Genetics 101
by Paul Patton
I. Introduction
Genetics studies how living organisms inherit features from their preceding generations. It tries to identify which features are inherited, and work out the details of how these features are passed from generation to generation.
In genetics, a feature of an organism is called a "trait." Some traits are features of an organ-ism's physical appearance, for example, a cat‘s eye-color, coat or color. There are many other types of traits and these range from aspects of behavior to disease resistance. Traits are often inherited but traits can also come from the interaction between inherited features and the environment.
Genetic information is carried by a long molecule called DNA that is copied and inherited across generations. Traits are carried in DNA as instructions for constructing and operating an organism. These instructions are contained in segments of DNA called genes. DNA is made of a sequence of simple units, with the order of these units spelling out instructions in the genetic code. This is similar to the orders of letters spelling out words. The organism "reads" the sequence of these units and decodes the instruction.
Not all the genes for a particular instruction are exactly the same. Different forms of one type of gene are called different alleles of that gene. Mutations are random events that change the sequence of a gene and therefore create a new allele. Mutations can produce a new trait, such as turning an allele for black hair into an allele for white hair. The appearance of new traits is important in evolution.
The Punnett square is a diagram that is used to predict the outcome of a particular cross or breeding experiment. It is named after Reginald C. Punnett, who devised the approach, and is used by biologists to determine the probability of an offspring having a particu-lar genotype. The Punnett square is a summary of every possible combination of one maternal allele with one paternal allele for each gene being studied in the cross. In this example, both organisms have the genotype Bb. They can produce gametes that contain either the B or b alleles. (it is conventional in genetics to use capital letters to indi-cate dominant alleles and lower-case letters to indicate recessive alleles.). The probability of an individual offspring having the genotype BB is 25%, Bb is 50%, and bb is 25%.
It is important to note that Punnett squares only give probabilities for genotypes, not phenotypes. The way in which the B and b alleles interact with each other to affect the appearance of the offspring depends on how the gene products (proteins) interact. For classical dominant/recessive genes, like that which determines whether a cat has black hair (B) or blue hair (b), the dominant allele will mask the recessive one. Thus in the example above 75% of the offspring will be black (BB or Bb) while only 25% will be blue (bb).
II. The Feline Alphabet Soup
a. Genes Dealing with Feline Colors/Patterns - see chart below
- Agouti - The gene has two alleles, 'A' which isdominant and produces the ticking on tabby cats & 'a' which is recessive and produces the solid or self colored cats. It should be noted that the agouti gene only affects the Black gene and has not affect on the Orange sex-linked gene coloration.
- Black - The Black or Brown Group, depending on whose literature your reading, has three alleles. 'B' which is dominant and produces a Black coat; 'b'(brown) which is recessive to 'B' and produces the chestnut coloration; and 'bl'(light brown) which is reces-sive to both 'B' and 'b' and produces the cinnamon coloration.
(Author's Note: the superscript character of 'bl' is a lower case 'L', where the superscript font cannot be used, it is written as 'bl'.Chocolate vs. Chestnut - some breeds use one, some breeds us the other, it my tables I use Chestnut and when combined with the Albino Series genes, I use Chocolate).
- Albino Series - This series of five alleles includes: 'C' which is dominant and produces the full expression of coat color; 'cb' which produces the Burmese colors and is co-dominant with the Siamese and dominant to the remaining alleles; 'cs' which produces the Siamese colors and is co-dominant with the Burmese allele and dominant to the remaining alleles; 'ca' which produces a blue-eyed albino which is recessive to the prior alleles and dominant to the final allele; and 'c' which produces the true pink-eyed albino which is recessive to all the alleles. (Author's Note: I have left the 'ca' and 'c' alleles out of the color breeding charts).
When combining the alleles
- Dilution - The Dilution or Dense pigmentation Group, depending on whose literature your reading, has two alleles. 'D' which produces the dense colors (i.e. black, chestnut, cinnamon and red) and is dominant and 'd‘ which produces the diluted colors (i.e. blue, lavender, fawn and cream) and is recessive to 'D'.
Inhibitor - The dominant inhibitor gene, 'I', prevents color from being fed into the growing hair shaft. This results in the coat color being only expressed in the tips of the hair shaft with a white under color near the skin. The gene is variable expressive resulting in range from the smokes colors to the chinchilla colors.
- Orange - All of the red cats show the sex-linked orange gene, 'O'. This gene is carried by the X chromosome. The sex of the parent introducing the orange gene must be taken into consideration during breeding. The males have only one X chromosome while females have two. This results in the males having only one 'O'(red) or 'o'(non-red) allele and the females having two alleles: 'OO'(red), 'Oo'(tortoiseshell) and 'oo'(non-red).
Tabby - Newer studies show the possibility of three genes associated with the tabby pattern:
1. The first determining if the cat is ticked tabby or not (Ta vs. ta)
2. The second determining mackerel vs. classic tabby pattern (Mc vs. mc, provisionally)
3. The third determining spotted pattern vs. non-spotted ( Sp vs. sp , provisionally)
The ticked gene is dominant over all others, the spotted gene is recessive to ticked, but dominant to mackerel and mackerel is dominant to classic (Author’s Note: These are the Tabby symbols as laid out by Lorraine Shelton in Robinson's Genetics for Cat Breeders & Veterinarians, 4th Edition).
- White spotting - The dominant piebald allele, 'S' and the corresponding allele for non-spotting, ‗s‘ are an example of incomplete dominance. 'ss' would be non-spotted and the 'Ss' usually mildly or moderately spotted with white appearing on the stomach, chest, face and legs of a cat. The 'SS' cat would be a 'high' white (van). The white spotting gene is highly variable and cats that are 'SS' and 'Ss' could have some overlapping characteristics.
- Dominant White - The white cat is created by a dominant white gene, 'W'. A cat homozy-gous for this gene, 'WW', will produce all white kittens, whether or not it is bred to another white. A heterozygous white, 'Ww', when bred to another heterozygous white or a cat of another color, will produce some white kittens & some colored kittens. The white gene is dominant to all other patterns and colors.
b. The Rest of the Soup
There are other genes that play an important part in the physical characteristics of any breed of cats and I am not really going to go into details on the them. For example: ‗L‘ for shorthair – ‗l‘ for longhair; ‗Hr‘ for full coat – ‗hr‘ for hairless; etc. You should be aware that they do exist and play an important part in our breed.
III. In Closing
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NOTES
1. I use the term Chestnut vs Chocolate in my charts.
2. The superscript character is a lower case 'L' for 'light brown, in some usage it is impossible to superscript the character and the symbol is written as 'bl'
3. There are two other allele of the Albino series, ca - blue-eyed albino & c - pink-eyed albino
4. These are the Tabby symbols as laid out by Lorraine Shelton in Robinson's Genetics for Cat Breeders & Veterinarians, 4th Edition
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