Chapter 14: Mendel and the Gene Idea

Vocabulary flashcards covering Mendel's laws of inheritance, probability rules, non-Mendelian inheritance patterns, human genetic disorders, genetic crosses, and stages of cell division.

Law of Segregation

Mendel's first law stating that the two alleles for a single gene separate into different gametes during gamete formation (meiosis), occurring mechanistically during Anaphase I when homologous chromosomes separate.

Law of Independent Assortment

Mendel's second law stating that allele pairs for different genes segregate independently of one another during gamete formation, applying to genes located on different non-homologous chromosomes or far apart on the same chromosome.

Multiplication Rule

A rule of probability stating that the probability of two or more independent events occurring together in a specific combination is determined by multiplying their individual probabilities.

Addition Rule

A rule of probability stating that the probability of any one of two or more mutually exclusive events occurring is determined by adding their individual probabilities.

Incomplete Dominance

A pattern of inheritance where neither allele is completely dominant, resulting in a heterozygous phenotype that is an intermediate blend between the two homozygous phenotypes (e.g., pink flowers resulting from red and white snapdragons).

Codominance

A pattern of inheritance where both alleles affect the phenotype in separate, distinguishable ways without blending in the heterozygote (e.g., $I^A$ and $I^B$ alleles in ABO blood groups).

Multiple Alleles

The condition where a gene exists in more than two allelic forms within a population, such as the $I^A$, $I^B$, and $i$ alleles governing ABO blood groups.

Pleiotropy

The ability of a single gene to exert multiple phenotypic effects, such as the sickle-cell allele affecting hemoglobin structure, red blood cell shape, organ health, and malaria susceptibility.

Epistasis

A type of gene interaction where the phenotypic expression of a gene at one locus alters or masks the phenotypic expression of an independently inherited gene at a second locus.

Polygenic Inheritance

An additive effect of two or more genes on a single phenotypic character, resulting in continuous variation across a spectrum (such as human skin color or height).

Character

A heritable feature that varies among individuals, such as flower color.

Trait

Each specific variant for a character, such as purple or white flower color.

True-Breeding

Referring to organisms that produce offspring of the same variety over many generations when self-pollinating.

Hybridization

The mating or crossing of two true-breeding varieties.

P Generation

The parental generation consisting of true-breeding (homozygous) parent individuals from which $F_1$ hybrid offspring are derived.

F1 Generation

The first filial generation consisting of hybrid (heterozygous) offspring derived from a cross between true-breeding parental ($P$) individuals.

F2 Generation

The second filial generation consisting of offspring resulting from interbreeding or self-pollination of the hybrid $F_1$ generation.

Gene

A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA.

Allele

Any of the alternative versions of a gene that account for variations in inherited characters and may produce distinguishable phenotypic effects.

Locus

A specific place along the length of a chromosome where a given gene is located.

Dominant Allele

An allele that is fully expressed in the phenotype, masking the presence of a recessive allele in a heterozygote.

Recessive Allele

An allele whose phenotypic effect is masked in a heterozygote and is expressed only in homozygous individuals.

Genotype

The genetic makeup, or set of alleles, of an organism (such as $PP$, $Pp$, or $pp$).

Phenotype

The observable physical and physiological traits of an organism, determined by its genetic makeup.

Homozygous

Having two identical alleles for a given gene (such as $PP$ or $pp$).

Heterozygous

Having two different alleles for a given gene (such as $Pp$).

Punnett Square

A diagrammatic device used to predict the allele combinations and phenotypic ratios of offspring resulting from a genetic cross.

Monohybrid Cross

A cross between two individuals that are both heterozygous for a single character ($Pp \times Pp$), yielding a $3:1$ phenotypic ratio and a $1:2:1$ genotypic ratio.

Dihybrid Cross

A cross between two individuals that are each heterozygous for two characters ($YyRr \times YyRr$), yielding a classic $9:3:3:1$ phenotypic ratio.

Testcross

Breeding an individual of unknown genotype with a homozygous recessive individual ($aa$ or $pp$) to determine whether the unknown parent is homozygous dominant or heterozygous.

Quantitative Characters

Heritable traits that vary continuously along a continuum rather than on an either-or basis, usually indicating polygenic inheritance.

Norm of Reaction

The range of phenotypic possibilities produced by a single genotype under varying environmental conditions.

Multifactorial

Referring to phenotypic characters influenced by both genetic components (genotype) and environmental factors.

Pedigree

A family tree diagram using conventional symbols showing the occurrence and inheritance of heritable characters in parents and offspring over multiple generations.

Carrier

An individual who is heterozygous at a given genetic locus for a recessively inherited trait ($Aa$), remaining phenotypically normal but capable of passing the recessive allele to offspring.

Cystic Fibrosis

A human recessively inherited genetic disorder caused by a mutant allele for a chloride channel protein, characterized by excessive mucus production and organ impairment.

Sickle-Cell Disease

A recessively inherited human blood disorder caused by a single amino acid substitution in hemoglobin, causing red blood cells to deform into a sickle shape under low oxygen.

Huntington's Disease

A human genetic disease caused by a dominant allele, characterized by uncontrollable body movements and progressive neurological degeneration, typically onsetting between ages $35$ and $45$.

Complete Dominance

The situation in which the phenotypes of the heterozygote and the dominant homozygote are indistinguishable.

Amniocentesis

A prenatal diagnosis technique in which amniotic fluid is obtained by aspiration to detect genetic and congenital defects in a fetus.

Chorionic Villus Sampling (CVS)

A prenatal diagnosis technique in which a small sample of the fetal portion of the placenta is removed for genetic analysis.

Mitosis Stages

The sequential phases of somatic nuclear cell division—progressing from prophase to prometaphase, metaphase, anaphase, telophase, and cytokinesis—resulting in two genetically identical cells.

Meiosis Stages

The sequential two-step cell division process comprising Meiosis I (homologous chromosome pairing and segregation) and Meiosis II (sister chromatid separation) to generate four genetically distinct haploid gametes.