Page 56 - Issue 2_2018

Pedigree Analysis and

How Breeding Decisions Affect Genes

by Jerold S Bell DVM, Clinical Associate Professor of Genetics, Tufts Cummings School of Veterinary Medicine

To some breeders, determining which traits will appear in the off-
spring of a mating is like rolling the dice--a combination of luck and
chance. For others, producing certain traits involves more skill than
luck-- the result of careful study and planning. As breeders, you must
understand how matings manipulate genes within your breeding
stock to produce the kinds of offspring you desire. First comes under-
standing dogs and cats as species, then as genetic individuals.

There is little similarity between a Chihuahua and a Saint Bernard, or
between a Himalayan and a Sphynx. However, we must understand
that while established breeds are separate entities among themselves,
they all are genetically the same species. While a mating within a
breed may be considered outbred, it still must be viewed as part of
the whole genetic picture: a mating within an isolated, closely related,
interbred population. Each breed was developed by close breeding
and inbreeding among a small group of founding ancestors, either
through a long period of genetic selection or by intensely inbreeding There are ways to measure the genetic diversity of a population. One
a smaller number of generations. This process established the breed’s method is to measure the average inbreeding coefficient (or Wright’s
characteristics and made the individuals in it breed true. coefficient) for a breed. The inbreeding coefficient is a measurement of
Pure-breeds have closed stud books. This means that the diversity of the genetic relatedness of the sire and dam. If an ancestor appears on
genes in the breed is fixed. Genes cannot be gained through breeding, both the sire and dam’s side of the pedigree, it increases the inbreed-
only lost. In some cat breeds, cats who meet the phenotypic standard ing coefficient. The inbreeding coefficient gives a measurement of
of the breed may be introduced into the gene pool. This, of course is the total percentage of variable gene pairs that are expected to be
an added source of genetic diversity for the breed. homozygous due to inheritance from ancestors common to the sire
and dam. It also gives the chance that any single gene pair can be
homozygous.

The types of matings that you choose for your breeding animals will
manipulate their genes in the offspring, affecting their expression.
Linebreeding is breeding individuals more closely related (a higher
inbreeding coefficient) than the average of the breed. Outbreeding in-
volves breeding individuals less related than the average of the breed.
Linebreeding tends to increase homozygosity. Outbreeding tends to
increase heterozygosity. Linebreeding and inbreeding can expose del-
eterious recessive genes through pairing-up, while outbreeding can
hide these recessives, while propagating them in the carrier state.

Most outbreeding tends to produce more variation within a litter. An
exception would be if the parents are so dissimilar that they create a
uniformity of heterozygosity. This is what usually occurs in a mismat-
When evaluating your breeding program, remember that most traits ing between two breeds, or a hybrid, like a Cockapoo. The resultant
you’re seeking cannot be changed, fixed or created in a single genera- litter tends to be uniform, but demonstrates “half-way points” between
tion. The more information you can obtain on how certain traits have the dissimilar traits of the parents. Such litters may be phenotypically
been transmitted by your animal’s ancestors, the better you can pri- uniform, but will rarely breed true due to the mix of dissimilar genes.
oritize your breeding goals. Tens of thousands of genes interact to pro-
duce a single individual. All individuals inherit pairs of chromosomes; One reason to outbreed would be to bring in new traits that your
one from the mother, and one from the father. On the chromosomes breeding stock does not possess. While the parents may be geneti-
are genes; so all genes come in pairs. If both genes in a gene pair are cally dissimilar, you should choose a mate that corrects your breeding
the same gene (for instance, “aa” or “AA”) the gene pair is called homo- animal’s faults but phenotypically complements its good traits. It is
zygous. If the two genes in a gene pair are unlike (for instance, “Aa”) not unusual to produce an excellent quality individual from an out-
the gene pair is called heterozygous. Fortunately, the gene pairs that bred litter. The abundance of genetic variability can place all the right
make a cat a cat and not a dog are always homozygous. Similarly, the pieces in one individual. Many top-winning show animals are outbred.
gene pairs that make a certain breed always breed true are also ho- Consequently, however, they may have low inbreeding coefficients
mozygous. Therefore, a large proportion of homozygous non-variable and may lack the ability to uniformly pass on their good traits to their
pairs--those that give a breed its specific standard--exist within each offspring. After an outbreeding, breeders may want to breed back to
breed. It is the variable gene pairs, like those that control color, size individuals related to their original stock, to attempt to solidify newly
and angulation that produce variations within a breed. acquired traits.

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