Chapter 4.2

Comprehensive vocabulary flashcards covering epistatic gene interactions, complementation analysis, pleiotropy, sex-linked and sex-influenced traits, penetrance, expressivity, cytoplasmic inheritance, and genetic anticipation.

Epistasis

The situation in which one pair of alleles can mask or stop the phenotypic effects of another pair of alleles at a different locus, leading to modified dihybrid ratios based on sixteenths such as $9:3:4$, $9:7$, or $12:3:1$.

Epistatic Gene

The specific gene that masks or suppresses the expression of another gene in an epistatic interaction.

Hypostatic Gene

The gene whose phenotypic expression is masked, hidden, or suppressed by an epistatic gene.

Recessive Epistasis

A form of gene interaction where homozygous recessive alleles ($2$ copies) at the epistatic locus block the expression of the hypostatic gene, producing a characteristic $9:3:4$ phenotypic ratio.

Bombay Phenotype

A condition resulting from recessive epistasis where individuals homozygous for the recessive $h$ allele ($hh$) fail to synthesize compound H, preventing the addition of terminal sugars and resulting in an O blood phenotype regardless of ABO genotype.

Dominant Epistasis

A gene interaction in which a single dominant allele at the epistatic locus is sufficient to block the expression of the hypostatic gene, yielding a characteristic $12:3:1$ phenotypic ratio.

Complementary Gene Interaction

An epistatic mechanism where at least one dominant allele at each of two separate gene loci ($A\_B\_$) is required to produce a specific phenotype (such as purple flowers in sweet peas), resulting in a $9:7$ phenotypic ratio.

Novel Phenotypes

New phenotypic categories in the $F_2$ generation produced by gene interaction that differ from both parental strains, such as spherical fruit shape ($6/16$) resulting from crossing disc ($AABB$) and long ($aabb$) summer squash in a $9:6:1$ ratio.

Complementation Analysis

An analytical experimental procedure used to determine whether two independently isolated recessive mutations that produce identical phenotypes reside in the same gene (allelic) or in different genes.

Complementation Group

A collection of all mutations that are located within the same gene, representing allelic variations of that locus.

Heterogeneous Trait

A phenotypic trait where a similar mutant phenotype can be produced by a mutation in any one of multiple different genes involved in a common structure or cellular function.

Pleiotropy

A genetic phenomenon where the expression of a single gene mutation results in multiple distinct phenotypic effects across different body systems.

Marfan Syndrome

An autosomal dominant disorder caused by a single mutation in the gene encoding the connective tissue protein fibrillin, resulting in pleiotropic defects in the eye lens, bone structure, and aorta lining.

Porphyria Variegata

An autosomal disorder in which affected individuals cannot adequately metabolize the porphyrin component of hemoglobin, leading to toxic porphyrin accumulation and pleiotropic symptoms like abdominal pain, muscular weakness, fever, racing pulse, insomnia, and vision issues.

X-Linkage

The transmission and expression of genes located exclusively on the X chromosome that lack corresponding homologous alleles on the Y chromosome.

Hemizygosity

The condition of possessing only one copy of a gene in an otherwise diploid organism or cell, such as genes on the X chromosome in $XY$ human or Drosophila males.

Crisscross Inheritance

An X-linked inheritance pattern in which phenotypic traits controlled by recessive X-linked alleles are transmitted directly from homozygous mothers to all of their sons.

Sex-Limited Inheritance

A pattern of inheritance where the phenotypic expression of an autosomal trait is absolutely restricted to one biological sex, though the responsible genes are present in both sexes.

Cock-Feathering

An autosomal recessive sex-limited trait in chickens where longer, more curved, and pointed plumage is expressed exclusively in homozygous recessive ($hh$) males under the influence of sex hormones.

Sex-Influenced Inheritance

An inheritance pattern where the sex of an individual alters the expression or penetrance of an autosomal gene, causing heterozygous individuals to express one phenotype in males and a contrasting phenotype in females.

Male Pattern Baldness

A sex-influenced autosomal trait where the allele $B$ behaves as dominant in males ($BB$ and $Bb$ are bald) but behaves as recessive in females ($Bb$ is not bald; $BB$ is bald but less pronounced).

Penetrance

The percentage of individuals with a specific mutant genotype that actually exhibit the associated phenotype, calculated as: $\text{Penetrance} = (\frac{\text{number of individuals expressing phenotype}}{\text{number of individuals with genotype}}) \times 100$.

Expressivity

The range, severity, or extent to which a mutant phenotype is expressed among individuals sharing the exact same genotype.

Position Effect

A change in the phenotypic expression of a gene caused by its relocation to a different physical chromosomal environment, such as proximity to condensed heterochromatin.

Heterochromatin

Chromosomal regions (including centromeres and telomeres) that remain condensed and genetically inert, which can suppress or silence the expression of adjacent genes.

Temperature-Sensitive Mutations

A category of conditional mutations whose phenotypic expression varies according to environmental temperature, causing enzymes or gene products to function normally at one temperature but lose activity at another.

Permissive Conditions

Environmental conditions (such as specific temperatures) under which a conditional, temperature-sensitive mutant gene product remains functional and supports a normal or viable phenotype.

Restrictive Conditions

Environmental conditions (such as elevated temperatures) under which a temperature-sensitive mutant gene product loses function, arresting organismal development or displaying a mutant phenotype.

Nutritional Mutations (Auxotrophs)

Mutations in microorganisms that prevent the synthesis of essential nutrient molecules, requiring the missing nutrient to be provided in the diet or growth medium for survival.

Extranuclear Inheritance

The transmission of genetic traits determined by DNA located outside the nucleus within cytoplasmic organelles, such as mitochondria ($\text{mtDNA}$) and chloroplasts ($\text{cpDNA}$).

Endosymbiotic Theory

The evolutionary model explaining that mitochondria and chloroplasts originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells and developed a symbiotic relationship.

Maternal Inheritance

A uniparental inheritance pattern where organellar DNA is transmitted exclusively from the mother via the egg's cytoplasm to all offspring, with affected males unable to pass the trait down.

Four O'Clock Plant Variegation

A cytoplasmic maternal inheritance model identified by Carl Correns, where leaf coloration (white, green, or variegated) in Mirabilis jalapa depends entirely on the maternal ovule phenotype due to random segregation of chloroplasts.

Delayed Onset of Phenotypic Expression

The postponed manifestation of an inherited genetic disorder until specific later developmental stages (infant, preadult, or adult), as seen in Tay-Sachs disease, Lesch-Nyhan syndrome, and Huntington disease.

Genetic Anticipation

A phenomenon where an inherited genetic disease exhibits earlier onset and progressively increased severity in successive generations due to the expansion of an unstable trinucleotide repeat sequence.

Myotonic Dystrophy (DM1)

An autosomal dominant muscular dystrophy exhibiting genetic anticipation, caused by variable repeat expansions of a $3$-nucleotide sequence within the DMPK gene from $5$ to $35$ copies in normal individuals to $150$ to $2000$ copies in affected individuals.