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College of Cat Genetics: Part X
by Patricia Turner

Study Unit 10: Three Gene Pair Crosses

The correspondence found between the results of crosses such as those involving two gene pairs described with Blue Rex and Black Shorthair in an earlier Study Unit and those expected according to independent assortment also holds true for crosses involving large numbers of gene pairs.

In the experiment with three gene pairs, Mendel used the following:

Section A.

  1. Smooth and wrinkled seed shape.
  2. Yellow and green seed colour.
  3. Violet and white flowers.

The cat breeder may like to follow the experiment by the use of gene pairs more familiar:

Section B.

  1. Intense colour and dilution symbolized D and d respectively.
  2. Normal and Cornish Rex coat symbolized + and r respectively.
  3. Piebald white spotting and solid colour symbolized S and + respectively.

Where more than one normal (or wild type) alternative is involved and where the opposite in letter size is not conventional practice the plus sign alone may be confusing.  Where more than one pair of genes are being traced, the plus sign is usually added as a superscript to denote the normal alternative, i.e. r+ for the normal alternative to Cornish Rex.  In heterozygotes with the normal allele present at a given locus, the mutant allele is written first.  Alleles are separated by /; non-alleles by a space.  In the two sets of three gene pairs mentioned above, the first listed are dominant.

Mendel's experiment with three gene pairs will be reconstructed with the three gene pairs in Section B, although the results may be applied to any three sets of gene pairs known to be free from linkage.

To commence this experiment, the intensely coloured cat which, in this case, is black with a straight coat and white spotting is mated to a dilution cat which, in this case, is blue with Cornish Rex coat and no white spotting.  For the purpose of this experiment type and extent of white spotting are not considered.  The F1 kittens have the phenotype of the Black and White Shorthair dominant parent, d/D.r/r+.S/s+

 

 

The F1 kittens are mated on maturity, or as has been previously described, to others of similar genotype but from different parents.  They give rise to F2 kittens with 8 phenotypes and 27 presumed genotypes in the following approximate ratio:

  1. 27 Black and White SH (8 genotypes)
  2. 9 Black SH (4 genotypes)
  3. 9 Black and White Rex (4 genotypes)
  4. 9 Blue and White SHs (4 genotypes)
  5. 3 Black Rex (2 genotypes)
  6. 3 Blue SH (2 genotypes)
  7. 3 Blue & White Rex (2 genotypes)
  8. 1 Blue Rex (1 genotype)

Both the genotype and the phenotype ratios in the F2 can be derived by considering that 8 different types of gametes are formed by the F1 hybrid: Dr+S; Dr+s+; DrS; Drs+; dr+S; dr+s+; drS; drs+.

As in the mating involving two gene pairs each of the two gametes has an equal chance to combine with any other gamete thus producing 8x8 = 64 expected combinations.  Because of dominance and the fact that some phenotypes are produced by more than one genotype, only eight phenotypes appear.  The number of phenotypes is always less than the number of genotypes and it can be seen that the Black and White Shorthairs resulting from the above crosses can be produced in 8 different ways by the use of the Punnetts Square as explained in previous Study Units.