Interactions BIOL 261

Haplosufficiency

A condition in which a single functional wild-type allele provides at least $50\%$ normal gene activity, which is sufficient to produce a wild-type phenotype, causing loss-of-function mutations to be recessive.

Haploinsufficiency

A condition where having only one functional copy of a gene ($50\%$ dosage) is inadequate to produce a normal phenotype, causing a loss-of-function mutation to act as dominant.

Dominant negative mutation

A mutation where a defective polypeptide subunit incorporates into a multimeric protein complex and prevents the entire complex from functioning properly.

Gain-of-function mutation

A genetic change that confers an enhanced or novel activity onto a protein, typically inherited dominantly (such as constitutively active Ras driving cell proliferation).

Incomplete dominance

A pattern of inheritance where the heterozygous phenotype is an intermediate blend between the two homozygous phenotypes, producing a $1:2:1$ phenotypic ratio in a monohybrid cross.

Codominance

A pattern of inheritance where both alleles in a heterozygote are fully and simultaneously expressed without blending (e.g., $AB$ blood type or $Hb^A / Hb^S$ heterozygotes).

Recessive lethal allele

An allele that causes death only when present in two copies (homozygous mutant), leaving a modified $2:1$ phenotypic ratio among surviving progeny.

Expected phenotypic ratio among surviving progeny from a cross of two heterozygotes carrying a recessive lethal allele

$$2:1$$

Auxotroph

A nutritional mutant organism that has lost the ability to synthesize an essential organic compound and can only grow when that nutrient is added to minimal medium.

One gene-one enzyme hypothesis

The proposition formulated by Beadle and Tatum demonstrating that each specific gene encodes a particular enzyme or polypeptide in a biochemical pathway.

Biosynthetic intermediates in the Neurospora arginine pathway in sequential order

Precursor $\rightarrow$ Ornithine $\rightarrow$ Citrulline $\rightarrow$ Arginine

Phenylketonuria (PKU)

A metabolic disorder caused by a mutation in phenylalanine hydroxylase, preventing the conversion of phenylalanine to tyrosine and leading to the toxic buildup of phenylpyruvic acid.