Course: Science Subtest II
Lesson: Genetics and Evolution
Topic: Inheritance of Traits      Page 12 of 18  

Alleles - Variation of the Same Gene
Every gene has a specific location (called a locus) on a specific DNA strand. In organisms that reproduce sexually, traits are determined by the combination of genes from two parents. Variations of the gene (from different parents) are called alleles.

Allele Combinations
Homozygous and Heterozygous Alleles
The terms homozygous and heterogeneous refer to different combination of alleles for a specific trait.

When alleles are heterogeneous, (e.g. Yy), a dominant allele can prevent a recessive allele from being expressed. For example, yellow pea plants that are heterogeneous for color, will only express the dominant gene (Yy).

An allele is a different version of the same gene. The color of snapdragon flowers is determined by the combination of alleles from different DNA.