BIOL 202 - Basic Genetics
Lecture 3 - Chromosomal Theory of Inheritance
Reading: Ch. 2 p.47-65
Left off last time with Mendel who did work ahead of its time and gave a presentation about it.
He wrote to the foremost authorities on hybridization at the time - a man by the name of Carl
Nageli but Nageli said that he was wrong and told people to work with the plant that he was
working with. The only problem was, the plant that he was working with was asexual and didnʼt
go through meiosis so it didnʼt get him very far. And then, Mendel just gave up on science.
People were still interested in science though and they studied the mechanisms of cells and the
behavior of chromosomes during meiosis. People began to think that he was right and so then
three scientists independently came up with these crosses and this 3:1 ratio - and then they
discovered Mendelʼs paper.
The Discovery of Sex Chromosomes
• Sex chromosomes were not known at the time that Mendel worked. The discovery due to a
careful series of observations by Nettie Stevens.
• Stevens worked with the mealworm. These mealworms also have sex chromosomes (theyʼre
actually beetles - the worm is the larva stage but you donʼt have to know mealworm life cycle)
• These sex chromosomes (X and Y) play a central role in
pinning down the connection between Mendelʼs factors and
• Thomas Hunt Morgan (from Columbia!) was a
developmental biologists and he needed an experimental
organism and he worked with fruit ﬂies. We work with these
a lot today (think: Hutt!) Why? Because they replicate quickly
(about 2 weeks) so you can grow many generations in a
year (more than peas), theyʼre well known organisms and
you can grow thousands of them very easily so you just give
them yeast and theyʼll live. Theyʼre also easy to cross.
• This was a wise choice as well because there are only four
pairs of chromosomes in the fruit ﬂy.
• Morgan was the ﬁrst one to establish a large genetics program
using fruit ﬂies.
• There is natural variation in fruit ﬂies. There is discrete
variation which is useful in genetics.
• One day, Morgan was looking in the lab and he discovered
something interesting. The normal eye type for fruit ﬂies was
red but he discovered one with a white eye (see the gross
picture to the right). So what did he do? He created pure
breading lines and then crossed them.
• Fruit ﬂies have an X and a Y. However, where it differs from
humans is that itʼs not the presence of Y that determines
whether itʼs a male or a female - itʼs the dosage of the X
chromosome. There are no sex determining factors on the Y
chromosomes. So a single X or XY would give you a male fruit
ﬂy. The female on the other hand has to have two Xʼs (XX).
Sarah Margareta Ibrahim"Friday, January 11th 2013