A gene is segment of DNA that holds the code for a specific trait.
An allele is the different forms of a single trait like hair color or skin tone.
A phenotype is a physical trait, and a genotype is the genetic make-up of genes, which we show with letters.
A dominant trait is a trait that will always be expressed if at least one of its genes are present. This is represented by a capital letter.
A recessive trait is a trait that will only be expressed if both of the alleles present are recessive. This is represented by a lowercase letter.
Homozygous is when both alleles code for the same trait. This is also known as purebred. If there are two dominant traits, than it is homozygous dominant, represented by two capital letters. If there are two recessive traits, than it is homozygous recessive, represented by two lowercase letters.
Heterozygous is when both alleles code for a different trait. This is also known as hybrid. This will be represented by one capital letter next to lowercase letter.
A monohybrid cross is when you use a Punnett Square to represent the prediction of the offspring’s phenotypes and gentotypes for one trait.
A dihybrid cross uses a Punnett Square to represent the prediction of the offspring’s phenotypes and genotypes for two traits.
Non-Mendelian Crosses:
Incomplete dominance is when neither allele completely dominates the other. For example, a white flower and a red flower would produce a pink flower.
Co-dominance is when both alleles are fully expressed. For example, a black feathered chicken and a white feathered chicken would produce a black and white speckled chicken.
Biology EOC Review 