Final answer:
Mutations affecting the phenotype only under certain circumstances are called conditional. Recessive traits are seen in individuals that are homozygous recessive. Dominant negative mutations interfere with the function of the wild-type gene product.
Step-by-step explanation:
Mutations that affect the phenotype only under certain circumstances are called conditional. In genetics, certain mutations may only impact an organism's phenotype in specific environments or conditions, which is why they are termed as such. For example, a temperature-sensitive mutation might only cause a phenotype to be expressed at higher temperatures. This contrasts with unconditional mutations, which affect the phenotype regardless of external conditions.
A recessive trait will be observed in individuals that are homozygous for that trait. This means that the phenotype associated with a recessive allele will only be seen when an individual has two copies of that allele, a condition that is known as being homozygous recessive. In a genetic context, heterozygous individuals carry one dominant and one recessive allele, but it is the dominant allele that will be expressed in the phenotype.
Dominant negative mutations are a form of gain-of-function mutations which are dominant, and they can interfere with the function of the wild-type gene product. These mutations can be particularly harmful as they may act as competitive inhibitors, reducing the normal protein's function.
The idea that alleles can be either dominant or recessive is foundational in genetics. Dominant alleles will express their effect even if there is only one copy present in the genotype (heterozygous), while recessive alleles require two copies (homozygous) for their effects to be observed phenotypically.