12 Intro to Genetics - GEN BIO 115

Gene

Unit of hereditary information.

Allele

Variant or alternative form of a gene.

Character

Observable, heritable feature (e.g., hair color).

Trait (Character State)

Detectable variant of a character (e.g., red hair).

Genotype

Genetic makeup of an organism; what alleles are present.

Phenotype

Observable physical or physiological traits.

Relationship Example

Gene → Character → Hair color; Allele → Trait → Red hair.

Gregor Mendel (1822–1884)

Austrian monk; first to determine basic rules of inheritance in eukaryotes; experiments with pea plants laid foundation for genetics.

Why Mendel Used Pea Plants

Inexpensive, easy to grow, short generation time, many identifiable traits, easy to control pollination, many varieties available.

True-Breeding Lines

Always express the same phenotype after self-fertilization; Mendel bred each line for 2 years before experiments.

Blending Inheritance Hypothesis

Old idea that parental traits blend in offspring to create intermediate phenotypes.

Mendel’s Experimental

Crosses Mated true-breeding plants with contrasting traits (P generation) to observe offspring (F1 and F2 generations).

P Generation

Parental generation in a genetic cross.

F1 Generation

First filial generation (offspring of P).

F2 Generation

Second filial generation (offspring of F1).

Mendel’s F1 Results

F1 offspring resembled only one parent—no intermediate phenotype appeared.

Mendel’s F2 Results

Both parental traits reappeared; observed consistent 3:1 phenotype ratio.

Mendel’s Conclusion

Blending hypothesis rejected; traits are inherited as discrete “heritable factors” (genes).

Particulate Inheritance

Idea that parents pass on discrete units (genes) that retain their identity in offspring.

Mendel’s Model Component 1:

Alleles Genes come in different versions called alleles.

Mendel’s Model Component 2:

Diploidy Each organism has two alleles for each gene—one from each parent.

Mendel’s Model Component 3:

Dominance Dominant allele determines phenotype if present; recessive allele expressed only if no dominant allele.

Mendel’s Model Component 4:

Principles of Heredity Law of Segregation and Law of Independent Assortment.

Law of Segregation

Two alleles for a character separate during gamete formation.

Example of Segregation (Seed Color)

Y = yellow, y = green; genotypes can be YY, Yy, or yy.

Homozygous

Having two identical alleles for a gene (e.g., YY or yy).

Heterozygous

Having two different alleles for a gene (e.g., Yy).

Law of Independent

Assortment Genes on different chromosomes assort independently during meiosis due to random orientation of tetrads in metaphase I.

Genetic Variation

Sources in Meiosis Independent assortment and crossing over.

Genetic Cross

Method used to predict genotypes and phenotypes of offspring by tracking allele combinations.

Purpose of a Genetic Cross

Determine possible gametes, predict offspring genotypes, and calculate genotype/phenotype frequencies.

Monohybrid

Individual heterozygous for one character (e.g., Yy).

Monohybrid Cross Cross between heterozygotes for one gene; demonstrates Mendel’s laws.

Parental (P) Cross Example

True-breeding YY × yy.

Punnett Square

Tool for predicting possible genotypes and their frequencies in offspring.

Reason for 3:1 Ratio

Segregation of alleles during meiosis.

Probability in Genetics

Laws of segregation and independent assortment reflect basic probability principles.

Genetic Ratios

Can be expressed as fractions, decimals, or percentages.

Multiplication Rule

Used for independent events; “and” means multiply probabilities.

Multiplication Rule

Example Aa × Aa; probability of homozygous recessive = 0.5 × 0.5 = 0.25.

Addition Rule

Used for mutually exclusive events; “or” means add probabilities.

Addition Rule

Example Ee × Ee; probability of heterozygous offspring = (0.5×0.5)+(0.5×0.5)=0.5.

Use of Probability Rules

Simplifies prediction of outcomes in complex crosses (e.g., dihybrid or trihybrid) without huge Punnett squares.

Dihybrid Cross

Involves two genes; 16 possible combinations in offspring.

Trihybrid Cross

Involves three genes; can be analyzed with probability instead of a massive Punnett square.