Genetics needs to bring out a clean line of white mice. He has only gray mice (A) as the initial forms. How

Genetics needs to bring out a clean line of white mice. He has only gray mice (A) as the initial forms. How can he get the cleanest line in the shortest possible way?

1) If among gray mice there are heterozygous individuals, then when mating in the first generation, white mice will appear.
2) They must be crossed in the future.

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