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SCENARIO: You are especially fond of ticking in your breed,
however, the "pick" puppy in your most recent litter is the only one
of six puppies with no ticking. To what can this be attributed?
ANSWER: The effect of dominant and recessive genes.
In Part 1: Chromosomes and Genes we learned the
following:
- Chromosomes are made up of genes, which provide the instructions for how a
puppy will look and act.
- Except in the case of twins, no two dogs are ever genetically the same
because the genes a sire and dam pass on to each puppy are always a unique,
one-of-a-kind composition of the genes they each received from their
ancestors.
- In the fertilized cell from which a new puppy will develop there are two
sets of genetic instructions, one "building plan" provided by 39
chromosomes from the sire and the other by 39 chromosomes from the dam.
We are now ready to address the issue of which of the 2 sets
of genetic instructions will be followed in the "building" of our new
puppy. This brings us to the role of dominant and recessive genes.
GETTING A HANDLE ON MENDEL!
Successful breeders have long recognized the necessity of
understanding how genes are involved in the passing of a trait from one
generation to the next. The key to breeding better dogs lies in learning how to
"arrange" genes, which are the carriers of heredity and which
determine a dog’s size, conformation and temperament. Gregor Mendel’s work
with garden peas and other plants laid the ground work for understanding this
phenomenon. Mendel’s work established that traits do not blend. Breeding tall
plants to short ones did not produce medium-size plants. The lesson here: don’t
breed an overshot dog to an undershot one and expect to get a scissors bite!
DOMINANT AND RECESSIVE GENES
One of the major conclusions that emerged from Mendel’s
work that affects our breeding of dogs is that genes are inherited in related
pairs, one from each parent. Mendel discovered some genes over-rule the activity
of others. These are called DOMINANT genes and geneticists depict them with an
upper case letter. For example, we know that the gene T for ticking (color spots
in white patches) is dominant and over-rules the gene t for nonticking. Genes
that are over-ruled are called RECESSIVE genes and are represented by lower case
letters. The alternative forms of a particular gene, in this case, T for ticking
and t for nonticking, are known as alleles.
Remember, one member of each gene pair comes from each
parent. In our example, whether a puppy has ticking will depend on which two
genes it inherits. If his sire passes on to him the T gene for ticking and his
dam the t gene for nonticking, the puppy will inherit the gene pair Tt. It will
have ticking because the T gene is dominant and over-rules the activity of the t
gene. If it inherits the gene pair TT it will also have ticking because both
genes are dominant for ticking. If it inherits tt it will have nonticking
because there is no dominant T gene in the pair. In any gene pair there are only
3 possible combinations. Using our example they would be: TT, Tt and tt.
HOMOZYGOUS AND HETEROZYGOUS GENES
We need to understand two funny sounding words: HOMOZYGOUS
and HETEROZYGOUS. When both genes in a pair are the same, either dominant (for
example, TT) or recessive (for example, tt), we say the dog is HOMOZYGOUS
or PURE for that trait or character and must pass this
characteristic on to a puppy. If the genes in a pair are different (in our case,
Tt) the dog is HETEROZYGOUS for that trait and could pass either the
recessive or the dominant gene to offspring. As we learn more about genes we
will see that the goal in breeding lies in trying to arrange desirable genes in
homologous pairs. Having two "good" genes in the same pair in both
the sire and the dam guarantees us that one "good" gene from each
parent will always be inherited by a puppy.
GENOTYPE AND PHENOTYPE
The term genotype refers to the genetic make-up of an animal.
It refers to the letter symbols describing the gene pair. The term phenotype is
used to describe the external appearance resulting from a gene’s action. Thus
the genotype for dogs with ticking is either TT or Tt, while the genotype for
nonticking is always tt. Every breeder needs to understand that a dog is
really two different entities: what we see on the outside (the phenotype) does
not always predict what genes he is carrying on the inside (genotype). If a
dog happens to carry the Tt gene pair he himself will have ticking, When bred to
a female carrrying either TT or Tt, however, he is capable of producing puppies
with nonticking (see Figure 1).
THE PUNNETT SQUARE
Geneticists and breeders frequently use a diagram called a
Punnett square to predict the expected outcome of individual breedings. Genes or
traits that can be contributed by one parent are listed on the top of the
diagram; at the left are listed genes that may be contributed by the other
parent. Possible combinations that can be produced in the offspring are found in
the squares formed by the intersection of the columns and rows. Figure 1 shows
the mating of Emma to Joe and the expected outcome relative to ticking and
nonticking. Joe and Emma both carry the Tt gene pair; they can each contribute a
T or a t to each puppy.
Figure 1
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Possible Combinations In
4 Puppies |
|
(Emma Tt) |
|
Egg
with T |
Egg
with t |
|
(JOE
Tt) |
Sperm
with T |
Rover
TT |
Eddy
Tt |
Sperm
with t |
Spot
Tt |
Jane
tt |
LET’S REVIEW!
Referring to the Punnett Square and the text, see if you can answer the
following questions. Answers follow below.
- Does Joe have ticking?
- Does Emma have ticking?
- Which of the four puppies will have ticking?
- Which puppies will have nonticking?
- Of the two parents and 4 puppies, who is homozygous for these traits? Who
is heterozygous?
- Which puppies have the genotype for nonticking? Which have the genotype
for ticking?
- Of the 4 puppies, which are capable of producing nonticking in their
offspring?
IF THINGS WERE ONLY THIS SIMPLE!
The above discussion has centered on single genes and how they are involved
in the passing on of a trait.
In the next column we will address the more complicated issue of traits that
are controlled by multiple gene pairs.
ANSWERS
Joe has ticking.
Emma has ticking.
Rover, Eddy and Spot will have ticking.
Jane will have nonticking.
Rover and Jane are homozygous. Joe, Emma , Eddy and Spot are heterozygous.
Jane has the genotype for nonticking. Rover, Eddy and Spot have the genotype
for ticking.
Eddy, Spot and Jane are capable of producing nonticking in offspring mated
to a Basset with the t gene.
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