Final answer:
Null alleles are nonfunctional mutations in genes that do not produce a gene product. In a heterozygous state, they are masked by a functional allele, allowing carriers to pass the allele through the gene pool. Homozygous recessive individuals for a null allele can suffer lethal consequences if the gene is essential, altering expected genotypic ratios.
Step-by-step explanation:
Null alleles are a type of genetic variant where the allele is nonfunctional due to a mutation. Such alleles fail to produce the functional product that they typically would, due to deletions, insertions, or point mutations that disrupt the gene function. In diploid organisms, which possess two alleles for each gene, the presence of one null allele (heterozygous condition) is often masked by a functional wild-type allele. Therefore, the individual being a carrier does not exhibit the phenotype associated with the null allele, because the wild-type allele is dominant and can provide enough product to maintain normal function. This is why the null allele can remain in the gene pool of a population; it gets passed down through carriers without affecting the viability of individuals carrying one copy.
However, when an individual is homozygous for the null allele (possessing two copies), it can be detrimental or even lethal if the gene is essential for survival. The severity of the phenotype depends on the function of the gene in question and the stage of life at which the gene is required. An example of such a scenario is the concept of recessive lethal alleles, which can be carried by heterozygous individuals but can cause death in homozygous recessive offspring. This is because the two nonfunctional alleles do not produce the needed gene product essential for life. As a result, the expected genotypic ratio among surviving offspring from two heterozygous parents is 2:1 rather than the Mendelian 3:1 ratio, since homozygous recessive offspring would not survive to be counted among the surviving progeny.