Copyright information:  information content copyright owned by Cat World expires 70 years from November 1973 at which time the information minus the research notes may be placed in the public domain.
Research note:  How do we ensure accurate copyright information provided?  How can we enhance the copyright experience -- by providing email addresses for those people who need to find out more about the copyright and by providing a link to copyright law.  Note in this article the use of phenotype pictures to enahnce the information experience.  Does it make understanding the concepts and principles easier?
 

College of Cat Genetics: Part IV
by Patricia Turner
 

Study Unit 4
 


 
 

The 1:3:1 Ratio
 
Some genes are not completely dominant over each other and this is true of the genes at the albino locus which are listed down the scale from full colour to siamese and possibly albino.  In this series burmese is only partially dominant to siamese.

As the burmese/siamese heterozygote (Tonkanese) is reasonably well known, the experiment showing the ratio expected will be reconstructed with burmese and siamese.

The gene for burmese is symbolized cb while the gene for siamese is symbolized cs.  As cb is incompletely dominant over cs, the heterozygote is intermediate in effect.

A cross is made between between a burmese and a siamese, both homozygous.

P1 cbcb                 x            cscs
Segregation into gametes cb           cb                  cs        cs
F1 genotypes
cbcs cbcs
cbcs cbcs
The mating between cbcb x cscs giving the F1 genotypes of cbcs and thus all kittens of intermediate effect.

In order to produce the F2, a mating is planned between the F1 littermates or between cats of genotype like them.  Each of these cats produces two types of gametes and again four types of fertilization are possible.
 

Genotype Phenotype
1 cbcb Burmese
2 cbcs Tonkanese
1 cscs Siamese
Ratio 1:2:1
T
O
N
K
A
N
E
S
E
TONKANESE

cb cs
cb cbcb
Kitten 1
cbcs
Kitten 2
cs cbcs
Kitten 3
cscs
Kitten 4
It should be remembered that the ratios relate only to the two alleles under investigation and that type and other factors are not included.  Since type is due to the joint action of many genes the F1 and F2 kittens in the above examples would be of indeterminate type.

The Backcross

If a cat that is heterozygous for any one characteristic is mated to either of the two parental types, the mating is called a backcross.  As has been described, the heterozygote forms two types of gametes while the homozygote forms only one.  So fertilization results in a genotype ratio of 1:1.

The factors of dominance, partial dominance and recessivity then dictate the phenotype ratio.

If an F1 cat of the mating between the original blue and black cats is then backcrossed to its blue parent or to another blue, the results will be as follows:
 

Genotype Phenotype
2 dd 2 Blue
2 Dd 2 Black
Ratio 1:1
B
L
A
C
K
BLUE

d d
d dd
Kitten 1
dd
Kitten 2
D Dd
Kitten 3
Dd
Kitten 4

If, on the other hand, the backcross of an F1 heterozygous black cat is made with a dominant partner of the original cross (i.e. the black), the results worked out by the Punnett Square will be as follows:
 

Genotype Phenotype
2 DD 2 Black 
2 Dd 2 Black 
Ratio all Black
B
L
A
C
K
BLACK

D D
D DD
Kitten 1
DD
Kitten 2
d Dd
Kitten 3
Dd
Kitten 4