by Todd D.L. Woods, MD
Without a working knowledge of the genetics of producing a better Siberian, the breeder is limited to combining dogs together that seem to physically complement each other when genetically the mixture may be unsound. At the very least, in order to avoid certain genetic faults, an understanding of recessive, dominant, homozygous, heterozygous, variable penetrance, line breeding, in breeding, polygenic recessive, etc. needs to be understood, if not becoming fully second nature. It is the goal of this and following articles to familiarize readers with genetic terminology and to instruct the Siberian fancier on those traits that have proven modes of inheritance. Even if certain patterns of inheritance are not known, a knowledge of genetics can only assist a dedicated Siberian fancier who wishes to responsibly contribute back to the breed by producing a litter of puppies. Why know about genetics? I heard recently about two breeders being uncertain as to what color the puppies out of 2 red Siberians would be. Most of you know that 2 reds Siberians will produce red puppies, or possibly a red piebald or a red splash coat but that you can't get black and whites or greys if both parents are red. (As a twist that will be explained later, you could also get whites with liver points.) But did you know that the reason the breeding of 2 reds results in red offspring is because reds are homozygous for the autosomally recessive red factor? Knowledge like this helps breeders determine or control both trivial qualities (like coat color) in the breedings that we plan and more importantly, the cornerstone qualities like temperament, angulation, back length, foot shape, tail set and carriage, etc. Genetics as a science was not understood until the monk, Gregor Mendel, made some insightful observations with peas around the year 1900. Serendipitously, pea genetics are very straight forward and intuition allowed an understanding of simple recessive and dominant traits. From this work, the field of genetics began. Much was understood about breeding good animals before Mendel discovered the "wheel", but new concepts were able to be developed and old wives' tales discarded once the concept of recessive traits were accepted. For instance, colors that were thought to result from genetic "weakness" resulting from inbreeding like the fawn and blue color in Dobermans and the red in Siberians could now be understood as a combination of recessives rather than a loss of "strong" outcross traits. All traits are carried or coded for on alleles. Alleles are the building blocks of genes. Therefore all traits are carried on a gene or a combination of genes. Genes are bodies of DNA that are made up of many alleles each specifying certain proteins that determine the trait(s) in question. Genes (and the alleles that make them up) are inherited on chromosomes (each chromosome consisting of many genes) that are donated from the mother (egg or ovum) and father (sperm). The dog has 78 chromosomes arranged in 39 pairs, one of which determines the animal's sex (though other traits are also carried on the sex chromosomes). One chromosome of each pair of chromosomes comes from the mother and the other chromosome of the pair comes from the father. There are always 2 alleles that code for the same trait (e.g. 2 alleles that code for coat color). These alleles are arranged within genes on each of two chromosomes that make up one of the 39 pairs. One (allele, gene, and chromosome) is inherited from the mother and one from the father. Now we can take a diversion from precise terminology, since the difference between alleles, genes, and chromosomes is understood, and use the term allele and gene almost interchangeably. Since multiple alleles exist in a given gene, it is customary to refer to a "gene coding for a certain trait" rather than "an allele on a gene coding for a certain trait." You need to understand the difference between alleles and genes for later understandings in genetics but the customary use of the word gene is often not as precise as it should be--don't be confused by this. To continue with the discussion, while there are 2 genes (actually alleles) that code for the same trait, they are not necessarily identical (e.g. one may code for a red coat and the other may code for a black coat). This is where the terms recessive and dominant become operational. If one gene is dominant (e.g. black) and the other is recessive (e.g. red) then the end product (e.g. the dog's coat color) will be revealed as the dominant trait (e.g. black) and the recessive gene will be silently held to possibly reveal itself in subsequent generations. Therefore dominant genes for physically apparent traits are always revealed in the animal's physical appearance (phenotype) whereas recessive genes may or may not be revealed, depending on whether the gene it is paired with is dominant or also recessive. Phenotype is the expression of a combination of genes. Genotype is the actual genetic make up of the individual. In the example above if the animal's "coat color genes" are both red, then the animal will be red (red phenotype-- the genotype is homozygous red) because there is no dominant gene to hide the red gene's expression of coat color. Now if we take the example of two red parents, both having two recessive red genes, the offspring, receiving one gene for coat color from each parent, can only receive red recessive genes (since the parents cannot carry the black color gene or they wouldn't be red) and hence all the offspring will be red. The inheritance of coat color is different from the inheritance of coat color distribution though, so other genes will determine if the red coat color will be distributed as a splash coat, piebald, white (restriction of pigment from the hair shaft) or irish (i.e. solid on the top and white underneath and on the legs), as is the most common color distribution in Siberians. More on that and more terminology in the next installment. The building blocks in this column are extremely basic and there are exceptions to the general principles outlined here that I will attempt to detail later. Despite the exceptions, learn these terms cold to begin to understand the genetics of breeding a better Siberian Lessons in this column: Dominant: a trait, that when present on a gene in a single dose, will mask the presence of another. It is usually depicted by upper case letters (e.g. B for black). Recessive: a trait that needs to be present in duplicate in order to indicate its presence. In a single dose it will be masked by a dominant gene. It is usually depicted by a lower case letter (e.g. r for red). Thus the B for black combined with a r for red would be depicted Br, which would code for a black coat color. Chromosome: gene carrying body in a cell, there are 39 pairs, or 78 total, in dogs. Gene: building block of chromosomes made up of DNA. Allele: building block of genes made up of DNA. Autosome: any chromosome other than the sex chromosomes. There are 38 pairs of autosomes and 1 pair of sex chromosomes that make up the 39 total pairs of a dog's chromosomes. An autosomal trait is one that is carried in a gene on an autosome. A sex linked trait is one that is carried in a gene on one of the sex chromosomes (almost always the X, rather than the Y chromosome). Phenotype: The visible expression of traits. Genotype: The actual genetic structure. Homozygous: both copies of a trait on a gene are identical. Heterozygous: copies of a trait on a gene are different.
