(version 3 September 1999)
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Dioecious species (two houses) separate sexes
Monoecious species (hermaphrodites)
Sex chromosomes vs. autosomes
Homogametic sex -- that sex containing two like sex chromosomes
Butterflies and Birds, ZZ males
Heterogametic sex --- that sex containing two different sex chromosomes
In most animal species these are XY males
Butterflies and birds, ZW females
Grasshopers have XO males
In the heterogametic sex: The two sex chromosomes are very different, and have special pairing regions to insure proper pairing at meiosis
(Details on pp 709-712 --- NOT required reading!)
Single Y = male, so XXY, XYY, XXXY all male
Extremely rare cases of XX males, as these have a small region of the Y chromosome transposed to the X
Likewise, rare XY females
testicular feminization (lack androgen receptor)
X-autosome ratio determines sex (p 704-709)
XX AA; XXX AAA = female (1:1 X:A)
XY AA, XO AA, XXY AAAA= male (1:2 X:A)
Problem: gene difference in males vs females
Mammals: Inactivate one X (X inactivation)
Drosophila : Overexpress X in male (hyperactivation of X)
Details (not required reading) on pages 698-700. some detailed references from the current literature
Differential behavior of identical genes, depending on which parent contributed them.
In extreme cases, some genes are transmissed from one sex in an active form and in the other sex in an inactive form
Prader-Willi syndrone
located on an autosome (Chromosome 15)
The maternal copy is inactived.
Individuals with two maternal copies have PWS
XA = Locus on X chromosome
XX females, ZZ males "normal" genetics
XA XA, XaXa = AA & aa homozygotes
XA Xa = Aa heterozygote
XY males, WZ females
XA Y, XaY = hemizygous for A, a
XR = red, Xr = white; Red dominant
In females:
XR XR , XR Xr = red-eye female
Xr X r = white-eyed females
In males:
XR Y = red-eye male
Xr Y = white-eyed male
cross (purebreeding) red-eyed females to white-eyed males
F1: All males and all females have red eyes
Reciprocal cross: white females crossed to red males
F1: All males are white, all females red
What about the F2?