For every trait we have, we can talk about the way it is expressed, as well as our genetic make-up that gives us that trait.  The way it is expressed is our phenotype, while the way it is encoded in our genes is our genotype.  The phenotype is the easy term to understand.  I have brown hair.  I have brown eyes.  A pea plant has purple flowers.  A pea plant has wrinkled peas.  Those are all ways to describe phenotypes.

What is a genotype and how do we describe it?

    As I already mentioned, for every pea plant trait, there are 2 alleles (2 versions of the gene) that code for it in our cells.  Are those two alleles the same?  They can be.  Are they different?  They can be that, too.  We each get one of our alleles for a specific trait from our father and the other allele for that same trait from our mother.  That's how come we are such a nice blend of our parents.  Let's examine how that works for a pea plant trait.

    Let's start with true-breeding plants for our P generation, where one plant has purple flowers and the other has white flowers.  If they are true-breeding, then both of the alleles within each plant must be the same-- the purple-flowering plant has to have 2 purple-flower alleles, while the white-flowering plant has to have 2 white-flower alleles.  It is customary to represent all the alleles for one gene with a single letter-- usually the first letter of the dominant trait.  So if we are going to describe the flower color genotype, we would be using the letter "P" because the dominant version is with purple flowers.  It is also customary to capitalize the dominant allele, so that "P" represents a purple-color allele, while "p" represents the white-flower allele.  Now we can write out the cross showing the genotypes (at the right):

In order to make the F₁ generation, each parent plant has to give one of its alleles.  Therefore, the F₁ generation plants all will receive one P allele from the parent that only has P (purple flower) alleles to give, and one p allele from the parent that only has p (white flower) alleles to give.  All of the F₁ generation plants will then have the genotype of:  Pp.  One of each.  Since purple is dominant, all of them will express the P allele and have purple flowers (see Figure 9.6 in your book).

Finally, there are a couple of terms for all of these letterings.  If the two alleles are different (e.g., Pp), we call that heterozygous.  If both alleles are the same (e.g., PP or pp), we call that homozygous.  And, to be even more specific with the homozygous genotypes, when both alleles are dominant, that is called homozygous dominant; likewise, when both alleles are recessive, that is called homozygous recessive.

© 2006 STCC Foundation Press, content by Dawn A. Tamarkin, Ph.D.

Last changed: September 05, 2007