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by Theresa Garton, M.D.

Part I - "Genetics 101"

Take the Quiz

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 INCOMPLETE DOMINANCE.

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).

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

Genetics 101 Quiz

For each question, choose the most likely mode of inheritance.

Objective: answer all 5 questions correctly.
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1. Mary takes a trip to Tibet, and finds there a breed of dog with purple tufts of hair between their toes. She finds this trait charming, and purchases two, a male and a female, to take home and reproduce. In her first litter, she has the following result:
2 male puppies with purple tufted toes
2 female puppies with purple tufted toes
1 male puppy with no toe tufts
1 female puppy with no toe tufts

A. Autosomal Dominant(Complete)
B. Autosomal Dominant(Incomplete)
D. Autosomal Recessive
E. Sex-Linked Recessive
F. Sex-Linked Dominant

2. Tom has a dog, Spot, who is named for the red spot on the top of his forehead. Tom wants another male dog with this spot, and so breeds him to a bitch of the same breed. In the litter of 10, he is disappointed, because none of the 6 males has a red spot. However, all 4 of the females do. He breeds each of the 6 males, but none of the puppies from Spot's sons ever has a red spot. He breeds the first of the 4 bitches to an unrelated male, and gets:
2 females with no spot
3 females with a red spot
4 males with no spot
2 males with a red spot.

A. Autosomal Dominant(Complete)
B. Autosomal Dominant(Incomplete)
C. Autosomal Recessive
D. Sex-Linked Recessive
E. Sex-Linked Dominant

3. John takes a trip to Africa, and finds a type of dog with ears that curl into corkscrews shapes. He wants to start a breed based on this trait, and so purchases a male and female to reproduce at home. He decides he will breed the two together to expand his population. After five litters, all puppies are found to have corkscrew ears. To "set type," John then breeds the first generation offspring together, and repeats again for another generation. All dogs in subsequent purebred generations have corkscrew ears. John then crosses out to another breed to start adding back some genetic diversity, and obtain other traits he wants in his new breed. No puppies in the first crossbred generation have corkscrew ears.

A. Autosomal Dominant(Complete)
B. Autosomal Dominant(Incomplete)
C. Autosomal Recessive
D. Sex-Linked Recessive
E. Sex-Linked Dominant

4. Dick has an unusual physical trait. His ear lobes are a bright orange color. He marries and has children. Neither of his two daughters has orange ear lobes, nor does his son. His son Derek marries, and has 10 children, 6 girls, and 4 boys. None of these has orange lobes. However, when his daughters begin providing him with grandchildren, Susan has 3 girls, none of these have orange ear lobes. But Rebecca has 3 boys, and one of these has orange ear lobes.

A. Autosomal Dominant(Complete)
B. Autosomal Dominant(Incomplete)
C. Autosomal Recessive
D. Sex-Linked Recessive
E. Sex-Linked Dominant

5. Jane is planting her flower garden, and wants a certain type of daisy to fit in a certain area. Unfortunately, the daisies she has now are much too tall, and all the daisies she finds in the store are too short. She decides to breed her own daisies, and crosses the tall ones with the short ones. The results are 15 daisy plants. Of these, 4 are the same height as the tall parent, 3 are the same height as the short parent, and 8 are in the middle.

A. Autosomal Dominant(Complete)
B. Autosomal Dominant(Incomplete)
C. Autosomal Recessive
D. Sex-Linked Recessive
E. Sex-Linked Dominant

Check the Answers.


1. A - Autosomal Dominant

2. E - Sex-Linked Dominant

3. C - Autosomal Recessive

4. D - Sex-Linked Recessive

5. B - Autosomal Dominant- Incomplete


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