Ch5: Sex Chromosomes

sexual differentiation

differences in male/female body form

dimorphism

phenotypical differences to show dif sexes

heteromorphic chromosomes

XY

____, not _______ ultimately determine sex

genes ; chromosomes

What was discovered in the sperm of Protenor & Lygaeus insects in 1891?

Insects’ sperm sometimes lacked an “X-body,” showing some sperm had X, some didn’t.

• heterogametic sex = M

• homogametic sex = F

Klinefelter syndrome

• 47,XXY karyotype (extra X).

• tall w/ long arms & legs, large hands & feet

• Testes fail to produce sperm

• occur due to nondisjunction during meiosis (chromosomes dont seperate)

Turner syndrome

• 45, X karyotype (only 1 X)

• female external genitalia, but no ovaries

• underdeveloped breasts

• occur due to nondisjunction during meiosis (chromosomes dont separate)

mosaics

some of your somatic cells display two dif genetic cell lines w/ dif karyotypes

(some 45.X & others 46.XX)

Triplo-XXX

47,XXX

• female w/ underdeveloped, sterility, and/or intellectual disability

47,XYY karyotype condition characteristics

• males over 6 ft

• may have subnormal intelligence & personality disorders

bipotential gonads

the early stage gonads (before 6 weeks) that can develop into either testes or ovaries

By the ___ week, the gonadal ridges form either ovaries or testes

5th

formation of male parts

medulla develops into testes

• Y chromosome has at least 75 genes

  • 95% of Y chromosome doens’t recombine w/ X = MSY (male specific region of Y)

  • SRY (sex determining gene of Y) activates 6-8 weeks

    • SRY gene encodes protein TDF = testes formation

formation of female parts

• cortex of ridge forms ovarian tissue

• X chromosomes has 900 - 1400 genes

• mullerian duct forms oviducts, uterus, cervix, & vagina

pseudoautosomal regions (PARs)

tips of Y that match w/ X and recombine during meiosis

• this pairing region is critical for segregation & combining

Transgenic mice research

• Inserting SRY gene into XX mice makes them develop as males.

  • TDF is the “switch.”

human sex ratios

• primary sex ratio: embryos conceived

• secondary sex ratio: actual live births

Dosage compensation

females have 2 Xs so they could produce twice as much product for x linked genes:

• solution = X inactivation (one x becomes inactive) : Barr Body

Barr Bodies

condensed inactive X chromosome after X inactivation

Lyon Hypothesis

Inactivation of X is random, but once an X is chosen to be shut off in a somatic cell, all its future copies keep that choice.