Genetics

  
 

   
 
 
  Click on the bold italic words to access the  Glossary of Terms  
 

THE BASICS

  
 

A cat's visible characteristics such as coat colour, pattern and length, eye colour, body type, tail length, ear size and shape,  temperament etc, vary remarkably between breeds, and, in the "moggy:/mixed breed population, kittens from the same litter can be so different  in colour, coat and shape that it can be difficult to comprehend that they have the same parents. On the other hand, kittens can also all look similar to each other or even look like a clone of one of the parents.

There is in fact,  often a remarkable similarity between parents and their offspring, and between siblings, but no matter how near  the likeness, a closer inspection usually highlight many subtle differences that also exist  between very similar individuals.

 
 

                              CELLS, CHROMOSOMES & INHERITANCE
 

 

 

 

 

 

 

 

 

 

Some of these similarities and differences are dramatically highlighted in the above photograph of the blue/cream tortoiseshell and white mother cat and her five kittens. The two solid black and solid red, dense pigmented kittens are totally different to their patterned, pastel coloured mother and siblings.  Furthermore, while the two dilute or pastel coloured kittens exhibit their mother's soft colouring, they differ remarkably in coat pattern and the distribution of coat colour. 

The above mentioned similarities and differences are due to inherited characteristics, or coded information carried in genes that are contained on chromosomes within every cell. Every normal body cell in the domestic cat contains 38 paired chromosomes. Genes may be dominant or recessive  and the visual expression of a certain characteristic, such as coat colour, depends on which of these variations the kitten inherits. 

The genetic information is passed from each of the parents to their offspring. Each of the parent's ova or sperm, referred to as a gamete, contains only one half  from each of the 38 paired chromosomes, resulting in total of 18 autosomes and 1 sex chromosome, half the genetic material contained in normal body cells. During fertilisation, the ova and sperm fuse together to form a zygote. Fertilisation blends the 19 chromosomes from egg and sperm from the parents therefore restoring the full complement of 38 chromosomes in every cell as the new kitten grows. The zygote cells continue to divide and replicate to develop into an embryo then a foetus and finally a male or female infant.

Kittens therefore inherit half their genetic material from their father, and half from the mother when the DNA/chromosomes fuse at fertilization. Female animals carry two XX sex chromosomes, and, male animals carry one X and one Y chromosome (XY) Because the kittens only inherit one of each pair, they always inherit one X chromosome from their mother and, either an X or a Y from their father. While the male to female ratio in a litter cannot be predicted on a small scale, the actual odds are mathematically calculated at 50/50,  or a 1:1 ratio because male sperm is made up of equal numbers of X or Y  chromosomes and each X or Y from the father has an equal chance of fertilizing the ova.

The following table provides a basic graphical representation male/female sex genes,  their symbolic expression, and the arithmetic calculation of the expected offspring from the mating. In this case it  will be a 1:1 ratio.

 

     
 
     

Father

    

X

Y
 

Mother

X

XX

XY

2  Female Kittens  = 50%

   2  Male Kittens  = 50%  = 1:1 ratio          

   
 

X

XX

XY
     

Kittens

    
 
 
     

 Table 1:

 Graphical representation showing  the sex genotype of both parents and the predicted 1:1 genetic ratio for the sex of the progeny of the mating
 

The animal's genetic makeup is referred to as the genotype and the outward appearance, or the characteristic we actually see, is referred to as the phenotype. If a kitten receives identical genes from each parent for a particular characteristic, that is, when a pair of chromosomes at a particular locus are the same, its genetic makeup is said to be homozygous for that trait. Alternatively, if the kitten receives a particular gene from one parent and an alternative form of that gene from the other parent, the genetic make-up is said to be heterozygous for that trait.  Therefore, the phenotype of two animals may be the same for say, black coat colour in sibling kittens, but, they may be genotypically different from what can actually be seen.  For example, the black coated kitten may also be carrying the gene for blue coat, but this characteristic is hidden, or can't actually be seen. These differences result from the action of a dominant  gene over a paired recessive gene for that characteristic.

Basic terms to remember:

  • There are always two matching sets of Chromosomes In all cells except sperm and egg cells, which only contain one set. One of half of each pair is inherited from each of the parents.

  •  Genes: sections of DNA along a chromosome that are positioned at specific points known as the locus

    • . Genes carry the information for all the inherited characteristics of the individual such as hair , skin or coat colour eye colour, body shape, size, etc.

  • Alleles are varieties of different genes that occupy the same position on  chromosome

    • , eg. blue and black coat alleles are a matching pair for coat colour, found at the same place on the chromosome, but are different in actual make-up as they carry different information about colour.

  • Dominance: the visual aspect of a characteristic when one gene dominates over another recessive or hidden gene, eg. black is dominant over blue coat colour, so the cat that is carrying both these alleles will be black even though it is carrying the blue (hidden) gene.

  • Modifiers: genes which have a modifying  effect on other genes in some way .

