Biol 61 – Patterns of Mendelian Inheritance
Our favorite Czech monk, Gregor Mendel, began playing around with his peas because he was interested in such
questions as where does variability in a population come from, and how are new forms and species generated.
scientists at the time thought that offspring simply had “blends” of parental traits.
Now, a number of scientists and
naturalists of the time were trying to address these questions, but what Mendel did was take better notes and actually
follow his genetic crosses for more than one generation.
This gave him the data that helped him formulate his ideas
about inheritance patterns and lead to our understanding of how chromosomes, containing genes that specify
particular traits, behave during mating events.
Let’s start with some basics.
What is a gene
A gene is a particular region of a chromosome that contains all
the information needed to make a polypeptide chain
(or in some cases just a polymer of RNA)
in the right place
at the right time in the right amounts
Chromosomes are long polymers of double-stranded DNA, and a gene is a
segment of this DNA that has the instructions for making the primary structure of a polypeptide (or an RNA
polymer), and for making it in the right tissue (muscle vs. liver vs. brain, for example, since cells in different tissues
can need different proteins in them so they can do their jobs), at the right time (I needed to have growth hormone
around when I was growing up, but I really don’t need to make it now that I’m old!) and in the right amounts
(muscle cells need a lot of the actin protein for contraction but liver cells just use some for their cytoskeleton).
location of a gene on a particular chromosome, its actual address, is called the
of that gene.
In general, a gene encodes the information for a particular polypeptide (or RNA, which we will get to in the next
But there can be more than one version of a gene
An alternate version of a gene is called an allele
example, let’s say the protein that is encoded by a particular gene has the amino acid sequence:
There might also be a second version of this polypeptide that had this sequence:
In this second allele of the gene, the glycine (G) has been changed to an arginine (R).
This is due to a mutation in
the DNA sequence of the gene encoding the information for making this polypeptide.
Sometimes one allele will
work just as well as the “normal”, or
, allele of a gene (we can use the term “wild-type” to mean the most
prevalent version of that gene in a population).
Sometimes different alleles can have a different level of function
relative to the wild-type version, and in some cases one allele could encode a protein that works better or much
worse than the wild type allele.
In our example, we are replacing a little non-polar amino acid for a big, charged
amino acid, and this might affect how the polypeptide folds, and so affects the protein’s function negatively.