Mendel and the Gene

1) Wild type – Individuals with the most common phenotype.

2) Meiosis

3) Proposing that a gene is on the X chromosome is called X-linkage. Proposing that a gene is on the Y chromosome is called Y-linkage. Proposing that a gene is on either sex chromosome is called sex-linkage.

4) The + symbol always stands for the wild type trait.

5) Non-sex chromosomes are called autosomes. Genes on non-sex chromosomes are said to show autosomal inheritance.

6) Studying the organism yeast. Yeast is a fungus, it is a single celled eukaryote, it has a nucleus, it has chromosomes that pair up, its cells to a first order approximation are a lot like your cells. How do you grow yeast? Takes growth medium that has a lot of rich medium. Put yeast cell in the broth. I get culture of yeast.

6) put them on a petri plate that again has nutrients. Dilute it, take a little of the broth, and spread it, so that there are individual cells scattered randomly. and I get a colony all of which descend from a single cell. Called colonies.

7) Life cycle of yeast – has a diploid stage, it can undergo mitosis. Yeast has 16 chromosomes. Humans have 23. yeast also undersgoes meiosis to produce spores. They come in 2 flavors in A and alpha, like male and female. This is identical to the human genetic cycle. turns out that, yeast can also undergo mitosis as a haploid, the haploid cells of yeast, they can continue to grow indefinitely. Your gametes cannot. This is very convenient for geniticists.

8) Yeast can grow on minimal media. with very few macromolecules. It needs a carbon source, which is some sugar that it can ferment, it needs a source of nitrogen, phosphorus, salts and water. Why is it able to grow? It makes them. It is extraordinarily self reliant. You are not.

9) Yeast is not stupid, if you give it amino acids, it will use it. yeast is able to use rich media. Why not always manufacture? Save ATP, = energy. How does it make argenine?

10) Biochemist will find an enzyme that makes argenine. Geniticist will find one that cannot make it. Geneticists wants mutants. Find them by going on a mutant hunt. Grow yeast on a medium that has argenine, and pour them on a plate that has minimal medium w/o argenine. Those that can grow are the ones we are not interested in.

11) Yeasts that are able to grow on minimal media are called prototrophs. Yeasts that need help are called oxotrophs. One way, find those that grow on minimal + argenine, but cannot grow on minimal. We will end up with argenine auxotrophs.

12) The yeast cells we plated, were they haploid or diploid. – haploid. Because, we’ll need a mutation in both copies for the diploid, assuming oxytrophy for argenine is a recessive trait.

13) Suppose we get n colonies, arg1, arg2, … arg n that show oxotrophy for argenine. How many distinct genes does this affect? Are these mutants all in the same gene? A geneticist wants to know what are all the ways to disrupt a cell so that it cannot make argenine.

14) Geneticist do a variety of tests. Tests of recissivity or dominance. Whether the oxotrophy for argenine is dominant or recessive for each colony. Make a cross with wild type. Test each from 1 to n.

15) Lets assume all are recessive. How do I tell if they are on the same gene or not? This is called Test of Complementation. If two mutations complement each other’s effect, they are on different genes. Suppose arg 1 and arg 2 are on different genes. Cross them.

13) So we can make a complementation table. Rows contain arg1 to arg n, and so do colums.The groups that fail to complement define mutations in the same gene. These are called complementation groups because they don’t complement each other. Actually it should be called failure to complement groups.

14) Tests of Epistasis – Suppose a biochemist wanted to test his hypothesis about manufacturing of argenine. A goes to B goes to C goes to argenine. Suppose specific genes were necessary to catalyze specific steps of this biochemical process. The geneticist and biochemist could collaborte with each other to study whether these mutants affected each step of this pathway.


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