Lecture 17: Chromosomal Theory of Heredity, Sex Chromosomes and Sex Determination

A review (along with self-test) can be found on the Cell biology page of the U of A biology project.

Walter Sutton (US grad student), Theodor Boveri (German Biologist) suggested that

Chromosomes contain the units of heredity

Sutton was motiviated 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

• Showed independent assortment of chromosomes using observations of a heteromorphic chromosome pair (from a grasshopper) at anaphase

Studies strains of Jimsonweed


Strains of containing different extra chromosomes different in fruit appearance. 12 stains containing different extra chromosomes and 12 different appearances.

• Used red and white-eyed mutants in Drosophila, see pg. 99- 102 (you will not be responsible for this reading, but it is interesting)
  

  X^R = red, X^r = white; Red dominant

  In females:

  X^R X^R , X^R X^r = red-eye female

  X^r X ^r = white-eyed females

  In males:

  X^R Y = red-eye male

  X^r Y = white-eyed male

Sex Chromosomes and Sex Determination

Sex! (Chromsomes and determination)
Which sex are you?

Dioecious species (two houses) separate sexes



Monoecious species (hermaphrodites)



Sex chromosomes

Sex chromosomes vs. autosomes



Homogametic sex -- that sex containing two like sex chromosomes

In most animals species these are females (XX)

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

Pairing of Sex chromosomes

In the homogametic sex: pairing like normal autosomes chromosomes

In the heterogametic sex: The two sex chromosomes are very different, and have special pairing regions to insure proper pairing at meiosis

Fun ways for Sexual determination -- Humans

Single Y = male, so XXY, XYY, XXXY all male

There is an interesting New York times article on XXY individuals.

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)

Fun ways for Sexual determination -- Flies

X-autosome ratio determines sex

XX AA; XXX AAA = female (1:1 X:A)

XY AA, XO AA, XXY AAAA= male (1:2 X:A)

Dosage Compensation

Problem: gene difference in males vs females

Mammals: Inactivate one X (X inactivation)

Drosophila : Overexpress X in male (hyperactivation of X)

X inactivation

In each female cell in mammals , one X is picked at random and inactivated


Inactivated X is called a Barr body


Thus, somatic tissues are mosaics


Calico cats: Orange and black alleles

Imprinting

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 defected paternal copy have PWS

Individuals with two maternal copies have PWS

Causes insatiable hunger. St. Petersburg Times article on the life a a prader-Willi patent (More details on the pathogy of this disease can be found at the Online Mendelian Inheritance in Man website.)

Sex linkage

X^A = Locus on X chromosome

XX females, ZZ males "normal" genetics

X^A X^A, X^aX^a = AA & aa homozygotes

X^A X^a = Aa heterozygote

XY males, WZ females

X^A Y, X^aY = hemizygous for A, a

Red/white eye color in Drosophila

X^R = red, X^r = white; Red dominant

In females:

X^R X^R , X^R X^r = red-eye female

X^r X ^r = white-eyed females

In males:

X^R Y = red-eye male

X^r Y = white-eyed male

Genetics with sex-linkage

One key indicator of sex-linkage Reciprocal crosses give different results

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?