The following information regarding Mating a Merle to a Merle and Patterned Whites can be found in the book "All About Aussies" by Jeanne Joy Hartnagle-Taylor (in the section "The Mating Game"):
Mating a Merle to a Merle
"When two merled Aussies are mated together, the puppies will inherit either merled genes or nonmerled genes from their parents. Some puppies will inherit a nonmerled gene from each parent, resulting in a solid-colored puppy. Other puppies will receive a nonmerled gene from one parent and a merled gene from the other parent, resulting in merled puppies. The remaining 25 percent of the litter (one out of four puppies) will receive two merled genes, one from each parent. When Aussie puppies inherit two merled genes, the result is a double (defective) merled white. These puppies are almost white in appearance and may or may not have a few merling spots or diluted speckles. These individuals can have defective organs, and their hearing and sight are almost always impaired. In some cases, the eyes are not completed formed or the puppies have no eyes. The eye rims, lips, nose and pads are without pigment except where some merling is present. The defective merled whites occur in approximately 25 percent of the litter.
Defective puppies have no effect on normal-colored puppies in the litter. The remaining puppies (approximately 75 percent) that are solids and merles are totally normal because they have inherited either two nonmerled genes or one nonmerled gene and one merled gene. The only time when defective (double) merled whites, known also as homozygous whites, occur is when there is a concentration of the merling genes by mating two merled individuals together. It can be totally avoided by breeding solid-colored individuals to merled individuals. Whenever you cross two merled Aussies, it is necessary to cull the defective whites, which are identified by the appearance of white and diluted coloring."
Patterned White Aussies
"Too often, novice or inexperienced breeders confuse the defective merled whites with "mismarked" or patterned white Australian Shepherds. These individuals can appear in a litter in which two solid Aussies or a solid and a merled Aussie or two merles, are bred together. These "white" puppies are not defective like the double merled whites, nor are they a product of the merling gene. These Aussies may have solid-colored bodies with white spots (not to be confused with merling spots), or they may have white bodies with colored spots, or more than half of their bodies may be white. The head is always colored, and the ears and eyes are surrounded by full pigment other than white."
Lethal White (printable) Info Packet
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