I found the section about sex determination pretty interesting. Humans have either two X chromosomes (if you're female) or one X and one Y chromosome (if you're male). But this isn't the case for all animals! How and why are sex chromosomes different? This article does a great job of explaining everything. Hermann Henking first studied wasp sperm cells in 1891. He observed that some cells had 12 chromosomes and other had only 11. He also noticed that the 12th chromosome acted different than the other 11. Puzzled, Henking called the 12th chromosome the "X element" because of its "unknown nature."
Henking found that the "X element" couldn't be found in female grasshoppers and hypothesized that the "X element" must help determine the sex of insects. Over ten years later, Nettie Stevens studied numerous beetle species and their inheritance patterns. She also hypothesized that chromosomes had something to do with sex.
Besides the XX-XY system that we have, there are also the XX-XO and ZZ-ZW systems!
XX-XO system
- Found in insects such as crickets and grasshoppers
- Females carry two X chromosomes (XX) and produce gametes with X chromosomes
- Males carry only one X chromosome (XO) and produce some gametes with X chromosomes, some gametes without sex chromosomes
- The number of X chromosomes determines maleness
ZZ-ZW system
- Found in birds, snakes and some insects
- Females carry the mismatched chromosome pair (ZW)
- Males carry the identical pair (ZZ)
- Similar to humans' XX-XY system, except that females have the mismatched pair
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| The picture above shows the Punnett Squares of different sex determination patterns |
Another topic in this chapter I found interesting were sex linked genes. The X chromosome of humans is a lot bigger than the Y chromosome. The X chromosome carries over 1000 genes while the Y chromosome carries less than 100 genes. That's a huge difference! It helps to explain why many genes are found on the X chromosome but not on the Y. These genes are called X-linked genes. The book mentioned Morgan's crosses of Drosophila melogaster and I decided to go on PubMed to research this more in-depth. This article describes how eye color mutants contribute to our knowledge of enzymatic pathways and vesicular transport. It also mentions the studies of mutations aiding organogenesis. In case you didn't know, organogenesis is the development and production of the organs of a plant or animal.
The eye (particularly of the Drosophila melanogaster) has one of the greatest experimental fields and phenotypes. The fly eye is comprised of unit-eyes called ommatidia. I looked it up, and each omnatidium has photoreceptor cells, in addition to support cells and pigment cells. Omnatidium structure is very complex, and any minor mutations will physically appear on the ommatidium. Studying eye mutants helps scientists to further understand signal transduction, cell polarity and programmed cell death. Drosophila melanogaster is a great experimental system because of its multiple identifying genes.
One section of the chapter that I had to spend a little extra time studying was pedigree analysis. Pedigree analysis involves looking at a pedigree for a specific trait and analyzing it for multiple generations of a family. It takes some time and practice to understand a pedigree, but once you understand a few concepts, everything is a piece of cake!
The video above gives some great pedigree analysis practice. And if you look in the bottom corner, something may look familiar. Screencast-O-Matic! Haha I guess other people do use it. The lady in this video takes you step-by-step at looking at different pedigrees. What was great about this video is it starts really basic, and doesn't show a giant confusing pedigree. It only shows small pedigree sections, so you won't get confused. It also talks about the difference between autosomal and sex-linked traits on a pedigree. While doing the ConnectPlus homework, I had to determine the types of traits shown on different pedigrees. After reading the book, I wasn't sure which was which. But after watching this video, I understood the concept. It is a little on the long side, but it's a good quality video. Even if you understand pedigrees well, I would still recommend watching it because it's wonderful practice.
That's all for this chapter's post!
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