Principles of Genetics I
D.H. “Denny” Crews, Jr., Ph.D., P.A.S.
The science of genetics is a specific discipline within the life sciences
which is relatively young. The original experiments of the famous Austrian monk
and founder of the science of genetics, Gregor Mendel, were conducted in the
1860's and went largely unnoticed until the "rediscovery of Mendel" about 1900.
The molecular structure of DNA was not established until the Watson and Crick
model in 1953. It is unique that the vast number of advances in the study of
genetics have occurred recently: since the post World War II years. The theories
of Mendel, based on his work with the garden pea, are still the basis for
introductory study in the field of genetics.
The basic unit of hereditary expression, the gene, is a structural unit.
Genes are located on chromosomes: the small, threadlike molecules of DNA located
in the nucleus of cells. Genes on chromosomes determine the characteristics of
the individual, such as hair coat color, size, metabolic function, reproduction,
the occurrence of genetic disease, and other traits that we can see or measure.
Many traits are also influenced by non-genetic factors, referred to as
environmental effects.
Animals that reproduce sexually produce sex cells, or gametes. Females
produce eggs and males produce sperm. The sex cells contain a sample one-half of
the chromosomes, or genetic information, of the individual. When the sperm of
the male parent fertilizes the egg of the female parent, the complete genetic
makeup of the progeny, or offspring, is determined. This pairing of genetic
information equally from each parent means that individuals contain the genetic
information passed on from their ancestors. Generally, one-half of the genes
come from the father and one-half come from the mother, except in cases
involving sex linkage and extra chromosomal inheritance.
Genes occur in pairs on the chromosome; one member of the pair from each
parent. Genes are usually represented with letters and symbols. At a single
"locus" where a single gene is located, the genotype of the individual would be
described perhaps as Aa. This representation indicates that the individual has
one "A" gene and one "a" gene. This
is an example of heterozygous genotype. Heterozygous genotypes are those
where the two paired genes are not alike. The alternative, a homozygous
genotype, is when the two genes at the locus are alike, such as would be the
case with the genotypes aa or AA. In the case where only two genes exist, there
are three possible genotypes: AA, Aa, and aa. The chromosomal location (where A
and a are located) may control the color of the individual's eyes, or some other
simple trait. Most traits, however, are controlled by many genes at many
locations on the chromosomes. The number of unique genotypes quickly becomes
very large as the number of genes and locations increase. This type of
"genotypic variation" accounts for the diversity we see among different species
and the variation we can see within the species.
Dogs (Canis familiaris) have 78 chromosomes, organized into 39 pairs.
Thirty-eight of these pairs are called autosomal, and one pair is made up of the
sex chromosomes that essentially determine whether individuals are male or
female. Since dogs reproduce sexually, 39 chromosomes come from the father and
39 come from the mother. Dogs are also a multiple-birth species, meaning that a
female may give birth to several puppies during whelping. This implies that
puppies born in the same litter can be identical or fraternal twins. These
designations are usually lumped together in the category of littermates.
Littermates are, of course, sets of full brothers and sisters. Full brother and
sisters can occur among puppies from different litters, but only if they have
the exact same parents.
Traits that are largely under the control of genes (little environmental
influence) are said to be highly heritable. This is the situation where puppies
tend to be more similiar to their parents with respect to a certain trait.
Selection of dogs (making planned matings to change the average value of a
trait) works best when traits being selected are highly heritable. Improvements
in highly heritable traits can be accomplished more quickly than in traits that
are lowly heritable. Generally, traits such as mature size and conformation are
highly heritable. Traits that are not highly heritable are heavily influenced by
non-genetic factors, or environmental influences. Selection for these traits is
usually not very successful, or is very slow. Traits with low heritability are
also traits that tend to show high levels of heterosis, or hybrid vigor.
Examples of lowly heritable traits would be those associated with reproduction
and survivability. Hybrid vigor has occurred when the performance of the
offspring is different from that of the average of their parents. The actual
mechanism of hybrid vigor is not well understood, however, matings can be made
to increase the beneficial effects of hybrid vigor. Mating of animals which are
not alike in their pedigree or that are not related are those which will
generally exhibit higher levels of hybrid vigor. Closely related animals do not
exhibit high levels of heterosis.
Crossbreeding in plants and animals is done to take advantage of both hybrid
vigor and complimentarity. Complimentarity is the term used to describe the
"combining" of desirable traits from two breeds or lines that are not related.
Hybrid vigor is also maximized when animals are mated that have little or no
genetic history in common.
When animals that are related are mated together,
the offspring are termed "inbred". Inbreeding in animals will eventually lead to
the reduction in vigor of offspring, lowered productivity and fertility, and
will increase the incidence of genetic defects. Common inbreed matings include
brother-sister matings and father-daughter or mother-son matings. Inbreeding,
like crossbreeding, is a form of mating, but inbreeding and crossbreeding are at
opposite ends of the mating system spectrum in terms of genetic variability.
Crossbreeding increases variability and close inbreeding, generally, reduces
variability in the progeny.
The choice of a mating system depends on the goals of the system. For
example, the production of new breeds involves crossbreeding whereas inbreeding
might be used to make an established breed more uniform. The advantages and
disadvantages of various mating systems should be understood before any one
system is used.