Recognize progeny ratios that imply the existence of recessive lethal alleles.

Explanation:

From Mendel’s Law of Segregation we can expect, as a result of a monohybrid cross, a phenotypic ratio of 3:1 in the offspring.

If you get a different ratio such as 2:1, and notice that a particular genotype does not appear in the offspring, you can suspect that genotype is lethal.

Mendel’s Law of Independent Assortment on the other hand says that the offspring resulting from a dihybrid cross will have a phenotypic ratio of 9:3:3:1.

If you get numbers that don't match the expected ratios, and are smaller numbers like 4:2:2:1, and notice that a particular combination of alleles does not appear, you can suspect that allele is recessive and lethal in an homozygous individual.

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