This is part one of what will be an ongoing series. I will
review in this article basic genetic principles, to aid in understanding certain inherited
health problems in boxers, as well as aid in selecting desired conformation and other
traits. At the end of the articles are questions to test your knowledge and understanding.
This is not an easy subject to summarize in one or two pages, so please bear with me!
CHROMOSOMES are structures found in the nucleus of
each cell. Chromosomes are in turn made of GENES. The chromosome is like a chain,
with the genes making the links. Each gene has a specific site, or LOCUS, on a
specific chromosome. The gene is made of a biochemical called deoxyribonucleic acid or
DNA for short. Two special proteins - purine and pyramidine - make up the DNA. The
sequence of these two proteins makes a binary code which instructs the body in making up
the organism. Genes are inherited in discrete units that are passed down intact, if all
goes well, from generation to generation.
The number of chromosomes depends on the species. The
chromosomes come in a variety of shapes and sizes, but there are always two of each type.
Dogs have 39 pairs, or 78 individual chromosomes.
For each gene locus, there are a variety of choices as to
which specific gene will occupy the space. For each site, the different choices are called
ALLELES. In a matched pair of chromosomes, one allele will occupy the specific site
on one of the pair; another allele will occupy the site on its partner. These alleles may
be identical, or each of the pair may have a different allele. If the two alleles are
identical, then the trait they represent will appear, or be expressed.
If the two alleles are different, then there is some
question as to which of the alleles will be expressed. If one of the alleles is expressed,
to the exclusion of the other, then that allele is said to be DOMINANT. The one
that is hidden, or not expressed, is called RECESSIVE. Geneticists use capital
letters to describe dominant genes, lower case letters to represent recessive ones.
Sometimes, when the two alleles are different, the trait expressed will be something
between the trait described by either gene, this is called INCOMPLETEDOMINANCE.
There is a special pair of chromosomes, the sex
chromosomes. This pair consists of at least one "X" chromosome, the other
of the pair is either another "X" chromosome, or a "Y"
chromosome. In mammals, an organism with two X chromosomes is female, an organism with an
X and a Y chromosome is male. Interestingly, in birds this configuration is reversed, with
an XX individual being male. The Y chromosome is almost identical to the X chromosome,
except that it is missing one of the arms of the X, so that it resembles the letter Y.
Recessive traits require two copies of the gene to express
themselves. Dominant traits will express themselves no matter whether the organism also
has a gene for the recessive trait. The exact combination of alleles is called GENOTYPE.
The physical appearance of the organism is called the PHENOTYPE.
All chromosomes that are not sex chromosomes, are called AUTOSOMAL.
A trait that would normally be located on the missing arm of the Y chromosome is called
SEX-LINKED. These traits do not follow the rules of the autosomal traits. Any trait on
the above described arm of the X chromosome will be unopposed on the Y chromosome of the
male, so sex-linked recessive traits only require one copy to express themselves in males
(but not females).
References:
Willis, Malcolm. "The Genetics of the Dog". Howell Book House, 1989. ISBN
0-87605-551-X
Baer, Adela S. "The Genetic Perspective". W.B. Saunders Company,1977. ISBN
0-7216-1471-X
