-Regarding tutorials on Monday, bring your iClickers! It’s going to be a
M/C question and it will test on the plot to make sure you’ve actually
seen the movie. Then, there’ll be a S/A component which will be the
group part that you’ll do.
-Today’s lecture is about changes in chromosome number. In the past
few lectures, the topic of discussion was small mutations. Now, let’s
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
-Most of our cells are somatic cells which are diploid. The only cells
that aren’t are the gametes which are haploid. So we have in our cells,
two copies of each chromosome. And that’s called diploid.
-There are some particular organisms that have one set of chromosomes
and they’re 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 that’s 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, that’s called trisomy. And two
extra chromosomes is called tetrasomy.
-Monosomy: Missing 1 chromosome.
-Trisomy: Having an extra chromosome.
-Tetrasomy: Having 2 extra chromosomes.
-Now, we’re going to learn about the consequences of this and where
they’re 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. There’s 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 b/c if you have
only 1 copy, any gene that’s defective will be observed. So in the slide
where it says “unmasks recessive lethals,” it means that any gene that’s
found in 1 copy if it’s defective, then you’re 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 b/c you only have 1 copy.
And that’s why it’s usually lethal b/c it shows these recessive traits.
-If the individual survives to adulthood, there’s no meiosis. You should