The genes for coat colour (a single specific characteristic) are carried on two matching alleles positioned on the same locus on a gene. As stated above, these two alleles, although identical for determining coat colour, may be different from each other. For example one of the alleles may be dominant for one colour and the other recessive for another colour. The genetic coding for colour may also be influenced by other genes that modify. The himalayan/colourpoint gene is a modifier that inhibits the expression of colour except at certain points on the cat. White is a gene that is actually not a colour but a lack of colour that totally masks the expression of all other colours even those known to be dominant genes.

The following diagram shows what happens genetically when mating two adult cats that are homozygous for a particular trait or characteristic such as coat colour, This mating results in pure breeding or what breeders call "breeding true" over subsequent generations.  Pure, or genetically identical kittens are always produced from matings between parents that are homozygous for a particular trait regardless of whether the two parents are genetically the same for the dominant or for the recessive gene. That is, two homozygous dominant black adults will always produce black kittens and two homozygous recessive blue adults will always produce blue kittens.

 

  
 

 

  

 

  
 

Diagram 1: Dominant  Black Coat gene is expressed symbolically as BB and the recessive blue coat gene is expressed symbolically as bb.  All offspring from a mating between parents that are homozygous for coat colour results in kittens that are genetically and phenotypically identical to their parents.

 
 

                    A

  

                    B

  
     
                         C  

            A:

 

Homozygous Dominant Black coat  parent crossed with Recessive Blue Coat parent results in all phenotypically black coated, genetically heterozygous kittens that are carrying the recessive but unexpressed blue coat gene.
   

             B:

 

Heterozygous Black coat parents produce litters in the following ratio:  1/2 heterzygous (black coat), 1/4 homozygous dominant (black coat) and 1/4 homozygous recessive (blue coat) kittens = 3:1 ratio

 C:

Heterozygous Black coat parent crossed with a homozygous recessive blues coat parent produce litters in the following ratio: 1/2 heterozygous black coat kittens & 1/2 homozygous blue coat kittens.+

Diagram 2:  Graphic representation depicting the genotype and phenotype of kittens produced from matings carrying variations of the genes for the Dominant Black (BB) and Recessive Blue Coat (bb).

 

                           Successive Generation Table
       

 Diagram 3: Graphic representation of kittens produced from subsequent litter generations that are mated back to each other. The first filial generation is referred to as F1 and the second filial generation as F2.

Cornish Rex Coat  (see Diagram 1)

The above rules for determining the ratio of black to blue kittens in a mating between parents that are homozygous (pure) for either one of those coat colours, can be used to determine the outcome of matings between subsequent generations of rex and straight coated kittens.  The straight/normal coat (RR) is dominant over the  recessive (rr) rex coat. Therefore using the above diagrams, the genetic symbol B (black) can be substituted with R (normal) and the b (blue) can be substituted with r (rex). The formula is the same for all dominant/recessive combinations.

There are differences however between dominant and recessive carriers. A prime example of this is the difference between the Cornish Rex and Devon Rex breeds. When these two breeds are mated, the kittens all have straight coats. 

Rex Genotypes: The Cornish and Devon Rex  are genetically quite different and labelled for scientific purposes as: Gene 1 Rex (Cornish) and Gene 2 Rex (Devon). Genetically the two breeds are currently known as: Cornish (rrReRe) and Devon (RRrere). A Cornish x Cornish mating  produces kittens that are genetically the same for coat type as their parents (rrReRe), and kittens from a  Devon x Devon mating have the same coat as their parents (RRrere). But a Cornish x Devon mating results in straight coated kittens with a RrRere genetic descriptor. Both breeds are genetically recessive to normal coat and independent of each other. The German Rex, however, is genetically compatible with the Cornish and breed true for rex coat when they are cross mated.

However,  there are many other genes that do not fall into this simple dominant and recessive category. Some of these  genes act as modifiers or polygenes, others are not fully penetrant, while others mask the action of dominant genes.  Several further categories of gene include those that are linked to the X or Y chromosome resulting in sex-linked gene expression, inhibititor genes that mask the action of another gene, limited penetration genes and a finally, a category that causes diseases including lethal genes. 

The following links provide a great deal of information on these variations in genetic makeup and inheritance.            

                                            ONLINE GENETIC LINK
         *  FELINE GENETIC PRIMER  & FREE Feline Genetics Software
 
  Free program for predicting phenotypes of cats based on known genotypes, and calculating mating predictions. From the makers of Breeders Assistant Pedigree Software
 

Cat Genetic Resources
   

                    Extra Reading & Excellent Resources

  

*

 SIMPLE GENETICS FOR CAT JUDGES & BREEDERS:  by Diana F Arnold
  *  : Cat Gentics Links  
  * CAT COLOURS: FAQ: Common Colors  
  * CAT COLOR GENETICS  
  * THE PIGMENT PARADE:   by Lorraine Shelton of Featherland Cattery  
  * COAT COLOURS:   Penmarric Cornish Rex Cattery  
* Feline Color Genetics Links
   * * MESSY BEAST: excellent resource with many sites listed  
 

 

 

 

 
 
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