Page 21 - Aussie Magazine - 2023 Issue 2

Mosaicism
Some of the most exciting test results which
explained so many previously unexplained FIRE
irregularities we often see in some Merle patterns
and differences in phenotype from parent to
offspring, were the mosaic results. I remember
very well my astonishment at seeing that first
mosaic result, “WHAT IS THAT?!” And then that
moment of profound revelation when suddenly it
all came together and made sense. Mosaicism -
“Somatic Mosaicism” or “Somatic Mutation” is
the presence of two or more types of cells with
different genotypes present in the body of one
individual dog. Merle mosaicism results from
the shortening of the poly-A tail in one cell in the ones with the longest base pair numbers - the
the early stages of embryo development. This alleles inherited from each parent.
mutation is then replicated during cell division.
The shortened length/allele will be present However in some very interesting cases the
in only some of the adult cells and in different mutated cell has replicated at a higher than
parts of the body. From 308 Merle dogs tested, normal rate or even at such a great rate that
56 are mosaics - an average of 18% or 1 out of there is a larger fraction of the shortened allele
every 5.5 dogs having 3 or more different alleles then that of the original allele inherited from the
on the M locus, indicating that mosaic results are parent. This original allele from the parent then
not uncommon. becomes the “minor” allele and the mutated/
shortened allele is the “major” allele. The
NOTE: Merle’s poly-A tail is not unique in this expression of the mosaic dog’s phenotype will
sense of shortening. It is common for the tail of depend on how the mutated cells replicated and
all SINE’s to shorten. In this way researchers can the fraction of each allele present in the dog. In
estimate the age of the insertion - the longer most cases it is impossible to know that the dog
the tail then the more recent the insertion. is a Mosaic Merle based on phenotype alone,
For example “at” - Tan Points on the A Locus however in cases where the cell containing the
is surmised to be an older SINE mutation as shortened allele has replicated at a higher rate
the poly-A tail length is very short and stable the phenotype of the dog can be dramatically
at 99 - 100 bp. However, judging the age of altered from what would be expected from the
the Merle’s poly-A tail in this manner will not 2 original alleles inherited from the parents.
apply as the shortening has not been left to Further, when a mutation occurs very early in
nature. As breeders we have artificially kept development, it may be present in both somatic
the longer visible lengths of M and Mh in play and germline cells. Somatic cells occur in the
by intentionally breeding for the trait. If Merle body only; they include all cells other than
had been left to nature the visible pattern would reproductive. Germline cells are found only in
likely be gone by now as every dog’s length the gonads - the ovaries of a female where eggs/
became Mc. ova are produced and testes of the male where
sperm are produced. A germline mutation alters
Typically, only a small proportion of cells contain the genetic make-up of the reproductive cells,
the shortened allele - this is referred to as the meaning that the cells containing the mutated/
“minor” allele/s and represented in test results shortened allele may be present in either the
with the use of [square] brackets. The two alleles male’s semen or the female’s eggs. In this way a
with the higher peaks on the chromatogram having germline mutation can affect the progeny of the
the larger fraction of cells are referred to as the mosaic Merle dog and subsequent generations
“major” alleles. In most cases the major allele/s are of that offspring.

A Worldwide Magazine for the Aussie • Issue 2/2023 | 21