- Basic Genetics - Basic Genetics
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(DISCLAIMER - This article is for information purposes only. This information, in no manner or form, is intended to take the place of a Veterinarian/Client relationship. Consult your Vet for your animal's health!)
Genetics 101
Genetics is the term given to the sudy of heridity. Heridity is the process by which characteristics are passsed on from parents to offspring (progeny), so that the progeny resembles the parents. The main concept in genetics is that inheritance is controlled by a set of genes, which are discrete physical molecules (DNA, chemically known as deoxyribonucleic acid) linked together in long polymer chains to form chromosomes.
The first geneticists were interested in how genes were transmitted from parents to progeny during reproduction. They were also interested in how different genes act together to control what we saw in a variety of traits (e.g. height, color, shape, size, etc, known as phenotypes - "observed traits"). Gregor Mendel established a theory of the "Laws of Genetics" which he demostrated through painstaking experimentation the relationships between sets of genes and the physical characteristics that were seen. Although this work was done mid-1800's (Mendel lived in a monastary and his work wasn't really recognized until about 1900), the principles of the Laws of Genetics reamain essentially intact today as what we recognize as the fundamental principles of genes and inheritance. The importance of Mendel's contribution cannot be overemphasized, and we still talk of Mendelialn inheritance patterns today. Mendel's experiments removed the mystery of heridity, and demonstrated that genetic inheritance followed predictable rules.
A change in emphasis occurred in the 1930's, where science started to recognize the chemistry of the organism. This fundamental change brought about the realization that genes, like any other components/molecules in an organism, could be studied directly by physical and biochemical methods. Thus a new branch of genetics, called molecular biology, was "formed" with its orientation being to understand the chemical nature of genes.
This new approach led to new concepts and ideas, and soon geneticists ceased to regard individual genes as only units of inheritance, and began to realize that they were units of complete "biological information". These genes contained the entire complement of information required to construct a living, functioning example of that organism. The aim of Geneticists and Molecular Biologists in the last fifty years has been to understand the manner in which these detailed information is made available to the living organism.
DNA, the essence of inheritance
It has been well established that DNA is the chemical basis of genes. James Watson and Francis Crick performed famous experimentation in the early 1950's which led to the discovery that DNA was a double helix (i.e. the genetic material coiled around each other). Watson and Crick shared the Nobel Prize in 1962 with this discovery.
DNA is, in fact, a long polymeric molecule made up of of individual nucleotides joined in chain-like fashion to form one strand of the DNA double helix. This nucleotide is actually a complex molecule made of three distinct chemical components: a sugar molecule (specifically, 2'-deoxyribose in the case of DNA), a nitrogenous base molecule (specifically there are four "varieties": adenine, guanine, thymine, and cytosine), and a phosphoric acid molecule. The sugar joined with the nitrogen base is called a nucleoside. The nucleoside joined with the phosphoric acid molecule is called the nucleotide.
Individual nucleotides monomers (single nucleotide molecules - "mono" means single) are joined together to form a polymer, by attaching one nucleotide monomer to the next in a sequence. Realizing that any one position of a nucleotide in a DNA chain could be one of four nucleotides ("A" for adenosine, "G" for guanosine, "C" for cytidine, and "T" for thymidine), you begin to realize that the DNA chain is coding a very specific sequence depending on the combinations and lengths of the nucleotides strung together. This is the variablility of DNA that enables the genetic material to exist in an almost infinte number of forms.
During reproduction, the DNA chain is copied, and those daughter copies are split and separated into the egg and sperm sexual gametes. The individual gametes from the two parents fuse to produce the fertilized ovum, creating a new combination in the next generation.
Phenotype vs. Genotype
In genetics, what is seen is described as "observable traits", called the phenotype. Underlying this phenotype is a set of set of genes which are expressed in biochemical and physical terms giving us these observed traits. DNA strands contain two alleles for any specific gene. This gene "set" (the two alleles) is called the genotype, and each single allele may come in alternative forms.
