Chapter 4.1

Comprehensive vocabulary flashcards covering the key terms, mechanisms, and examples of non-Mendelian inheritance and gene modifications from Chapter 4.

Gene interaction

A condition in which a single phenotype is affected by more than one set of genes, or where the phenotypic expression of genes at one locus depends on the presence of genes at other loci.

Wild-type ($wt$) allele

The allele that occurs most frequently in nature and is typically, though not always, dominant.

Loss-of-function mutation

A mutation that leads to a reduction or complete elimination of the wild-type function or activity of a gene product.

Gain-of-function mutation

A mutation that enhances the function of the wild-type gene product or increases the quantity of gene product produced.

Neutral mutation

A mutation that causes no observable change to the phenotype and has no effect on the evolutionary fitness of an organism.

Complete dominance

A condition in which the phenotype of the heterozygote is identical to the phenotype of the dominant homozygote.

Incomplete dominance

A mode of inheritance where the heterozygous phenotype is an intermediate blending that falls within the range between the two homozygous phenotypes, yielding identical genotypic and phenotypic ratios.

Codominance

An inheritance pattern in which both alleles in a heterozygote produce distinct, detectable gene products, resulting in the simultaneous phenotypic expression of both alleles without blending.

Tay-Sachs disease

A human fatal lipid-storage disorder inherited in an autosomal recessive manner, characterized by the absence of hexosaminidase A activity and onset between $3\text{--}6$ months of age.

Threshold effect

A phenomenon where normal phenotypic expression is attained whenever a minimum quantity of gene product (typically $50\%$ or less) is present, as observed in healthy carriers of Tay-Sachs disease.

MN blood group

A human red blood cell antigen system governed by codominant alleles ($L^M$ and $L^N$), where heterozygous individuals ($L^M L^N$) express both M and N antigens.

Multiple alleles

The existence of three or more alternative forms of a specific gene within a population, which can only be studied at the population level.

Isoagglutinogen ($I$)

The gene locus designating antigens responsible for the human ABO blood groups, represented by alleles $I^A$, $I^B$, and $i$.

H substance

The precursor carbohydrate structure formed by the addition of fucose via the $FUT1$ allele, to which terminal sugars are attached to form A or B antigens.

Bombay phenotype

A rare genetic condition where an individual homozygous for a mutant $FUT1$ allele fails to synthesize H substance, preventing the expression of A or B antigens and functionally resulting in type O blood.

Essential genes

Genes required for basic survival; organisms must possess at least one functional copy to live, whereas homozygous null mutations are lethal.

Dominant lethal allele

A lethal allele that causes death in heterozygous individuals carrying just one copy of the allele, such as the allele for Huntington disease.

Huntington disease

An autosomal dominant disorder caused by the $H$ allele, characterized by delayed adult onset (typically around age $40$), progressive neurological degeneration, dementia, and death.

Recessive lethal allele

An allele that results in death only when present in the homozygous form, shifting standard progeny ratios from $3:1$ to $2:1$ among surviving individuals.

$A^Y$ allele

A mutant agouti locus allele in mice that acts dominantly to cause a yellow coat color in heterozygotes ($A A^Y$), but acts as a recessive lethal in homozygotes ($A^Y A^Y$).

Heterogeneous trait

A mutant phenotype resulting from mutations occurring in any of multiple different genes, such as hereditary deafness, which involves over $50$ separate genes.