14-15 Patterns of Inheritance 2009

14-15 Patterns of Inheritance 2009 - Biol 61 Patterns of...

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Biol 61 – Patterns of Mendelian Inheritance 2/13/09-2/23/09 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. Many 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). The location of a gene on a particular chromosome, its actual address, is called the locus of that gene. In general, a gene encodes the information for a particular polypeptide (or RNA, which we will get to in the next chapter). But there can be more than one version of a gene . An alternate version of a gene is called an allele . For example, let’s say the protein that is encoded by a particular gene has the amino acid sequence: MVTRFS G GKLYSWPGART There might also be a second version of this polypeptide that had this sequence: MVTRFS R GKLYSWPGART 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 wild-type , 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. For
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14-15 Patterns of Inheritance 2009 - Biol 61 Patterns of...

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