Genetic allele types generally have a heirarchy of expression when seen in their phenotype. The allele that is dominant is the allele which, if present in a single dose, will mask the presence of other gene allele types. A allele that is recessive is an allele which needs to be present is a double dose to be observed. A recessive allele, when present in a single dose, will be maksed by the existance of a dominant allele.
Very often, "letter" symbols are used to designate each of the individual alleles of the gene set, so you can easily see the genotype variation within a inherited condition. The dominant version of the gene allele is generally expressed with a capital letter (e.g. "B"), whereas the recessive version of the gene allele is generally expressed with a small case letter (e.g. "b"). Some alleles exist in multiple variants, in which case there generally is a relative dominance/recessive heirarchy, and "letter designations" will have superscript notations to give that extra variant its identity.
For example, let's hypothetically say in simple terms that eye color was controlled by one gene location, and you have two alternatives to a gene. Brown eye (being dominant) can be expressed as "B", while blue eye (being recessive) can be expressed as "b". Since there are two alleles at this gene, the genotypes can be among the following "BB", "Bb", "bB", or "bb". Since "B" is dominant, eye colors in the example will be brown for "BB", and "Bb"/"bB". Blue eye, since "b" is recessive, will only be observed when the genotype "bb" exists. Where the genotype contains two of the same alleles (in this case either "BB" or "bb") it is called homozygous. Where the genotype contains different variants of the gene alleles (in this case either "Bb" or "bB"), it is called heterozygous.
Mendelian Inheritance Patterns
Mendel's contribution to genetics cannot be overemphasized, and it's not nostalgia that geneticists still discuss him in revered tones. His detailed experiments led to the understanding of inheritance patterns of simple observed traits, and the understanding of how dominant and recessive alleles segregated during gamete formation phase of reproduction, and then recombined to produce a new genetic combination during the fertilization.
This understanding is still used today (but with some further sophistication to explain more complex modes of inheritance in some cases), and one of the most common forms is the Punnett Square, which helps visualize the phenotypes arrising from the genotypes. Anybody who has dabbled at inheritance patterns will immediately recognize the square, and for the sake of space only a brief example will be shown:
Simple allele distribution in a single observable trait
Female Gamete allele types | ||||||||||
Male Gamete allele type
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It can be easily seen where the phenotype distributions come from, where knowing the underlying genotypes. One can do a Punnett Square for single trait inheritance, plugging in the male parent's genotype allele segregation on the left side, also plugging in the female parent's gentotype allele segregation on the top side, and filling in the cells with the resulting answers to obtain the potential genotype/phenotype of the first generation progeny (F1). These squares can be continued on the next generation(s) (F2, F3, etc.).
Although it is more complex than can be handled here, you may be able to see that by knowing the relative ratio's of the various hybrid crosses, one can attempt deduce potential genotype information on prior generations from phenotypes of progeny, if sufficient information is available.
To reiterate, In simpe terms Phenotype is the **observed** traits which are genetically determined (possibly with some environmental "help"). Genotype is the underlying **DNA types** which "express" into what you see. What you see in your dog (phenotype) - in coat color, coat texture, physical stature, some disease states, etc. - is determined by the genes which are part of the dog's DNA. In some cases, a single gene locus (location) may determine what the trait is that you see. In that case, it is often simple to determine the underlying genotype from knowing the relative dominance/recessive character of the gene, and by knowing ancestors, siblings, and progeny.
In many cases, multiple genes control the trait, and it doesn't follow simple Mendelian distribution. In those cases, it can be very hard to determine the genotype from the phenotypes that you observe, even if you have a large amount of data from related dogs.
Other Considerations - Molecular Genetics
It is beyond the scope of the Web to go into detailed information regarding other molecular genetic issues, and how the molecular biology handles this knowledge. Inheritance patterns of "dominant with limited penetrance", "polygenic", "linkage", "base pair vs transcription error mutations" are all important, but complex.
We would refer you to our page on Genetics - Further Reading if you have the desire to obtain more detailed knowledge on these complex issues. Understanding these issues, however, allows you to better understand how to breed better dogs.....