Last time you learned
the concepts of dominant and recessive genes, autosomal and sex
chromosomes, and homozygous and heterozygous genotypes. This time these
terms will take on more meaning as the implications of a heterozygous
black and white male Br (B=dominant black and r=recessive red) bred to a
homozygous red rr (as all recessive traits must be in order to be
phenotypically evident) female. Sperm and eggs each carry half of the
parents' genetic material. In other words, each of the 39 pairs of
chromosomes that were described in the last article, splits into 39 single
chromosomes, each half going into one sperm or egg. The distribution of
the chromosomes is both ordered and random. It is ordered in that one of
each pair separates, for instance the two 14th chromosomes, so that each
egg gets a 14th chromosome, rather than one egg getting two 14th
chromosomes and not getting any 15th chromosomes. It is random in that
which of the 14th chromosomes the egg receives (the one that that parent
inherited from its sire or dam) is by chance. In this way the chromosomes
that will ultimately make it to an egg will come from both grandparents.
The egg, for instance, may get the first copy its 14th chromosome from the
maternal granddam and the second copy of its 14th chromosome from the
maternal grandsire. The same random and ordered process occurs in the
production of sperm. When a sperm and an egg join, each of which consist
of a combination of single chromosomes, 1 through 39, from each respective
grandsire or dam, the result is 39 pairs of chromosomes. This makes up a
complete set of 78 chromosomes which can then become a new, genetically
unique puppy. If we trace the heterozygote black (Br) dog and homozygote
red (rr) bitch breeding proposed in the first paragraph above we can see
how the production and combination of sperm and eggs results in both red
and black offspring. (It may also produce grey puppies because another
gene can cause the B to be distributed in such a way as to make the
overall coat color appear grey--there is not a grey gene per se). Half of
the dog's sperm will carry the B gene and half will carry the r gene. All
of the bitch's eggs will carry the r gene since this is the only coat
color gene she carries. Custom and common sense allows us to use a table
to predict the product of the combination of these two individuals:
| Dog's genes | |||
|
Bitch's |
B | r | |
| r | Br | rr | |
| r | Br | rr | |
The dog's sperm will be 50% B and
50% r; the bitch's eggs will be 100% r. Setting these up in the table
above we can see how, when the dog's sperm unite randomly with the bitch's
eggs, as occurs in fertilization, half of the produce will be Br (black
and white or greys) and half will be rr (copper or more dilute). Of course
if we do not know the genotype of the dog (whether he is BB or Br) then we
could get a different result if a homozygous (BB) black and white dog (who
may look the same- same phenotype-as the firstly proposed sire) is bred to
the same red bitch. This time we would get.
| Dog's genes | |||
|
Bitch's |
B | B | |
| r | Br | Br | |
| r | Br | Br | |
All the progeny will be black and white or greys though all the progeny will carry the red factor. It is easy to calculate now the predicted percentages of certain traits, assuming they are simple genetics like coat color, if you know the genotype of the proposed parents. Simple genetics is in contradistinction to polygenic and variable penetrance genetics as we will discuss later. As we have discussed coat color genetics in Siberians, there being only two genes, the dominant B for black and the recessive r for red, let's proceed to coat distribution factors. Remember that there is no gene for grey. Another gene that controls for pigment distribution on the individual hairs, can make a black dog grey, or a copper dog light red. This grey or lightening of red factor is usually dominant to the individual hair distribution factor that permits a black (Br or BB) Siberian to appear black or a red (rr) Siberian to appear copper. There are four coat color distribution (coat distribution is different from pigment distribution which determines grey from black) factors in Siberians. The Irish factor, far and away the most common pattern in Siberians, symbolized as si, is a color distribution factor that codes for white on underparts, typical masking on faces, without white located on the upper portion of the dog (i.e. neck or rump) and is dominant to Neck Marks, symbolized as sn. The sn or Neck marks factor allows white on the neck as a small dot up to a full shawl. The sn or Neck marks factor is dominant to Piebald (symbolized as sp) and White (symbolized as sw), but is recessive to Irish. The Piebald factor (sn) is dominant to White (sw). The White factor (sw) is recessive to the other three factors (si, sp, and sn). In other words, in order of decreasing dominance there are Irish (si), Neck marks (sn), Piebald (sp), and White (sw) distribution factors. To test your understanding of dominant and recessive traits answer the following questions: Can a Piebald carry the Irish factor? No! If it did it would be an Irish since Irish is dominant to Piebald. Can a Piebald bitch produce an Irish distributed offspring? Yes! If bred to an Irish factored dog, at least some of that dog's sperm will carry the Irish factor and when combined with the Piebald's coat distribution factor which could be homozygous piebald (sp,sp) or heterozygous piebald (sp,sw), the dog's Irish factor will be dominant and the coat distribution of those puppies will be Irish. Of course if the Irish distributed sire is heterozygous Irish (e.g. si,sn) then we would predict that half of the puppies will be Irish and half will be Neck Marks by the following table.
