Genetics: Mendelian Principles, Punnett Squares, and Inheritance Patterns

Gene

unit of hereditary information

Allele

variant of a gene

Character

observable, heritable feature

Trait (character state)

detectable variant of a character

Genotype

genetic makeup; what alleles are present

Phenotype

observable physical trait

Blending inheritance hypothesis

gametes contain sampling of fluids from parents; fuse, fluids blend, offspring will have intermediate phenotype

True-breeding

always express same phenotype after self-fertilization, no exceptions

3:1 phenotype ratio

consistent ratio observed in F2 generation

Particulate inheritance

characters determined by 'heritable factors' (now called genes)

Diploidy

diploid individuals inherit 2 copies of each gene - 1 from each parent

Dominance

dominant allele determines phenotype if present; recessive allele only affects phenotype unless no dominant allele is present

Law of Segregation

2 alleles for a character segregate (separate) during gamete formation.

Example of Law of Segregation

Seed color: Y and y, yellow and green.

Homozygous

Having two of the same allele at a locus.

Heterozygous

Having two different alleles at a locus.

Law of Independent Assortment

Genes on different chromosomes assort independently during meiosis.

Mechanism of Independent Assortment

Due to random orientation of tetrads during metaphase I.

Variation in Sexual Reproduction

Independent assortments + crossing over = lots of variation.

Genetic Cross

Method for predicting genotype of offspring.

Purpose of Genetic Cross

Tracking alleles to make and test predictions.

Monohybrid Cross

Cross between heterozygotes for one character.

True Breeding

1 homozygous dominant, 1 homozygous recessive.

Example of Monohybrid Cross

P: YY x yy.

Punnett Square

Tool to determine possible offspring and frequencies in a genetic cross.

F1 Monohybrid Cross Example

Yy x Yy.

Genotype Ratio in F1 Monohybrid Cross

1:2:1.

Phenotype Ratio in F1 Monohybrid Cross

3:1.

Probability in Genetics

Laws of segregation and independent assortment reflect basic rules of probability.

Multiplication Rule

Predicts combined probabilities of independent events.

Addition Rule

Predicts combined probabilities of mutually exclusive events.