Understanding General Canine Genetics
Embryonic cells divide again and again and again and each new cell contains all
the information required to make a entirely new puppy. Embryonic cells are
therefore referred to as totipotent.
A molecule called DNA is stored in the cell mitochondria which are contained in
a sack called the nucleus. It holds all the puppy’s genetic information and has
been called the instruction manual for all life. DNA makes an exact copy of itself
whenever a cell divides and it controls the manufacturing of proteins through a
process called protein synthesis. Each cell will have a combination of DNA from
both parents. Somewhere around the time there are 8 or 16 embryonic cells
(called "stem cells"), differentiation begins.
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
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.
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"
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).
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
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:
(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
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
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.
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
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
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