Sex:
--- understanding its biological significance
-- appreciating how genetics was used to understand
how it is determined.
… according to Jacob Bronowski in “The Ascent of Man” (1973)
Mendel himself was inspired by the clear-cut
difference between males and females and the
1:1 sex ratio
Costs of sex:
(1) Males dilute females’ genetic contribution
(the couple is the unit of reproduction)
(2) Seeking a mate and mating
takes time and energy -- and is dangerous
(3) Sexual conflicts arise
(remember the Haig hypothesis for imprinting)
(4) Sex and its consequence, recombination,
break up winning gene teams
Benefits of sex:
(1) Reduces mutational load
(escape “Muller’s ratchet”
-- irreversible loss of genes)
perhaps males particularly useful
(rationale for “maladaptations” from sexual selection)
(2) Free good mutations from bad genetic backgrounds
(3) Help to keep ahead of parasites
(there is no “optimal” genotype in the real world)
“Sex determination genes” determine two qualitatively
different things (a distinction not often appreciated, even by those
who study the genetic programming of sex):
population sex ratio
sexual dimorphism (developmental differences)
An extreme example of sexual dimorphism
Bonellia viridis
Female: 100 mm
Male:
1 mm
larva lands on rock
larva lands on adult female
ESD: environmental sex determination
relevant variables for ESD:
Host (Bonellia)
Temperature (turtles, alligators)
Neighbor density (parasitic wasps)
“Presence of male” (tropical fish)
vs. GSD: genotypic sex determination
Segregation of alleles (genes) determines sex
best for generating 1:1 sex ratios
apparant paradox:
Since females are rate-limiting for reproduction,
why see 1:1 sex ratio so often?
(as usual, Darwin had the answer first)
In the aggregate, both sexes contribute equally
to the next generation (every female needs a male)
hence, any minority sex
on average will make a
disproportionate contribution per individual
Natural selection will favor generation of the minority sex.
At 1:1, no minority sex!
Known for fruit flies: XX females XY males
…but what really determines fly sex?
Calvin Bridges (1916):
w -/w - (white eyed) Females X Males (red eyed) w+/Y
expected:
“exceptions”:
(primary)
x
w -/w+ (red) daughters
XX
w -/Y (white) sons
XY
white daughers (fertile)
red sons (sterile)
red XY
progeny are
“secondary”
exceptions
white daughers (fertile)
red sons (fertile!)
XXY
X(O)
(xxx & o/Y die)
XXY
XY(±Y)
for fruit flies:
normal: XX females XY males
abnormal: XXY females XO males
Sex-chromosome difference CAUSES
(triggers) different sexual development
Y chromosome does not detemine sex
(but is required for male fertility)
X chromosome number determines sex
XX females XY males
What about X-chromosome number matters?
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?
…again, genetic exceptions to the rule provide the answer
(autosomal genes)
px bw +
+ bw sp
Females
X
Males
px + sp
expected PROGENY:
px + & + sp
Parental types:
Nonparental types:
(recombinant)
( 6.5 cM)
+ + & px sp
XXX
AAA
ALSO: one unusually large ++ female X px bw sp Male
(1) Three, not two,
parental types recovered:
px bw +
+ bw sp
px + sp
(2) many intersexual
XXY
(sterile) progeny
AAA
(3) normal and jumbo females
X AA
X:A = 0.5, male
XX AA
X:A = 1, female
XX(±Y) AAA
X:A = 0.67, intersex
XXX AAA X:A= 1, female (large)
XA
X:A=1,
(dead) female
GENETIC MOSAICS
X-chromosome loss generates
“gynandromorphs”
XX AA zygote -->
XXAA cells / X AA cells
(XXAA)
(X AA)
Female
Male
XXAA zygote -->
XXAA cells/XA cells
(“loss” of an entire haploid set)
(XXAA)
(X A)
Female
Female
(XA never reaches
adult stage
but mosaics do)
X AA
X:A = 0.5, male
XX AA
X:A = 1, female
XX(±Y) AAA
X:A = 0.67, intersex
XXX AAA X:A= 1, female (large)
XA
X:A=1,
(dead) female
GSD by X:A ratio (balance)
The worm:
XX self-fertilizing hermaphrodite
XO male (heterogametic sex)
Origin of males:
(1) Spontaneous X-chromosome nondisjunction (rare)
to make “O” eggs (+ X self sperm)-> XO male
(2) Mating (outcross) of hermaphrodite to male:
X eggs join with X or O male sperm -> 50:50
The worm:
XX self-fertilizing hermaphrodite
XO male (heterogametic sex)
XX AAA X:A= 0.67 = male
XXX AAAA X:A = 0.75 = hermaphrodite
GSD by X:A ratio
HUMANS:
XX female
XY male
XXY Kleinfeler Syndrome
sterile male (1:1000 men)
XO Turner Syndrome
sterile female (1:2000-5000)
GSD by Active Y
dominant masculinizer
HOUSE FLIES:
m/m female
M/m male
GSD by dominant masculinizing allele M
(one of three different GSD systems
in the same species!)
Birds, moths and butterflies:
ZZ male
ZW female
female is
the 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)
Diploid (± royal jelly)
Eggs not fertilized --> Drones (males)
Haploid
GSD by “haplodiploid” system
But is the relevant variable ploidy?
Let’s encourage inbreeding among the honeybees:
increased homozygosity
suddenly: DIPLOID MALES!
a1/a2 heterozygotes: females (queens and workers)
a1 or a1/a1 hemizygotes and homozygotes: males
(fertilization)
a1/a2 Queen X a1 Drone --> a1/a1 & a2/a1
diploid
drones
GSD by a multiple allele system
--- highly “polymorphic” sex gene (many alleles)