How can you tell if there's a lethal allele when given a problem?

In a Belgian horses, coat coloration can either be chesnut brown or roan (brown with white mixed in). The coloration pattern in these horses is governed by a single gene. The roan allele is completely dominant, so heterozygous horses are roan-colored. When you mate several sets of heterozygous roan horses over a number of years, you get 99 roan-colored offspring and 53 chesnut-brown offspring. You begin to suspect that one of the alleles for the coloration gene may be lethal. Why does this data seem to indicate that a lethal allele is involved? Which allele appears to be lethal when two copies are inherited. Use the data provided to test the hypothesis that one of the alleles for this gene is lethal.

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