Question

How can lethal alleles affect phenotypic ratios?

Answer 1

Lethal alleles, though unexpressed in the general population because of the mortal outcome of having two sets of them, can have a visible effects in phenotipic ratios when linked to other genes.

Explanation:

In the first place, phenotypic ratio is mostly determined by the collection of expressed genes inside each organism in a population.

Secondly, two or more genes are not always segregated 100% randomly during meiotic gametogenesis. In most cases, two genes that are closely located in a given chromosome will have their alleles assignated to the same gamete. Another way of saying this is that if two alleles of two different genes have but a small distance separating them, they are likely to avoid crossing-over and thus end up together after chromosomic segregation.

This phenomenon is known as genetic linkage and is responsible for associated hereditary features. For example, wrinkledness in sweetpeas may be statistically correlated with yellow colour in seeds without being caused by the same gene if the two responsible genes are "neighbouring" genes.

Thus, if one allele of a specific gene is linked to a lethal allele, it is more probable for the carrier to die due to latter. If each individual with the former is prone to result in a miscarriage, then this allele will be remarkably underrepresented in the visible population, thus causing its phenotypic ratio to plummet below the predictions that should otherwise be fulfilled according to traditional Mendelian genetics.

Sources:

I study biology at the UPCH, here in Perú. You can Google "genetic linkage" and find more details and probably more material on its special relevance to lethal alleles.