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Some alleles are lethal as they cause the death of the individual that inherits them. Such alleles are called lethal alleles. The presence of dominant lethal alleles causes the death of the organism before an individual attains puberty.

Thus, dominant alleles are not transmitted to the next generations. However, some lethal alleles are expressed after the reproductive age of an individual. These alleles are late-acting lethal alleles.

Individuals with such alleles can transmit the alleles to their offspring, which is a rare condition. Thus, the frequency of dominant lethal alleles is much less in the population.

Recessive alleles cause lethal effects only when they are in homozygous condition. Thus, homozygous individuals cannot transmit the lethal alleles to the future generation. A heterozygous carrying one recessive allele and one normal allele can transmit the lethal alleles to future generations.

Thus, the frequency of recessive lethal alleles is higher in the population as compared to dominant lethal alleles.

It has been observed that people of the United States and other developed countries are getting married and starting families late in life. There is a high chance that the frequency of late-acting dominant lethal alleles would increase in the population in such conditions.

In most cases, individuals who possess dominant alleles die early in life before transmitting their lethal alleles to future generations. However, individuals who possess late-acting dominant lethal alleles can survive longer because the alleles are expressed later in life.

Thus, there are high chances that people having children late will transmit the lethal alleles to their children. As a result, the frequency of such late-acting dominant lethal alleles would increase in the population.