Genes and Alleles

There are several terms to be familiar with before continuing.

• Genes are genetic material on a chromosome that code for a trait. For example, you have a gene for eye color.
  
  
• Alleles are variations of genes. For example, you have the allele for brown eye color. Note that some alleles are dominant over others. That is, if a person inherits both the dominant and the recessive alleles, the dominant allele will be the one expressed.
  
  
• A genotype is the actual set of alleles an organims carries. For example, you have the genotype Bb since you have the allele for brown eye color (B) and the allele for blue eye color (b). An organism is said to be homozygous for a certain trait if both it carries two of the same alleles. It is homozygous dominant if it carries two dominant alleles and homozygous recessive if it carries two recessive alleles. The organism in our example is heterozygous -- it carries two different alleles.
  
  
• A phenotype is the expression of a gene. For example, since you have the genotype Bb with one dominant and one recessive allele, the dominant allele (B) will mask the recessive allele (b) and you will have the phenotype for brown eyes.