Walking Through Chapter 13

Hey Bios, tonight's post will be a walk through chapter 13. So grab your book and come along.

First of all, be sure to work on understanding the bold typed words. The bold typed words in chapter13-15 are central to your understanding genetics because they will be used freely and regularly. If you will look at my comment to Ana in the previous post you will see that I am questioning taking time to prepare vocabulary cards. I would like your opinion on this.

The chapter has three main sections; An Introduction to Heredity, The Role of Meiosis in Sexual Life Cycles, and Origins of Genetic Variation. The first section points out that understanding meiosis is really the first step in understanding genetics because genetics (heredity) is transmitted through gametes and meiosis makes gametes. It also discusses asexual and sexual reproduction in terms of the amount of variation passed to the next generation. See figure 13.1, this is a hydra budding. It is a type of asexual reproduction. A hydra is like a tiny sea anenome, notice the tentacles. Many simple animals have the ability to reproduce asexually.

Now we move to the second section, The Role of Meiosis in Sexual Life Cycles, on page 229 there is a description of how a human karyotype is made. Spend some time examining figure 13.3 and 13.4. Notice how the 2n and n stages alternate and whenever gametes come together (fuse) there is new genetic variation. Now the chapter moves on to meiosis. Study figure 13.6 and if you do not understand it, ask me questions. Notice that there is two divisions and at metaphase I the chromosome twins pair up rather than line up as in mitosis. Meiosis I is the reduction division because there is only one set of chromosomes in each cell when it is complete. Meiosis II is just about separation the chromatids and it results in four cells.

The final section is about how meiosis creates genetic variation. First, independent assortment is the "shuffling" of chromosomes at metaphase I. With 4 chromosomes there are (4) combinations and the 46 chromosomes of humans results in well, an astronomical number (see text)! Now that's variation. Second is crossing over the process in which homologous (twin) chromosomes exchange parts (See figure 13.9). This introduces additional shuffling. So with just independent assortment and crossing over there exists tens of millions if not more possible genetic variants in the gametes of one human. This leads us to the last source of variation - random fertilization. Think of it this way, if any gamete has an equal chance of meeting all the others, imagine the potential number of new genetic variants! And this does not even account for mutation! That is a lot of shuffling? Comments?

That's it for now. I will be up for just 1/2 hour more but will be looking for your comments. Gotta go get Kingston's bone ready for the morning and prep the coffee maker.

Mr. Baker