Lethal White

What Is A Lethal White?

Lethal white is a term often used to describe a homozygous merle. A homozygous merle has two merle parents and has inherited a copy of the merle gene from each of them making him a "double merle." Most merle dogs only have one copy of the merle gene and one copy of the solid gene. Since merle is dominant to solid the heterozygous merle is a normal merle. Heterozygous merles are genetically normal and no health problems are associated with a single copy of the merle gene. A double copy of the merle gene however is associated with several health problems.

At birth most homozygous merles are mostly white, or may appear to have a washed-out coat. In addition to the hearing problems associated with the lack of pigment around the ears/eyes, the lethal white has a high probability of having serious eye defects such as Collie Eye Anomaly, blindness, cataracts and others. Heart and liver disorders are also reported although references for this phenomena is lacking.

Only the homozygous merle puppies in a merle to merle breeding are known to be affected in this way. Non-merle and heterozygous merle puppies from the same litter are not affected. The puppy buyer is advised to be aware of these problems and willing to deal with them if considering acquiring a homozygous merle puppy.

What Are The Chances of Producing a Lethal White?

A Lethal White is only produced from the breeding of a merle female to a merle male. Avoiding merle to merle breeding is the surest way to avoid producing a Lethal White. Sometimes however a merle sire and dam are so compatible that a breeder may risk the chance of producing a lethal white for the opportunity of producing an exceptional litter. The explanation below offers statistical probability for producing a homozygous merle in a merle to merle breeding. Not every merle to merle breeding will produce a lethal white. These statistics are accurate when studying large numbers of breedings, but results within an individual litter may vary.

To examine the statistical probability of producing a lethal white from a merle to merle breeding, one can use a Punnett Square. A Punnett Square shows the possible combination of genes for dominant and recessive traits. As stated above, the merle gene (M) is dominant, while the solid gene (m) is recessive. It only takes one copy of a dominant gene to express that trait, however it takes two copies of a recessive gene to express a recessive trait. Therefore:

  • mm = solid color
  • Mm = one merle gene, one solid gene; merle is dominant to solid so the dog is merled
  • MM = homozygous (double) merle

Breeding two MM dogs together would produce an all merle litter as there are no solid genes in the equation. 100% of the litter would be homozygous merles and subject to all the woes associated it as seen in the following Punnett Square:


In the next example, the sire and dam are both heterozygous merles:

M m
Dam M MM Mm
m Mm mm

25% of the litter is homozygous merle (MM), 50% is heterozygous merle (Mm) and 25% are solid (mm). Again, this is statistical probability, results on a per litter basis may vary.

The breeding of two merles is advisable only by experienced breeders who are very familiar with the lines behind both sire and dam and the potential consequences of such a breeding. The breeder should discuss in detail the potential defects of the homozygous puppy who is being placed with a pet owner.

© 2000, Janet Boursier. No portion of this material may be reproduced or used by any individual or organization without the express written consent of the owner.