads twisted around each other (called a "double helix") with each thread held together by many bridges made up of four amino acids called adenine, guanine, cytosine and thymine.

This gives DNA the appearance of a spiral staircase with the amino acids forming the steps. Just like the order of the letters in a sentence, the order of the amino acids (called "bases") in a chromosome must be correct. Therefore, a defective gene is like a misspelled word. Genes are short stretches of DNA and they all have to be in a very specific location on the chromosome. They carry the instructions for the production of proteins which make up cells and direct them to develop a certain way or to perform a specific function.

Each gene provides the genetic instruction to make one protein or control one function. For example, the genes tell the cell to produce a certain chemical or to produce a specific characteristic like blue eyes. So, simply put, DNA is a set of instructions. Lots of DNA, protein, and other materials make up chromosomes and genes are discreet packets of DNA found along the chromosome.

Genetic Diseases

In the dog, there are 78 chromosomes existing of 39 matched pairs which make up the dog’s “genotype”. On the other hand, the dog’s “phenotype” is what the animal actually looks like and this can be influenced by both environmental and developmental factors. For example, a dog’s adult size is partially determined by his genotype but is also influenced by such factors as health and nutrition as a puppy.

Chromosomes are found in pairs and each gene on a given chromosome has a partner at the same position on the matching chromosome. Each member of this gene pair is called an allele and each allele pair influences a particular trait. If the two alleles are the same, they are “homozygous”. If they are different, they are “heterozygous”. Then there is epitasis. This refers to the circumstance when alleles at one location on the chromosome mask the action of another pair of alleles elsewhere on the chromosome.

Autosomal Traits:

The autosomal chromosomes contain the instructions for every puppy's body make up (size, color, height, etc.) while the sex chromosomes mainly determine gender. Puppies receive one-half of their autosomal set from each parent and one-half of their sex chromosomes from each parent. The gender of the puppy is determined by whether an X or a Y sperm fertilizes the female egg which contains an X chromosome from the mother. A female puppy will then have two "X" sex chromosomes and a male will have an "X" and a "Y" sex chromosome in addition to the 38 matched pairs of autosomal chromosomes. The fertilized egg is then referred to as a “zygote”.

The inheritance of genetic diseases, abnormalities or traits is described by both the type of chromosome on which the abnormal gene resides (autosomal chromosomes vs. sex chromosomes) and by whether the trait itself is "dominant" or "recessive".

Dominant inheritance occurs when a defective gene from ONE parent is capable of causing disease even though the matched gene from the other parent is normal. The dominant gene over-rides the normal gene and dictates the performance of the matched pair. If only one gene in the pair is abnormal and recessive, the disease does not occur or is only mildly present. A dog with a single defective recessive gene is called a "carrier" meaning the disease does not manifest itself in the carrier but that the defective gene for the disease can be passed on to offspring. Recessive disease usually only occurs when BOTH genes of an autosomal pair are abnormal.

In many instances, there is incomplete dominance: If either parent is affected, all puppies have a susceptibility to the disorder but not all will be equally affected. A dominant gene may also have incomplete penetrance. If penetrance is 75% for example, only about 75% of the puppies who inherit the trait will express it.

Since autosomal chromosomes are paired, there are 2 copies of each gene. If a gene is abnormal, it may code for an abnormal protein or for an abnormal amount of protein. If only one of the genes of a pair is defective, the normal gene of the pair may code for sufficient protein, so that no disease is clinically apparent. A puppy in which both genes of a pair are defective is referred to as homozygous (homo is Latin for the same).

Sex-Linked Traits:

Autosomal diseases (the most common mode of inheritance for genetic conditions in dogs) are inherited via the X autosomal chromosomes (since there are no Y autosomal chromosomes) while sex-linked diseases are inherited through either the X or the Y chromosome of the sex chromosomes. The mother will contribute an X sex chromosome to an egg and the father will either contribute an X or a Y sex chromosome to fertilize the egg. Since X chromosomes are so much larger than Y chromosomes, there are more X-linked traits then Y-linked traits.

If the trait is a sex-linked recessive trait, it means that an abnormal gene on the X chromosome from each parent is required to cause the disease. In order to have two X sex chromosomes, the puppy must be female. Because a normal gene on one X chromosome would protect the female from the recessive gene on the other, both X sex chromosomes must be defective for the disease to be expressed. In males, there is only one X chromosome. The Y chromosome is the other half of the XY gene pair in the male. Since the Y chromosome doesn't contain most of the genes of the X chromosome it cannot protect the male. Therefore, a single recessive gene on the male's X chromosome from the mother will cause the sex-linked disease.

Males and females can inherit X-linked traits since both have X sex chromosomes.
Since only males have a combination of X and Y sex chromosomes, only they can inherit Y-linked traits. But, because they also have an X chromosome, even recessive genes on the X chromosome of the male may be expressed. See the inheritance pattern of sex-linked genetic traits below where the small x represents the sex chromosome carrying the defective gene:

Inheritance Patterns:
(where the small red x = a defective gene
    and the large black X = a normal gene)

XX (mom) + XY (dad) = XX (female puppies) or XY (male puppies)
In this case, there are no defective sex genes from either parent to be inherited so male and female puppies will be entirely free from the defective gene.

Xx + XY = XX or xX (female puppies) or Xx + XY = XY or xY (male puppies)
In this case, the normal or defective gene from the mother makes the female puppy clear or a carrier and the X or Y sex gene from it’s father makes it a female or a male. The male puppy may be a clear or an affected. Males receive their X chromosome only from their mothers. Males cannot pass X-linked traits to their sons via the Y chromosome but can to their daughters via the X chromosome. There are fewer females with X-linked recessive disorders than males because even if they have one recessive gene, the normal (dominant) gene is the one which is expressed.

XX + xY = Xx (female puppies) or XX + xY = XY (male puppies)
In this case, the defective sex gene comes from the father and, again, the puppy is a carrier if it is female or clear if it is a male. Males transmit defective X genes only to their daughters and to all of their daughters. As you can see, it is not possible for a male to pass an X-sex-linked trait to its son or a Y-sex linked trait to its daughters.

Xx + xY = Xx or xx (female puppies) or Xx + xY = XY or xY (male puppies)
In this case, the puppy inherits either a normal gene or a defective sex gene from its mother and a defective or a normal gene from its father. The puppies will be either a carrier or affected female or a clear or affected male. Note: In autosomal recessive diseases, clear males are not possible from this pairing.