Genetics: Heredity and Gene Interactions

Vocabulary practice flashcards covering polygenic inheritance, extranuclear/mitochondrial traits, expressivity and penetrance, complementation analysis, and modified dihybrid epistasis ratios.

Pleiotropy

The phenomenon where a single gene influences multiple distinct phenotypic characters, such as albinism causing both a lack of melanin production and vision defects.

Polygenic Inheritance

An inheritance pattern in which a character's phenotype is dependent upon the genotypes of many loci, producing continuous phenotypic variation across a population.

Quantitative Trait Loci (QTLs)

Genes involved in polygenic inheritance that are described as 'many genes, each of small effect,' which together contribute to additive genetic variation.

Continuous Variation

A distribution of phenotypic traits along a continuous spectrum within a population, which historically contributed to Charles Darwin's incorrect hypothesis of blending inheritance.

Polygenic Dose Model Cross (Figure 3-15)

A dihybrid selfing cross ($R_1/r_1 ; R_2/r_2$) where phenotypes depend on the total number of additive allele doses ($4$, $3$, $2$, $1$, or $0$ doses), resulting in an overall progeny probability distribution of $\frac{1}{16} : \frac{4}{16} : \frac{6}{16} : \frac{4}{16} : \frac{1}{16}$.

Metric Character Distribution (Figure 3-14)

A continuous bell curve graphing the frequency of individuals in a population against a quantitative metric character, such as color intensity.

Human Mitochondrial Genome (mtDNA)

A circular extranuclear genome of $16{,}569\text{ bp}$ containing gene loci where specific mutations can lead to genetic diseases such as MELAS, LHON, MERRF, NARP, KSS, and PEO.

Mitochondrial Disease Pedigree (Figure 3-25)

A pedigree illustrating extranuclear maternal inheritance, where an affected female transmits the mitochondrial disease to all of her offspring, whereas an affected male transmits it to none.

Chloroplast Inheritance in Chlamydomonas

An extranuclear inheritance system in single-celled green algae where streptomycin resistance ($str^R$) is encoded by a chloroplast gene passed exclusively from the $mt^+$ parent, whereas nuclear mating type ($mt^+$ vs $mt^-$) follows Mendelian inheritance.

Penetrance

The rate or percentage at which a mutant allele is expressed in the phenotype of individuals who carry the corresponding genotype.

Incomplete Penetrance

A phenomenon in which an individual carries a mutant genotype but does not express the expected phenotypic trait.

Expressivity

The degree or intensity to which a mutant allele is expressed phenotypically in an individual.

Phenotypic Expression Diagram

A conceptual model differentiating variable penetrance (where some mutant individuals do not exhibit the phenotype) from variable expressivity (where mutant individuals express different shades or levels of phenotype intensity).

Causes of Phenotypic Ratio Fluctuations

Factors leading to deviations from expected Mendelian ratios, including environmental conditions, unknown gene interactions, incomplete understanding of mutant phenotypes, and epigenetic influences.

H Substance and ABO Blood Type Interaction

A gene interaction cross ($I^A I^B Hh \times I^A I^B Hh$) producing a phenotypic ratio of $\frac{3}{16}\text{ Type A} : \frac{6}{16}\text{ Type AB} : \frac{3}{16}\text{ Type B} : \frac{4}{16}\text{ Type O}$, because homozygous recessive $hh$ fails to synthesize H substance required for A and B antigen expression (Bombay phenotype).

Modified F2 Phenotypic Ratios Table

A summary of modified $F_2$ dihybrid ratios resulting from gene interactions, including $9:3:4$ (mouse coat color), $12:3:1$ (squash color), $9:7$ (pea flower color), $9:6:1$ (squash fruit shape), $13:3$ (chicken color), $10:3:3$ (white-spotted mouse color), $15:1$ (Shepherd's purse seed capsule), and $6:3:3:4$ (flour beetle color).

Recessive Epistasis

A gene interaction yielding a modified $9:3:4$ phenotypic ratio in a dihybrid cross, where homozygous recessive alleles at one locus ($aa$) mask the expression of alleles at a second locus.

Dominant Epistasis

A gene interaction yielding a modified $12:3:1$ phenotypic ratio in a dihybrid cross, where a dominant allele at one locus masks the expression of alleles at a second locus.

Complementary Gene Interaction

A duplicate recessive epistasis interaction yielding a modified $9:7$ phenotypic ratio in a dihybrid cross, where homozygous recessive alleles at either locus result in the mutant phenotype.

Duplicate Interaction (Cumulative Gene Effect)

A gene interaction yielding a modified $9:6:1$ phenotypic ratio in a dihybrid cross, where dominant alleles at either locus alone yield the same intermediate phenotype, but together produce a distinct phenotype.

Dominant Inhibition

A dominant and recessive epistasis interaction producing a modified $13:3$ phenotypic ratio in a dihybrid cross, where a dominant allele at one locus suppresses the expression of a dominant allele at a second locus.

Duplicate Dominant Epistasis

A gene interaction yielding a modified $15:1$ phenotypic ratio in a dihybrid cross, where a dominant allele at either of two loci produces the dominant phenotype.

Complementation Analysis Diagram

A experimental test cross demonstrating that mutations in separate genes produce wild-type offspring ($F_1$ winged flies via complementation), whereas mutations in the same gene produce mutant offspring ($F_1$ wingless flies due to lack of complementation).

Labrador Retriever Coat Color Epistasis

A recessive epistasis interaction between genes A and B where A_B_ yields black coat color, $A_bb$ yields brown ('chocolate') coat color, and homozygous recessive $aa$ yields yellow coat color regardless of B locus alleles.