Chapter 17- Sex Determination

•Pre-1900s- Sex considered to be determined by environment, age, nutrition•Early 1900s- Sex chromosome identified

1. Primary sex determination•Refers to “determination of ____________” i.e. _______ vs. _________

In Mammals-• Strictly determined by the __ chromosome

XY, XXY, XXXY, XXXXY are all ______•There is no “default state”- gonad development is __________

•Aristotle- Women are men with ______________ development

2. Secondary sex determination

•Refers to “____________________” i.e. penis and prostate vs vagina and uterus

Also may affect body shape and size, voice….

•Dictated by ____________ produced by ______ If remove ______- female body is the “_________”

Estrogen and testosterone play key roles

•Estrogen instructs Mullerian Duct to become ___________________

•Testosterone instructs Wolfian Duct to become ____________, vas deferens

•Testes also secrete ____ hormone to _________ Mullerian Duct

Gonad developmentGonads derived from ___________ ____________________, next to kidneys

Intermediate

Kidney, gonads

Gonad

nephros

Developing kidney

Nephric Duct

Fig. 14.18

Recall Kidney development from Ch. 14

Migrating nephric duct cells induce surrounding mesenchyme to form nephros (tubules)

The nephric duct is also called the_______________

Gonad development

• Unlike other tissues, the gonad has two options- an ______or a ________

Genital ridge Bipotential gonadSF1WT1LHX9

Kidney

Gonads

__________duct (nephric duct)

__________duct

Genital ridge

Sex cords

4 weeks

6 weeks

Gonad development

• Genital ridge forms by ________ in human

• Sex cords formed in gonad by _________

At this stage, the gonad is still ________________

• Now germ cells migrate into __________

•If male, _________ development continues, become ____________ that link to ___________________

•Testis cords remain solid until __________

• At puberty, testis cords are _____________ to form ______________ and germ cells differentiate into _________

If male:

Testis cords

Wolffian Duct

8 weeks

16 weeks The wolffian duct becomes__________ to carry sperm to ________

•Mullerian duct ___________

•If female :•sex cords _____________•ovaries to not connect to _______________

•The Mullerian duct becomes _______ uterus, cervix and _______________

If female:

Cortical sex cords

Wolffian Duct8 weeks

16 weeks

Mullerian Duct

•__________ sex cords are generated around germ cells

•Cortical sex cords plus surrounding mesenchyme will eventually become the __________ that envelop the ___________

•Wolffian duct _________Follicles

Fig. 17.4

Bipotential gonads

Male Female

Both Wolfian and Mullerian ducts are present

prior to _____________Wolfian

duct

Mullerianduct

Either Wolfian and Mullerian duct must be

degraded

But, what dictates testes vs ovaries??

Kidney

Another view of sex determination

Mechanisms of Primary sex determination in mammals

SRY is the ______________________ gene. •SRY expression directs male _____________

•Located on the Y chromosome

• encodes a ____ AA peptide• is a ___________________

•contains a HMG domain, common in many transcription factors•induces bends in DNA

_______ may be the ovary-determining gene

•Two copies of DAX results in ___ female

•DAX located on X chromosome

•SRY gene into XX mouse results in a ___________ mouse

•Is ___________ but not ____________ to dictate testes•Other genes include _______ and ______

Extra DAX

X Y X X YX

Testis

Male

Ovary

Female

Gonadaldysgenesis

Female

(Poorly formed gonad)

If insert second DAX gene on X chromosomeTwo copies of ____ overrides the _____ gene:

• ______ is also critical for ovary formation

Model to explain primary sex determination in mammals

SRY competes with _____ to activate or repress the ____ gene. • If X + Y : ____ favored, SF1 expressed, _____ produced

•SF1 may also inhibit ____ expression• If X (w/ 2____) + Y : SF1 repressed, ____ expressed, and ________ produced

Fig. 17.10

Mechanisms of _________ sex determination in mammals

Testes secrete two _________

1. ______- results in Mullerian duct degeneration Probably through apoptosis2. _______________- results in Wolffian duct

differentiation

_____________ insensitivity syndrome- lack testosterone receptor, thus have testes but no uterus/oviducts; hence looks female

Fig. 17.11

Ovaries secrete __________

Estrogen is required for development of both _________ and _____________ ducts

Estrogen also promotes ___________ development

a. Drosophila sex is determined by the number of ___ chromosome ratio (where A is an autosome)

X- maleXX- female

•The Y chromosome is active in _________ development

In contrast to humans,

b. Turtle and crocodile sex is determined by __________-

Fig. 17.20

% male

Temp

c. The slipper snail sex is determined by ___________

Individuals are always _____,

then can turn into _________

Sex determination in Drosophila

• XX = females

• XY = malesUnlike mammals, the Y chromosome does not determine sex

Answer: The X to autosome _____________ determines sex

How is the X:A ratio interpreted??

How is sex determined??

Answer: Through activation of the _____________ gene• It is a ____________ between numerator proteins (on X) and denominator proteins (on A)

__________ (X) proteins __________Sxl transcription

Sxl gene

___________ (A) proteins ________ Sxl gene transcription

• In males, the _________ proteins win out• In females, the __________ proteins win out

The Sxl protein acts on: Sxl gene “____” promoter (PL) to maintain

transcription Sxl mRNA to splice out a _____________

PEPL

UGA

1

2

12

__________gene to splice out __________3

1 2 3 4DNARNA AAAAA

3

Fig. 17.18

Sxl controls splicing of tra switch controls splicing of_________ (dsx)

•Double sex in produced in both sexes, but ____________ protein is generated due to alternate splicing.

Thus, the _____ protein type dictates _____

• A _____________ similar to dsx has been isolated

Female mRNA Pre- mRNA Male- mRNA

transformer

doublesex

Stop codon

Stop codon

Sex-lethal

Stop codon

Stop codon