| Irish Dog's genes | |||
|
Piebald |
si (Irish) | sn (neck marks) | |
| sp (piebald) | si, sp | sn, sp | |
| sw (white) | si, sw | sn, sw | |
This table demonstrates that 25% will be si,sp (which would be phenotypically Irish but carry the Piebald gene), 25% will be sn,sp (which would be phenotypically Neck Marks but carry the Piebald gene), 25% would be si,sw (phenotypically Irish but carry the White gene) and 25% would be sn,sw (phenotypically Neck Marks but carry the White gene). Therefore 50% would be phenotypically Irish (the si,sp and si,sw) and 50% would be phenotypically Neck Marks (the sn,sp and sn,sw). Is a white Siberian more or less likely to produce Piebalds? It is more likely to produce them if bred to a piebald, because its distribution factors (sw,sw) are recessive to the piebald's distribution factors (the piebald's being either sp,sp or sp,sw). A white siberian is less likely to produce piebalds than some Neck Mark or Irish Siberians because Neck Mark and Irish Siberians can carry the sp factor, hidden behind the dominant Neck Mark (sn) or Irish (si) factors. Please take note that when I refer to white, I am referring to a real white, with either black or liver points, and not to an albino or to an Isabella white. An Isabella white, is a creamy white that is caused by an "oxidizing" of color such that a black, grey or red Siberian becomes a creamy yellowish casted white--the so called Isabella white. This type of white is not determined by a distribution factor, it is determined by a melanin factor. The melanin factor is symbolized by C for dominant (the common Siberian gene) and cr for the "oxidizing" Isabella white gene. An Isabella white is cr,cr. An rr Isabella white has liver points (nose, eye rims and lips) and is a red Siberian in disguise. A Br or BB Isabella white has black points and is a grey or black and white Siberian in disguise. Before we leave simple dominant and recessive traits let me explain what variable penetrance means. Penetrance is a term to describe the extent to which a dominant allele (or gene) will reveal itself when present in only a single dose (e.g. si,sp). If this combination produced 100% Irish (as it does) then the Irish factor would be said to have 100% penetrance. If this combination produced Irish distributed coat color in 75% of individuals, the Irish gene would be said to have 75% penetrance. Simple dominant or Mendelian dominant traits are ones with 100% penetrance. Some genes are dominant with variable penetrance (e.g. probably some forms of epilepsy and temperaments) which then produces different implications for breeders dealing with these problems, because apparently unaffected individuals may nevertheless carry the undesirable genes. Coat color and distribution genetics can be considered to have simple or "Mendelian dominance". You will remember that in the first article, I stated that (for Gregor Mendel and the field of genetics) "Serendipitously, pea genetics are very straight forward." Pea genetics are simple dominants and recessives. This fact allowed insight to theorize and then prove the genetics of pea plant height. In honor of Gregor Mendel, the term Mendelian genetics is used to describe genetics that behave according to simple dominant and recessive models. If you become fluent in the terms and genetic factors in this and the first article, you will have a strong handle on the determination of coat color and distribution in Siberians and will have the beginning of the tools necessary to approach upcoming subjects like hip dysplasia, temperaments, cataracts, PRA (progressive retinal atrophy), glaucoma, etc.
Lessons in this column:
B Black dominant coat color symbol
r Red recessive coat color symbol
si Irish coat color distribution factor symbol
sn Neck Marks coat color distribution factor symbol
sp Piebald coat color distribution factor symbol
sw White coat color distribution factor symbol C Non "oxidizing"
melanin factor
cr "oxidizing" melanin factor that in homozygous form produces
Isabella white
penetrance: the extent to which a dominant factor reveals itself when
present in only a single dose
polygenic: relates to a trait that is controlled by several genes each of
which have a small effect but which have a considerable effect when
present together.
Mendelian or simple dominance: dominance with 100% penetrance.
