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By; Linda Witouski
Long time breeder,
exhibitor, an AKC Judge, and one who is also deeply involved with legislation to preserve the sport of purebred dogs and dog
shows for us all.
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This paper shall provide any reader with some
fundamentals of coat color genetics and clarifies the meanings of some often used genetic terms.
A single animal cell is the structure in which
life begins. All living things are composed of cells which start the life cycle as a single cell, and through a process
of cell division and specialization, the organism is formed. The dog reproduces by producing germ cells (gametes), sperm
(male) and eggs (female), which fuse to give rise to a single fertilized egg cell (zygote). The zygote then grows and
divides to form the embryo. The information to reproduce a complex many celled animal is thus transmitted through two
cells, sperm and egg.
The cell is made up of a cell wall inside
of which is found cytoplasm and a nucleus. Within the nucleus are contained the thread-like structures known as the
chromosomes. It is the chromosomes which carry the genetic material. They are arranged in pairs, one member of
each pair being derived from each parent. The dog has 78 chromosomes (diploid number) in each cell with the exception
of the sperm and egg cells which have only 39 chromosomes (haploid number), one from each pair. Hence,when the sperm
and egg fuse, the zygote will again have the diploid number of chromosomes, 78. the chromosomes contain DNA (Deoxyribonucleic
Acid), RNA (Ribonuycleic Acid), and protein. A gene is a particle on the chromosome which provides hereditary information
and is made up of DNA. A gene can be thought of as a set of coded instructions or blueprints. In order to understand
how DNA carries the genetic code, it is necessary to look at the structure of the nucleic acid molecule. (Watson &
Crick theorized the structure was that of a double helix which might be compared to two bedspring-like structures linked together).
They suggested sugar phosphate units forming two long chains linked together by a series of nitrogen bases: Adenine,
Thymine, Cytosine and Guanine - these chains forming the double helix. Adenine is always linked to Thymine and Cytosine
is always linked to Guanine. Information is coded in each gene in a specific form or sequence of the four kinds of nitrogen
bases within DNA. The sequence differs in each gene and thereby distinguishes one gene from another. Genes produce
chemical compounds called enzymes. These enzymes govern the embryological development and growth of all structures in
the body.
There are two types of cell division.
The first is MITOSIS, a process in which the somatic cells (those other than gamete) of an organism are formed. In mitosis,
there is a daughter cell produced which received an exact copy of the diploid number of chromosomes in the mother cell.
It is this method by which the somatic cells of the body reproduce. (). I'd like to add here, that the somatic cells
are those that are used for cloning, such as the infamous Dolly the Sheep. These cells are also the cells being
experimented on in hopes of "reprogramming" them to act like embryonic stem cells
The second cell division is MEIOSIS, a process
in which there is a reduction in the number of chromosomes from the diploid number to the haploid number. It is this
type of cell division by which spern and egg are produced. The processes of producing sperm and eggs by meiosis are
known as SPERMATOGENESIS and OOGENESIS, respectively.
Among the 39 pairs of chromosomes, there is
one pair which determines the sex of the animal. This pair is known as the SEX CHROMOSOMES. They differ significantly
from each other in size and shape, one being called the X chromosome and the other the Y chromosome. Males carry the
X and a Y - females only carry two X's. Hence, it is the sire that determines the sex of puppies because his sperm can
contain an X OR a Y chromosome. The female's egg cells can ONLY carry an X. Therefore, if an X carrying sperm fertilizes
the egg, the puppy is female.
The remaining 38 pairs of chromosomes are
known as autosomes. Each one is similar to its partner in size, shape, and function. In a pair of chromosomes,
each one is known as being HOMOLOGOUS to its partner. Homologous chromosomes have the same gene sites or locations on
them. These gene sites are known as the gene loci and only one gene may be found on each loci on a chromosome.
Alternate forms of a gene which occupy the same loci on homologous chromosomes and affect the same trait are known as alleles.
If these two genes are the same, then the animal is said to be HOMOZYGOUS for that pair. If they are different, then
the animal is HETEROZYGOUS for that pair. If he is heterozygous, one gene may be DOMINANT to the other. Dominant
genes mask RECESSIVE genes. As an example - inheritance of tail shape. A dog who is homozygous for the dominant
straight tail gene (SS) bred to a homozygous recessive screw tail (SS) would produce all heterozygous puppies (Ss).
GENOTYPE of the puppies, the genetic make up, would be a Ss while their PHENOTYPE, outward physical appearance, would be that
of a straight tail dog. Since the straight tail is dominant to the recessive screw tail gene, the straight tail gene
will MASK the screw tail and the puppies will have straight tails. One gene was received from each parent.
Expected outcomes are based on a infinite
number of breedings. In the above example, although SS x ss and ss x ss breedings would always result in the expected
ration, the Ss x ss could deviate from the expected 50:50 ratio if too small a sampling is made. It is the same as tossing
a coin. If we toss it a million times, we would probably get close to the expected 50:50 ratio, and if we toss it an
infinite number of times, we would get the expected ratio. However, if we only toss that coin twice, we might not get
what we should have theoretically based on the small sample. Certain cases of deviance may be accounted for by mutation.
MUTATION is a spontaneous genetic change or may be introduced by such things as radiation, temperature change, chemicals or
other causes.
This ends the fundamental and clarification
section.
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~
Copyright © MinPins-N-Blue
2001
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