Mendelian Genetics and Heredity

Trait

A characteristic, such as flower color or seed shape.

Gene

A location on a DNA strand that encodes information for a trait.

Allele

A specific version of a gene.

Genotype

An organism's specific combination of alleles.

Phenotype

An observable physical characteristic caused by the genotype.

Law of segregation

Alleles separate during gamete formation, so each gamete receives one allele.

Segregation and meiosis

Allele segregation mirrors the behavior of homologous chromosomes during meiosis.

Independent assortment

Allele segregation for one trait does not affect segregation for another trait.

Independent assortment and meiosis

Nonhomologous chromosomes align and separate randomly during meiosis.

Monohybrid cross: PP × pp

All F₁ offspring are Pp and display the dominant phenotype.

Monohybrid F₂ phenotype ratio

A Pp × Pp cross produces a 3:1 dominant-to-recessive ratio.

Monohybrid F₂ genotype ratio

A Pp × Pp cross produces a 1:2:1 ratio of PP, Pp, and pp.

Dihybrid cross

A cross that tracks two traits simultaneously.

YyRr gametes

Four types: YR, yR, Yr, and yr.

Dihybrid F₂ phenotype ratio

A YyRr × YyRr cross yields a 9:3:3:1 ratio.

Testcross

Mating a dominant-phenotype individual of unknown genotype with a homozygous recessive individual.

Testcross: all dominant offspring

Indicates a homozygous dominant unknown parent.

Testcross: 1:1 phenotype ratio

Indicates a heterozygous unknown parent.

Probability range

From 0 for an impossible event to 1 for a certain event.

Multiplicative law

Multiply individual probabilities for independent events occurring together in exact order.

Additive law

Add individual probabilities when multiple ways produce an outcome.

Incomplete dominance

Heterozygotes display an intermediate phenotype.

Incomplete dominance in snapdragons

Red × white produces pink F₁ flowers and a 1:2:1 F₂ phenotype ratio.

Multiple alleles

More than two alleles for one gene can exist within a population.

Rabbit coat-color dominance

C > cᶜʰ > cʰ > c.

Epistasis

One gene masks or interferes with expression of another gene at a separate locus.

Chromosome theory of inheritance

Genes are physically located on chromosomes.

Chromosomal crossover

Exchange of genetic segments between homologous chromosomes creates recombinant chromosomes.

X-linked inheritance: carrier mother's sons

Each son has a 50% chance of being affected by the recessive disorder.

X-linked inheritance: carrier mother's daughters

Each daughter has a 50% chance of becoming a carrier.

DNA mutations

Sequence changes can produce nonfunctional proteins or alter gene expression.

Examples of mutant allele effects

Developmental defects, parasite drug resistance, or lethal traits.

Mendelian and molecular genetics

Mendel's hereditary particles are genes whose expression produces proteins influencing phenotypes.