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Lecture 8

Lecture 8- Changes in chromosome number

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Human Biology
Michelle French

Thursday, January 29, 2009 -Regarding tutorials on Monday, bring your iClickers! Its going to be a MC question and it will test on the plot to make sure youve actually seen the movie. Then, therell be a SA component which will be the group part that youll do. -Todays lecture is about changes in chromosome number. In the past few lectures, the topic of discussion was small mutations. Now, lets talk about larger changes in the genome. -This slide illustrates the different changes in chromosome number you can have. The left side is showing euploidy and the right side is aneuploidy. -Most of our cells are somatic cells which are diploid. The only cells that arent are the gametes which are haploid. So we have in our cells, two copies of each chromosome. And thats called diploid. -There are some particular organisms that have one set of chromosomes and theyre monoploid. So that would be the normal situation for them. -As well, you can have alternative amounts or numbers of the chromosomes and this would be considered polyploidy or aberrant euploidy. In that case, you have three copies or four copies or five copies of the same chromosome. In the case of a triploid, they have 3 copies of each chromosome. In the case of a tetraploid, they have 4 copies of each chromosome. So thats called polyploidy. -Monoploid is one copy and diploid is two copies. -In aneuploidy, you have the normal set of chromosomes except in the case of 1 chromosome you usually either have one too many or one too few. So the idea here would be this would be the normal chromosome number (in this case, diploid) and the aneuploidy is called somy so in monosomy, the person is missing 1 chromosome. So notice on the right that we have the usual set of 2 copies for each chromosome but at #3, one chromosome is missing. So this is called monosomy. -If you have an additional chromosome, thats called trisomy. And two extra chromosomes is called tetrasomy. -Summary: -Monosomy: Missing 1 chromosome. -Trisomy: Having an extra chromosome. -Tetrasomy: Having 2 extra chromosomes. -Now, were going to learn about the consequences of this and where theyre found normally and abnormally. -Some particular organisms (male bees, wasps, ants) are monoploids. And they arise from parthenogenesis where they develop from an unfertilized egg into an embryo. Theres no fertilization. They have a single set of chromosomes and produce their gametes by mitosis so that they have the appropriate number of chromosomes. -This is usually lethal in any other organisms. This is bc if you have only 1 copy, any gene thats defective will be observed. So in the slide where it says unmasks recessive lethals, it means that any gene thats found in 1 copy if its defective, then youre going to see that effect. Therefore, any recessive genes that you would normally need 2 of to see a problem, here you would see a problem bc you only have 1 copy. And thats why its usually lethal bc it shows these recessive traits. -If the individual survives to adulthood, theres no meiosis.u should
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