Chapter 4

Sexual differentiation

successful fertilization depends on sexual differentiation in reproductive organisms

why is sexual differentiation more evident in complex life forms?

males and females have phenotypic dimorphism

heteromorphic chromosomes

sex chromosomes

characterize one sex or the other

XX/X0 sex determination

XX: female

X0: male

what animals is the protenor (XX/X0) sex determination found in?

invertebrates

XX/XY sex determination

- females are homogametic (XX)

- males are heterogametic (XY)

how are lygaeus sex determined?

whether the X egg is fertilized by the X or Y sperm

what are autosomes?

non sex chromosomes

ZW/ZZ sex determination

ZW: females

ZZ: males

how are ZW/ZZ sex determined?

- inverse of human system

- Z sperm fertilizes either Z or W egg

what uses the ZW/ZZ system?

birds

what uses the XX/XY system?

humans

How are protenor sex determined?

half sperm consists of X, other 0X

humans have __ autosomes

22

What has to develop in order for a human to be male?

female is default, Y chromosome has to develop

Y chromosomes house

genetic information for maleness

what occurs in the gonadal ridges to form a sex?

- presence of Y: testes

- absence of Y: ovarian tissue

when do the gonadal ridges advance? What is that advancement called?

- by the 5th week they form ovaries/testes

- bipotential gonads

PAR

- pseudoautosomal regions

- to allow the X and Y sex chromosomes to pair and properly exchange genetic material during cell division in males

MSY

- male specific region of Y

- houses genes containing male development (SRY)

SRY

- sex determining region of Y

- critical gene controlling sexual development

when does the SRY become active

6-8 weeks of development

When can females have a Y chromosome?

SRY gene is missing due to mutation

When can males have no Y chromosome?

SRY region has been translocated on X gene

TDF

- testis determining factor

- activates the genes for proteins needed for male development

SRY encodes for

TDF

Klinefelter syndrome

male has more than one x chromosome

- sufficient to determine maleness, reduced secondary sex traits and fertility problems

what is the common karyotype for Klinefelters?

47,XXY

Turner Syndrome

female only has one x chromosome

- short, skin flaps, underdeveloped breasts, shield like chest

what is the common karyotype for Turners?

45,X0

Poly-X syndrome

female has more than two x chromosomes

- often women perfectly normal: underdeveloped secondary sex

what is the common karyotype for Poly-X?

47,XXX

Jacob's Syndrome

male has more than one Y chromosome

- many perfectly normal, higher risk of intellectual disorders and diseases

what is the common karyotype for Jacobs?

47,XYY

primary sex ratio

males to females conceived in a population

secondary sex ratio

reflects proportion of each sex born

more males are born than females

true

Gene dosage

How much of a gene is expressed

Barr body

inactive X chromosomes that lies against nuclear envelope

what is the barr bodies goal?

to leave 1 active X chromosome

combats sex disorders

Lyon hypothesis

Inactivation of X chromosome is random

how does x inactivation work in cats?

Xᴮ = black fur allele

Xᴼ = orange fur allele

One X gets turned off → different cells use different Xs → black and orange patches = calico pattern.

Drosophila sex determination

ratio of x chromosomes to number of haploid sets of autosomes

what are the female/male ratios for drosophila?

female - 1:1

male - 1:2

What are the two sexes of C. elegans and their ratios?

Males (X:AA) and hermaphrodites (XX:AA), no females

What chromosome is present in drosophila but not c. elegans?

Y

What is the offspring ratio when males mate with hermaphrodites in C. elegans?

50% male, 50% hermaphrodite

TSD

temperature dependent sex determination

- reptiles

- incubation temp of eggs determines sex

androgens

male hormones (testosterone)

estrogens

female hormones (estradiol)

what does aromatase do to hormones in TSD?

converts androgens to estrogens