Lecture 35

Question 1

Nettie Stevens studied beetle chromosomes (and discovered things about sex determination) in what year?

What did she find out about the beetles?

Answer 1

1905

during metaphase I, most chromosomes pair with an identical partner (homologous)

but one pair was different in male beetles (males had less genes!)

Question 2

Why do X and Y chromosomes still pair during meiosis?

Answer 2

Millions of years ago X and Y were identical

Over time, Y lost most of its genes while X still has its genes

Question 3

XY system has evolved (blank) several times

Answer 3

independently (i.e. in mammals, flies, sago palms)

Question 4

The ZW system is present in what species?

Answer 4

birds, butterflies, some snakes

Question 5

Drosophila still uses a X-Y pair, through Y is mainly for (blank)

Answer 5

fertility, NOT sex determination

Question 6

How is sex determined in Drosophila? What is female? What is male?

Answer 6

determined by number of sis+ genes

female is sis+sis+

male is only one copy of sis+

Question 7

What is the sex determination and dosage compensation system of Drosophila?

Answer 7

one X is male
two X is female

if one X, increase gene expression

Question 8

What is the sex determination and dosage compensation system of mammals?

Answer 8

Y is male
no Y is female

if two X, inactivate one

Question 9

What is the sex determination and dosage compensation system of birds?

Answer 9

one Z is female

two Z is male

if one Z, increase gene expression

Question 10

How is meiosis different in Drosophila?

Answer 10

meiosis in females is normal (crossovers occur)

there are no crossovers or any chromosomes in male

Question 11

Female drosophila have twice as many X-linked genes as males. What is the mechanism of dosage compensation in Drosophila?

Answer 11

TF binds to promoter region of male drosophila X linked gene

makes it work twice as hard! (gene expression increases)

Question 12

How does meiosis occur in mammals?

Answer 12

females, normal (crossovers occur on all chromosomes)

males, crossovers occur on all autosomes, but ONLY at the pseudoautosomal region on sex crhomosomes

Question 13

Where is the SRY gene located?

Answer 13

Yp11.3

Question 14

How is sex determined in mammals?

Answer 14

SRY gene at Yp11.3

If the SRY+ gene is present on the Y chromosome, male

if no SRY+ gene is present, female

Question 15

What are the 5 steps of sex determination in mammals?

Answer 15

1) gonads develop into ovaries or tests at 7 weeks (testis and ovaries have several proteins, but the testis also has SRY)

2) ovaries and testes synthesize sex hormones + meiosis

3) sex hormones travel to other cells

4) sex hormones enter cells

5) sex hormones activate TF turn turn on target genes (such as, estrogen turns on estrogen receptor protein on female specific gene)

Question 16

In mammals, females have twice as many X-linked genes as males. What is the mechanism of dosage compensation?

Answer 16

decrease X-linked gene expression in females (turn one off, to Xi)