Download Lecture 2a: Sex: Chromosomes

Lecture 2a: Sex: Chromosomes
Sutton-Boveri hypothesis (1902): Walter Sutton (US grad student), Theodor
Boveri (German Biologist) suggested that chromosomes contain the units of
heredity.
Sutton was motivated by observations that sex seems to be determined by the
presence/absence of a chromosome in the great lubber grasshopper
(Brachystola magna).
Why? Because Mendel's particles parallel the behavior of chromosomes at
meiosis.
Support for Sutton-Boveri hypothesis: 1913 Elinor Carothers - showed
independent assortment of chromosomes using observations of a
heteromorphic chromosome pair (from a grasshopper) at anaphase
Sex chromosomes: 1916 Calvin Blackwell Bridges
(as opposed to autosomes)
Used red and white-eyed mutants in Drosophila: XR = red, Xr = white; Red
dominant
In females:
XR XR , XR Xr = red-eye female and Xr X r = white-eyed females
In males:
XR Y = red-eye male and Xr Y = white-eyed male
Homogametic sex - that sex containing two like sex chromosomes. In most
animals species these are females (XX); in butterflies and birds, ZZ males.
Heterogametic sex - that sex containing two different sex chromosomes. In
most animal species these are XY males. In butterflies and birds, ZW females;
grasshoppers have XO males.
Pairing of sex chromosomes:
In the homogametic sex: pairing like normal autosomal chromosomes; in the
heterogametic sex, the two sex chromosomes are very different, and have
special pairing regions to insure proper pairing at meiosis.
Dosage Compensation:
Problem: gene difference in males vs females. Mammals inactivate one X (X
inactivation - in each female cell in mammals , one X is picked at random and
inactivated); Drosophila overexpress X in male (hyperactivation of X) .
Imprinting:
Differential behavior of identical genes, depending on which parent
contributed them. In extreme cases, some genes are transmitted from one sex
in an active form and in the other sex in an inactive form, e.g., Prader-Willi
syndrome, located on an autosome (Chromosome 15). The maternal copy is
inactivated. Individuals with defected paternal copy have PWS, and individuals
with two maternal copies have PWS.
Sex linkage:
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