By Nanna M. Andersen | Updated: 03.07.23 Need a complete database of genetics with pictures? Try visiting Rat Variety Guide! Genotype The rat's full genetic makeup which includes all dominant and recessive alleles. We only work with a very small number of genetic codes in the hobby, so we usually only study genes that code for color, coat type, eye color, body type and markings. The genotype is made up of gene pairs, one gene from each parent. These genes have been given letters to identify them. The uppercase letters represent the dominant form of the gene, while the lowercase letters represent the recessive form of the gene which can be carried. Writing out genotype can be done a few different ways; let's try to write out a Black Self Dumbo Rex. Where do we start? I find it easiest to just start from color and go down the line to marking, ears and then coat. I would write this genotype as follows: aa HH dudu Rere. Some might add the dividers as such: a/a H/H du/du Re/re - the divideres tend to make it easier for beginners to read the genotype. Phenotype The rat's expressed genetics which can be seen by the naked eye. Phenotypes have names, like Agouti Berkshire Dumbo Rex. Many of the genes have been named in laboratories, but some genes have been found in the hobby and the breeder or club have named the gene. This is why we see varying names for varieties within the hobby. Variety Rats don't have breeds, it's a common beginner mistake to think so. Rats have varieties. This means that Dumbo, Rex, Russian Blues, ect. are all the same breed; Rattus norvegicus, also known as the Brown rat or the Norwegian rat. DNA This is where you will find all the rat's genetic material that is inherited from the parents. Chromosome Chromosomes is a structure that contains all the rat's DNA. Rats have 42 pairs of chromosomes in total - 21 chromosomes inherited from each parent. Locus (plural Loci) A term that refers to the place on a chromosome where a particular gene is located. A locus can have one or more genes associated with it, but we see most have one dominant gene and one recessive gene. An example of a locus could be the Agouti locus which is responsible for producing pigment. Genes and alleles A gene is a unit of hereditary information and an allele is a variant of the gene. Each gene is found at a specific locus, always in two copies, one copy from each parent. The copies, however, are not always the same. When the copies of a gene differ from each other, they are known as alleles. A gene may have multiple different alleles, though only two alleles are present at the gene’s locus in any individual. Homozygous (Homozygot) A rat will be homozygous when it has two copies of the same alleles on the same locus, for example, "A/A", "m/m", "r/r" or "Re/Re". Homo meaning identical. Heterozygous (Heterozygot) A rat will be heterozygous when it has two different alleles on the same locus, for example "A/a", "M/m", "Rb/rb" or "R/r". Hetero meaning not identical. Complete dominance A heterozygous state where we see a complete dominance by the dominant gene, meaning the dominant gene "conceals" the recessive allele allwing only the dominant allele to express. For example, "A/a" and "A/A" will be phenotypically identical because Agouti "A" is completely dominant over the non-agouti "a". Incomplete dominance (or semi-dominant) A heterozygous state where we see an incomplete dominance by both alleles, meaning both alleles are equally expressed in the phenotype. An example of an incomplete dominant gene is Rex (Re). The "Re" isn't completely dominant over "re" so together they create an equal mix of both alleles. Co-dominant A heterozygous state where we see a co-dominanc from both alleles, meaning they share dominance and will both be visible to the naked eye. I'm not familiar with any codominant alleles in the hobby, but if they were to exist it would be like have a rat the expressed both Agouti and Black at the same time. Recessive A homozygous state where the two recessive alleles create a diluted color. Recessive alleles can be carried which means that recessive genes may be expressed in the next generation or in future generations. Recessive genes are expressed only when appearing in a homozygous state, for example, "a/a" which produces Black (non-agouti). Epistatic and hypostatic genes A gene that suppresses the expression of another gene at another locus is called the epistatic gene, while the gene that's being suppressed is called the hypostatic gene. Epistatic genes can be either "dominant antagonistic", "recessive antagonistic" or "complementary". "Dominant antagonistic epistatic" means that one gene suppresses the expression of a gene at another loci. For example when a rat is C* based, the dominant C will not let Burmese or the Black Eyed gene express. This means C is acting as a dominant antagonistic epistatic gene to the hypostatic (suppressed) Burmese and Black Eyed gene. "Recessive antagonistic epistasis" is something we see in Albino rats, the recessive cc genotype suppresses the expression of the locus responsible for pigmentation; Agouti, meaning we always get a white rat with pink eyes. "Complementary epistasis" is when the two genes encourage the expression of each other, meaning the epistatic gene allows the hypostatic gene to express when the right conditions are met. I'm struggling to find a source for this in the genetics of rats but I'm assuming this is what happens when a rat has a recessive C locus genotype while also having the hypostatic dominant Burmese or Black Eyed gene. Or have the recessive Mink, Am Mink or Aussie Mink genotype while also having the hypostatic dominant Pearl or Merle gene. Different loci in the Norway rat
Hooded locus and other markings
Most markings are caused by alleles on the Hooded locus. There are 6 alleles on the Hooded locus and the dominance is as follows: H > H^re > h^i > h^e > h^n > h. There is a common misconception that there is only 2 alleles on the Hooded locus; the dominant "H" and the recessive "h", while Irish "h^i", Extreme "h^e", and Notch "h^n" are somehow modifiers that is laid on top of the recessive Hooded allele, but this is simply not true. There 6 alleles on the Hooded locus are: Self (H), Irish (h^i), Extreme (h^e) Notch (h^n) and Hooded (h). They can be paired up to create a total of 14 recognized markings: There is a few loci separate from the Hooded locus that all work together with the genes on the Hooded locus to express a new type of marking:
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Februar 2024
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