a second practice problem set (with answers) is on the course website
DESCRIPTION
A second practice problem set (with answers) is on the course website. The review session for the second midterm is on Thursday evening, April 10, from 7-9pm in ROOM 141 GIANNINI HALL. onwards with:. GSD : genotypic sex determination. Segregation of alleles (genes) determines sex. - PowerPoint PPT PresentationTRANSCRIPT
A second practice problem set(with answers)
is on the course website.
The review session for the second midtermis on Thursday evening, April 10, from 7-9pm
in ROOM 141 GIANNINI HALL
GSD: genotypic sex determination
Segregation of alleles (genes) determines sex
best for generating 1:1 sex ratios
onwards with:
Amazing variety of mechanisms(sex determination & sexual dimorphism is
the most rapidly evolving aspect of developmental programming)
XX (±Y) females X(±Y) males
What about X-chromosome number matters?
…again, genetic exceptions to the rule provide the answer
For fruit flies:
absolute number: 1=male, 2or more = female
odd vs. even (paired?) XX X=male?
number relative to ploidy (non-sex chromosomes)?
X AA male, but X A female?
X:A ratio
XX(±Y) AA X:A = 1, female
X(±Y) AA X:A = 0.5, male
XX(±Y) AAA X:A = 0.67, intersex (phenotypic mosaic)
XXX AAA X:A= 1, female (jumbo)
(dead)
For fruit flies:
Bridges' "exceptional" female:
female
Progeny from
X A X:A=1,
What about:
how could he know???
XX AA zygote --> XXAA cells / X AA cells
recall that:X-chromosome loss generates “gynandromorphs”
GENETIC MOSAICS revealed the sex of X A cells(nonmosaic XA animals never reach the sexually dimorphic (adult) stage)
(XXAA)
Female(X AA) Male
(XXAA)
Female(X A) Female
XXAA zygote -->XXAA cells/XA cells(loss of an entire haploid set)
(genetic "markers"allowed him to infer
the genotypes)
XX AA X:A = 1, female
X AA X:A = 0.5, male
XX(±Y) AAA X:A = 0.67, intersex
XXX AAA X:A= 1, female (large)
X A X:A=1, (dead) female
GSD by X:A ratio (balance)
GSD by X:A ratio (balance)
What is it about ploidy changes that affects the way thatX-chromosome dose determines sex?
"Obviously": female-determining genes on the Xand
male-determining genes on the autosomes
Balance between what?
"numerator genes" "denominator genes":
Obviously:
female-determining genes on the X("numerator genes")and male-determining genes on the autosomes("denominator genes")
Pure speculationon Bridges' part
(1921)
…but what else could it be?
speculation turned intotextbook "fact"
Fig. 18.21(your text fudges whether
hypothesis vs. fact but many othertexts present it as fact)
Obviously:
female-determining genes on the X("numerator genes")and male-determining genes on the autosomes("denominator genes")
1921 2007
Fig. 18.21
vs.maternal gene products
…and alters the
TIME of counting by Sxland hence the concentration
of the relevant female-determining gene products
and not what Bridge's guessed or what the textbooks say)
female-determining genes on X exist(including the master, Sxl)
O.K.
not O.K.
finallythe answer!
ploidy
The (round) worm (Caenorhabditis elegans):
XX self-fertilizing hermaphrodite
XO male (heterogametic sex)
(1) Spontaneous X-chromosome nondisjunction (rare) to make “O” eggs in the hermaphrodite (+ X self sperm)-> XO male
(2) Mating (outcross) of hermaphrodite to XO male:
X eggs join with X or O male sperm -> 50:50
Two ways to get males:
mostly morehermaphrodites
Worm X-chromosome counting is somewhat like that in the fly,but different in important ways:
XX self-fertilizing hermaphrodite
XO male (heterogametic sex)
XX AAA X:A= 0.67 = male (not a mosaic)
XXX AAAA X:A = 0.75 = hermaphrodite
again: GSD by X:A ratio
…and there do appear to be "male-determininggenes on the autosomes against which
"hermaphrodite-determining genes on X" are measured
GSD by X:A ratiois a remarkably rare sex-determination mechanism
among species
HUMANS:
XX female XY male
XXY Kleinfelter Syndrome
sterile male (1:1000 men)
XO Turner Syndrome
sterile female (1:2000-5000)
GSD by Active Ydominant masculinizer
HOUSE FLIES:
m/m female M/m male
GSD by dominant masculinizing allele M(no heteromorphic sex chromosomes)
(…actually, only one of three different GSD systemsthat operate in different races of the same species!)
