Genetics and Inheritance Overview

Monohybrid Cross

→ A single trait is examined. Ex. Round seeds (RR) crossed with wrinkled seeds (rr) create Rr seeds in the F1 generation and 3:1 ratios in the F2 generation

Dihybrid Cross

→ Two traits are examined. Ex. AaBb x AaBb, where offspring show the typical 9:3:3:1 ratio.

Incomplete Dominance

→A blending of traits. A white flower (WW) crossed with a red flower (RR) creates a pink heterozygote (WR)


→ Both traits are expressed. A white cow (WW) crossed with a red (RR) cow creates cows that are red and white, roan.


→The expression of one gene is dependent on a modifier gene. In labrador retreivers, yellow alleles (ee) when present block the expression of black (BB) and brown (bb).


→ One gene controlling many phenotypes (the opposite of epistasis). Example: Agouti coat color in mice is linked to obesity and tumors.

Polygenic Inheritance

→ Many genes control one phenotype with an additive effect. Ex: Seed color and range from light (aabbcc) to dark (AABBCC) with medium colors between.

Multiple Alleles

→ More than two alleles control one trait. Ex: ABO blood types where A and B are codominant, and O is recessive

Sex Linked Traits

→ Genes are located on the sex chromosomes. Females can be carriers but males will receive only one copy of the gene. Ex: Hemophilia, Eye color in Drosophila

Environmental Effects

→Modifications can occur from environmental factors . Ex. Coat color in rabbits and foxes changes as the temperatures change.


Dominant / Recessive; Homozygous / Heterozygous; Lethal Alleles
Genotype / Phenotype; Allele / Gene ; P / F1 / F2; Test Cross