German Shepherds and Genetics
Heredity
Like all living creatures, German Shepherds are made up of cells containing all the information (DNA) needed to make an individual. Approximately one half of this information comes from the sire and one half from the dam.
Dogs have 78 chromosomes, in 39 pairs, on which approximately 100,000 genes are located. Your German Shepherd has both a phenotype (physical appearance) and a genotype (genetic makeup). Phenotype can be influenced by both environmental and developmental factors. EX: A dog’s size as an adult is determined partly by its genes and partly by environmental factors such as its health as a puppy and the kinds of food it ate.
Each gene in a chromosome pair has a partner at the same position (or locus) on the matching chromosome. Each member of a gene pair is called an allele. A gene can have many alleles within a population but an individual dog can have only two alleles influencing a particular trait. If the two alleles are identical (AA or aa), the individual is homozygous at that locus; if the alleles are different (Aa), then they are heterozygous.
If the allele is dominant, only one copy is required to express the trait; if recessive then two copies are required. Upper case letters are used to represent dominant traits, lower case letters for recessive traits. Dominant traits are expressed if either AA or Aa are present for a particular characteristic. Recessive traits can only be expressed if aa for the characteristic is present. The heterozygote (Aa) will be a carrier—clinically unaffected but able to pass the harmful allele to offspring.
Example of Genetic Inheritance of Disease
Sex-linked characteristics
X and Y chromosomes are known as sex chromosomes, while all other chromosomes are called autosomes. Females have a pair of X chromosomes (XX) and males have one X and one Y chromosome (XY). Genes carried on autosomes can occur in both sexes, but genes carried on the sex chromosomes are transmitted only in conjunction with sex. Genes carried on the male’s Y chromosome cannot appear in a female because she can only carry X chromosomes. Few, in any important genes are located on the Y chromosome.
Defects
Inherited or Not? There are a number of things that can cause diseases or abnormalities. Some are genetic (a result of a mutation in a gene) and some are spontaneous (Ex: a mutation caused by toxins consumed by the bitch during pregnancy). An inherited defect is one in which the defective gene has been past down from one or both parents.
Patterns of Inheritance
Patterns of inheritance are defined as the process of transmitting traits from generation to generation. These patterns have not been established for many disorders that are believed to be inherited. The following are examples of some known patterns of inheritance that have been identified:
Autosomal Dominant. Only one copy of the gene, which may be inherited from either parent, is required to produce the trait. The parent with the dominant trait will pass the affected gene to approximately half its offspring, and the trait will be apparent in both the parent and the affected progeny.
Autosomal Recessive. This is the most common mode of inheritance for genetic conditions in dogs. To be affected, the animal must inherit two copies of the gene (ee), one from each parent. Dogs with the genotype EE (normal) or Ee (carrier) will be clinically normal but the carrier will pass the affected gene to approximately half the offspring. As long as carriers (Ee) are mated to normal animals (EE), the offspring will be unaffected but some will remain carriers. If two carriers are mated, some of the offspring will be affected. The breeding of close family members or the “popular sire” effect (breeding a sire with a harmful recessive gene frequently because of desirable traits) can cause a dramatic increase in the incidence of effected progeny.
Sex-linked Traits (X-Linked) as mentioned earlier, sex-linked chromosomes are labeled X and Y. A bitch who is a carrier for a harmful recessive gene (Xx) can pass the recessive gene (x) to her daughter and the daughter will be an unaffected carrier, but her sons who receive that same gene will be affected. Hemophilia (a blood clotting disorder) is the most common example of a sex-linked disorder.
Polygenic Traits (Inheritances) are controlled by an unknown number of genes, and gene expression can be influenced by a variety of other factors such as gender, nutrition, breed, rate of growth, and amount of exercise.
Because it is virtually impossible to determine the exact genotype for many of these traits, it is difficult to control defects with a polygenic mode of inheritance. Canine hip dysplasia is an excellent example of a polygenic trait. Documenting and publishing all data relating to polygenic inheritance disorders is paramount. It is only through thoughtful selection by breeders, using this information, that the incidences of these traits can be reduced in a breed.