You’ve probably learned in science class that Gregor Mendel discovered that traits are inherited by the transmission of genes from parents to their offspring. Genes are segments of DNA located on our chromosomes, and they are passed on from one generation to the next.

The gene for a specific trait, such as tasting the bitterness of a PTC test strip, can exist in more than one form or allele.  For each trait, there are typically two alleles. Alleles can be homozygous (identical) or heterozygous (different) for a particular trait.

When the allele pairs are identical, the genotype for that trait is identical and the phenotype (characteristic observed) is determined by homozygous alleles.

When the allele pairs are different, there are a few different outcomes, including complete dominance, incomplete dominance, and co-dominance.

In complete dominance relationships, one allele is dominant and the other is recessive. The dominant allele suppresses the recessive allele for that trait. The phenotype is determined by the dominant allele.

In incomplete dominance relationships, one allele is not completely dominant over the other allele. The result is a third phenotype where the observed characteristics are a combination of dominant and recessive phenotypes. If you use the all three taste test strips, you may find instances of incomplete dominance. For example, some PTC tasters may find Sodium Benzoate to taste sweet, while others may find it to taste bitter.

Finally, in co-dominance relationships, neither allele is dominant, but both alleles for a specific trait are completely expressed. This also results in a third phenotype where more than one phenotype is observed.

Next week, we’ll take a look at the human pedigree symbols and how to do a pedigree analysis.