Birds, moths and butterflies:
ZZ male ZW female
female isthe heterogametic sex
(compare: XY males)
GSD by feminizing W or Z:A ?
>>20% of all animals use a very different GSD system:
Eggs fertilized --> Queens (females) or workers (sterile)
Eggs not fertilized --> Drones (males)
Diploid (± royal jelly)
Haploid
GSD by “haplodiploid” system
But is the relevant variable ploidy?
Let’s encourage inbreeding (incest) among the honeybees:
suddenly: DIPLOID MALES!
GSD by a multiple allele system--- highly “polymorphic” sex gene (many alleles)
increased homozygosity
a1 or a1/a1 hemizygotes and homozygotes: malesa2 or a2/a2
a1/a2 heterozygotes: females (queens and workers)
a1/a2 Queen X a1 Drone --> a1/a1 & a2/a1 & a1 & a2
diploiddrones
(fertilization)
diploidworkers
& queens
haploiddrones
(perhapsher brother)
GSD via a Maternal Effect system:
(for a blowfly)
Female-producing mothers
Male-producing mothers
F / f
f / f
F / f & f / f daughters
f / f sons 1:1sex ratio
Genotype of mother
determines (or at least influences) thePhenotype of the progeny
f / fX males
f / fX males
A potential problem with many GSD systems (including our own):
XXAA
XYAA
female male
fruit flies: honeybees:
AA A
female male
fact: 20% of all fly genes are on Xfew of these are on fly Y
hence: males are monosomic for 1/5 of their genome
males are monosomic for entire genomehaploid
PROBLEM NO PROBLEM
(even 1% is rarely tolerated)
potentially genetically unbalancednot genetically unbalanced
fact: gene output is generally proportional to gene dose in metazoans
How eliminate the anticipated X-linked gene expressiondifference between the sexes?
(1) increase X-linked gene expression 2x in males
(2) decrease X-linked gene expression in females by 1/2
= X-chromosome dosage compensation
2a: reduce each X by 50%
2b: inactivate one X
fruit flies (“the fly”)
the worm
us mammals
XX XY
wa/wa = (same color as) wa/Y
YET:
Recall that Muller observed X-linked gene dose effect within a sex,but not between the sexes
wa/Y; Dp(wa)/+ > (darker, more “wildtype”) wa/wa
It must follow that:
wa/wa > (darker, more “wildtype”) wa/Df(w)(or wa/w-)>
o+
wa/Y < (lighter, less “wildtype”) wa/Y; Dp(wa)/+o
“leaky” (hypomorphic) mutant alleles twice as leaky in males vs. females
Infer: wildtype alleles twice as active in males vs. females to achieve balance
wildtype (normal) X-linked alleleswork twice as hard in males
as they do in females
= X-chromosome dosage compensationXX XY
Are the male genes working twice as hard, or instead are thefemale genes working half as hard? (is the glass half full or half empty)
Are the alleles on both the female’s X chromosomes even working?
YES
Can actually answer the question.
But first:
Muller knew from gynandromorphs: white gene functioning is“cell autonomous”:
a cell’s phenotype reflectsits genotype with respect
to the particular gene
w+/w- non-mosaic eye is solid red,not mosaic red and white
(XXAA)
Female(X AA) Male
Gynandromorph:
w+/w- w-
…alleles on both X’s must be active
Are male X-linked genes turned UPor are
female X-linked genes turned DOWN?