Final answer:
A geneticist producing an organism homozygous for a loss-of-function allele has created a mutation. An allele is a gene version that can influence traits, and phenotypes can differ despite the same genotypes due to dominance in alleles. Mutations can affect fitness and survival, with recessive lethal mutations being fatal when homozygous.
Step-by-step explanation:
When a geneticist produces an organism that is homozygous for a loss-of-function allele, the change is called a mutation.
An allele is a version of a gene that can exist in different forms and influence the traits of an organism. A type of allele called a recessive allele only affects the phenotype when an organism has two copies of that allele, making it homozygous recessive. Phenotype and genotype are not synonymous; two individuals can have the same phenotype but different genotypes. For example, a tall, green plant with a homozygous genotype for both traits would have the genotype TTGG (T for tall and G for green), and its phenotype would be tall and green.
Mutations can result in organisms with a reduced fitness, an improved fitness, or no change in fitness at all. In some cases, a nonfunctional allele that is recessive can lead to a condition where the homozygous individual does not survive, known as recessive lethal. Conversely, a dominant lethal mutation leads to death in heterozygous and homozygous individuals. An example provided in the study of Biology would be Huntington's disease, demonstrating that even a dominant lethal allele can be transmitted if the phenotype does not express until after reproductive age.