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Thought mitosis was bad? It gets worse. . . Meiosis •  Double division, divided into meiosis I and

meiosis II, producing four cells at the end. •  Before meiosis, a cell goes through the G and S

phases we talked about in the last lecture – When meiosis starts, the cell has two of each

chromosome, each with two chromatids. •  Meiosis I

– One of each pair of chromosomes goes to each daughter cell

•  Meiosis II – Each chromosome splits and one chromatid goes to

each daughter cell (just like mitosis).

Prophase I�Chromosomes become visible. . .

Prophase I�But something else happens: each chromosome pairs

up with the other member of its pair. . .

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Prophase I�. . . and then the paired chromosomes undergo

crossing over: they exchange equivalent pieces of each chromatid with each other.

Metaphase I�The chromosome pairs move to the center of the cell. . .

Anaphase I�The pairs split up, and the members of each pair move to

opposite sides of the cell

Telophase I / Cytokinesis�Two new nuclei form, and the whole cell divides

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Prophase II�Chromosomes appear and prepare for division

Metaphase II�Chromosomes line up on the metaphase plate

Anaphase II�Chromosomes break apart at the centromeres, and

chromatids move apart. . .

Telophase II / Cytokinesis�Chromosomes form new nuclei, whole cell divides

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End Result:�Four haploid cells. Follow this carefully:

•  Before meiosis: –  1 cell –  2 of each chromosome –  2 chromatids per chromosome

•  At the end of meiosis I: –  2 cells –  1 of each chromosome –  2 chromatids per chromosome

•  At the end of meiosis II: –  4 cells –  1 of each chromosome –  1 chromatid per chromosome

It was soon suspected that chromosomes must have something to do with heredity. . . but what?

. . there is an hereditary substance, a material bearer of hereditary

tendencies. . . contained in the nucleus of the germ-

cells, and in that part of it which forms the nuclear

thread, which at certain periods appears in the form

of loops or rods. August Weismann�

(1834-1914)

So where are we?

•  We know about mitosis and meiosis •  We know that genes seem to follow the same rules

as chromosomes—they come in pairs, and each offspring gets one of each pair from each parent. . .

•  Is it possible that genes are chromosomes? Or are they related in some way? –  In 1902, Henry Sutton proposed that genes must be

somehow carried on chromosomes. . .

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Thomas Hunt Morgan, a professor at Columbia

University in New York City, was interested in

this problem. . .

He and his students began studying genetics using common fruit flies, Drosophila melanogaster. (Why?

They’re easy and inexpensive to raise, they breed rapidly, and they have many easily-identifiable traits

that follow simple Mendelian rules.)

In 1909, one of Morgan’s students, Calvin Bridges, discovered a mutant fly—one with white eyes,

instead of the normal red eyes.

And if you cross a pure-bred red-eyed fly with a white-eyed fly, the offspring all have red eyes—so red eye

color (the wild type) is dominant to white. . .

x

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Cross two hybrid red-eyed flies from the F1 generation, and you get three red-eyed flies for every one white-eyed

fly, just as Mendel’s laws would predict. . .

x

except. . . all of the white-eyed flies were males. (Half of the males were red-eyed and half were white-eyed. None

of the females were ever white-eyed.)

You could only get a white-eyed female by crossing a white-eyed male with a hybrid (F1) female—in which

case 1/4 of the offspring would be white-eyed females. . .

But then if you crossed a white-eyed female with a purebred red-eyed male, you got only red-eyed females

and white-eyed males. What the heck’s going on?

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Sex Chromosomes •  Remember how I said that in diploid cells,

chromosomes come in pairs? Well, that's almost true. . .

•  But in many organisms (though certainly not all), one pair of the chromosomes varies, depending on the sex of the organism that has them. In fact, these sex chromosomes determine the individual's sex. –  In most mammals, females have two copies of a large

chromosome, called the X chromosome. – Males, however, have one X chromosome and one

different-looking Y chromosome.

Chromosomes from a female human. Note the 22 pairs of non-sex-related chromosomes (called autosomes) and one pair of X chromosomes.

Chromosomes from a male human. Note the 22 pairs of non-sex-related chromosomes, and one�X chromosome paired with one Y chromosome. Sex Linkage

•  Fruit flies just happen to have the same sex determination system as humans and most mammals – Females have two X chromosomes; males have an X

and a Y •  By 1911, Morgan hypothesized that the

explanation must be that genes were physically “carried on” specific chromosomes – Specifically, the gene for eye color must be carried on

the X chromosome—a phenomenon called sex linkage.

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Here’s how it works: The original white-eyed fly was a male, so let’s write its genotype as XwY. Mate it

with a red-eyed female (X+X+) and you get. . .

X+ X+

Xw X+Xw X+Xw (red-eyed female) (red-eyed female)

Y X+Y X+Y (red-eyed male) (red-eyed male)

Now if you cross a male (X+Y) and female (X+Xw) from the first cross, you get. . .

X+ Xw

X+ X+X+ X+Xw (red-eyed female) (red-eyed female)

Y X+Y XwY (red-eyed male) (white-eyed male)