Final answer:
Lethal alleles, both recessive and dominant, can lead to the death of an organism. Recessive alleles are fatal in the homozygous state, while dominant alleles can be lethal in both the homozygous and heterozygous states. Illustrations include recessive lethal Tay-Sachs and dominant lethal Huntington's disease.
Step-by-step explanation:
Only lethal alleles will directly lead to the death of an organism. These lethal alleles can be either recessive or dominant. Recessive lethal alleles cause death only when an individual inherits two copies (homozygous). An example of a disorder caused by a recessive lethal allele is Tay-Sachs disease, which is fatal in early childhood. Conversely, dominant lethal alleles are fatal whether the individual has one or two copies. However, if a dominant lethal allele manifests after reproductive age, as with Huntington's disease, the allele may be passed on before the onset of symptoms.
In the case of Huntington's disease, individuals with one copy of the dominant allele (Hh) will eventually develop symptoms, although this may not happen until mid-adulthood, after they have had children, potentially transmitting the allele. With recessive lethal alleles, when two carriers reproduce, the expected genotypic ratio among offspring would be distorted to 2:1 instead of 3:1, as homozygous recessive individuals would not survive.