This article is from the WSSF 2009 AFRMA Rat & Mouse Tales news-magazine.
By Ann Storey, N.F.R.S., England
From Pro-Rat-a, No. 125, Sept./Oct. 2001, the N.F.R.S. journal. Permission given to reprint article.
This article features the questions on basic genetics most relevant to rat breeders. It is part 3 of 3 of the N.F.R.S. Breeding and Showing Seminar series that took place July 22, 2001. See Part 1 “N.F.R.S. Breeding & Showing Seminar”; Part 2 “Breeding for Show & Exhibition” in the WSSF 2008 issue of AFRMA Rat & Mouse Tales.
Genetics is the study of genes. For many years, fanciers bred winners without a knowledge of genetics and many still do. However, in order to breed new varieties or those without a straightforward inheritance pattern, a knowledge of genetics is very helpful. The Edwardian fancy lost the Topaz* due to lack of knowledge.
A gene is a section of a DNA molecule that codes for a protein. This protein may be structural, that is for muscle or hair, or functional, such as an enzyme used in digestion.
Genes are found on the chromosomes. Chromosomes are molecules of DNA. Most life forms, with the exception of some viruses and prions, contain DNA in their cells. Different species have different numbers of chromosomes. Rats have 42. Chromosomes are present in pairs, therefore rats have 21 pairs of chromosomes. One of each pair comes from either parent.
The position a gene occupies on the chromosome is known as its locus (plural loci). On the other chromosome of the pair, the same gene will occupy the same locus. Sometimes an altered gene will occupy the locus. This gene will have an effect on the same protein as the normal gene, but will alter it in some way. This modified gene is known as the mutant or mutant allele. Genes are each given a code, written in italics, which is internationally recognized. The code is usually chosen by the first person to describe the gene in the scientific literature. It must not have been used previously for another gene. As the rat fancy has a number of genes not described in the literature, it has become common practice to make up a code. However, everyone should be aware that these are purely unofficial and if you use them, you should make sure that other people know this.
When the two genes on a pair of loci are identical they are said to be homozygous. For instance, a rat with two copies of the agouti gene, A, is said to be homozygous for agouti. When the two genes on a pair of loci are different, they are said to be heterozygous. For instance, a rat with agouti, A, on one locus of a pair and non-agouti, a, on the other, is said to be heterozygous.
In a heterozygous relationship, one of the genes is usually dominant, that is producing its effect, while the other is recessive and its effects are either hidden or not produced. For instance, a rat heterozygous for agouti and non-agouti, Aa, will look the same as a rat homozygous for AA. Therefore A is dominant to a and a is recessive to A.
In its simplest form a dominant gene is functional, that is producing its protein, while the recessive allele is non-functional. Sometimes the product of the recessive gene is functional but the effects are masked by the dominant product.
Genotype is the animal’s genetic makeup while phenotype is its appearance. Two animals may look alike, thus have the same phenotype but not have the same genotype. For instance, two agouti rats may look alike but one may be carrying non-agouti (self).
Many genes are not fully dominant and when in a heterozygous partnership may give a sort of half way house. For instance, Himalayan rats are a first cross between Siamese and pink-eyed white and have one dose of each gene. This is because both gene products are being produced and neither one is totally masking the other. Alternatively, not enough product may be produced, leading to a lighter colour.
There are several reasons why a dominant gene may not be expressed. The most common cause is probably because the gene is not switched on. Most genes have a switch sequence in front of them and if this is damaged or missing, the gene will not operate. The most common example in the rat is the pearl gene. This is dominant over Mink but does not always operate. For this reason, two supposed Mink rats can give birth to Pearls. This switch may also cease to operate later on, leading to Pearl and Cinnamon Pearls with blobs and streaks of Mink and Cinnamon in their coats respectively. This is similar to variegated plants which will suddenly revert and produce plain green shoots.
Mendel worked out the likely ratios of offspring produced when two mutants of the same gene were crossed (Mendel’s first law) and later when mutants of two genes were crossed (Mendel’s second law). The ratios can be worked out using the chessboard so loved by some writers of small livestock books. However, there is not room to go into this here and it is well explained in most GCSE biology text books, as well as Nick Mays’ book, The Proper Care of Fancy Rats.
The Fl generation means the first filial generation. That is the first generation, or offspring of the rats you have paired together. The F2 generation is the second generation, or the offspring of the Fl generation. Sometimes laboratory strains will be described as F20 and so on. This means that the strain is 20 generations old.
Mendel’s second law, about working with two genes, only works if the two genes are on different chromosomes. If they are on the same one, the ratios do not work. Lots of colour genes in the rat are linked and this can cause problems, as it can be harder to get a wanted colour out than expected. Linked genes are difficult to part. The two which should never be mated together are pink- eyed and red-eyed rats, for example, Silver Fawn and Topaz*. It will take a lot of matings to get the Topaz out.
A lethal gene is present where the homozygote dies either before or shortly after birth. We have several in the rat fancy including Chinchilla and Pearl. A semi-lethal is where the homozygote is impaired in some way, for instance lack of eyes in anophthalmic white hamsters. The heterozygotes are usually healthy.
The colours in mammals are formed by the pigment melanin. This is available in two forms, eumelanin, which makes black and brown pigment, and phaeomelanin, which gives rise to the yellows, oranges, and reds. Blue and lilac are caused by the reflection of light on the placing of black or chocolate pigment granules and silvering by air bubbles within the hairs.
The pigments are formed in cells called melanocytes from the amino acid tyrosine. Melanocytes are present in the skin, around the hair roots and in the eyes. The pigments (cells can produce both but not at the same time) are secreted into granules which are deposited into the growing hairs.
Melanocytes originate from embryonic nervous tissue, shortly after the neural tube (which goes on to form the spine and brain) has formed. They start to migrate as melanoblasts at about the 14th day of gestation and at this stage they are actively multiplying. They gravitate toward the skin and the surfaces of some organs. The first area to get melanocytes is the crown of the head. There is a limit to the time the cells have to migrate, possibly due to the increasing density of the skin which they have to travel through. Therefore those areas that are furthest away, the tips of the toes and the chest, are the most likely not to get any melanocytes. These will then be white. In marked rats there are factors which hold up migration for one or two days, which leads to larger areas of white.
* Note: Topaz = U.S. Fawn