Download peacock tail human brain

Genetic mosaics have zillions of uses
besides just facilitating mutant isolation
…and geneticists have ways of controlling
exactly when and where FLPase is generated
…and hence
exactly when and where mitotic recombination is induced
Sex:
--- understanding its biological significance
-- appreciating how genetics was used to understand
how it is determined.
Sex education
.. .from a genetical and evolutionary (= genetical) perspective
Forces in evolution
(and evolution is what genetics -- and life -- is really all about):
(1) Natural selection:
(this term is in your glossary)
(2) Sexual selection:
… 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
human brain = peacock tail ?
…but a tail with the capacity to
change the way life evolves
reproduction of the fittest
…remember, responses to changes in biological environment (including parasites)
are as important as responses to changes in physical environment.
reproduction of the sexiest
(this term is NOT in your glossary)
runaway sexual selection generally leads to
maladaptive ("unfit") sexually selected traits
Sex: Sexual reproduction
vs.
Asexual reproduction
Coming together
of genetic material
from (two) individuals
to form progeny that
combine genes
from all (both) parents
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symmetrical sex:
us
equal genetic contribution
from each partner
qualitative exceptions:
mitochondria & Y chromosome
asymmetrical sex:
Can there be sex without gender?
(males & females)
unequal genetic contribution
from each partner
bacteria
GAMETES (sex cells)
Recall:
Isogamous sex systems:
Sex: Sexual reproduction
vs.
Asexual reproduction
gametes from each parent are of equal size
Anisogamous sex systems:
Coming together of genetic material from (two)
gametes from each parent are of different sizes
individuals to form progeny that combine genes
Males: sperm/pollen (smaller)
from all (both) parents
Females: eggs/ovules (larger)
(parents have to be separate to qualify)
(hate to disappoint those who think
male/female labels are politically incorrect)
Sex is ancient and ubiquitous
Nevertheless:
bdelloid rotifers: no sex for millions of years
Evidence:
Costs of sex:
(1) Males dilute females’ genetic contribution
(the couple is the unit of reproduction)
no meiosis (genes are missing)
(2) Seeking a mate and mating
takes time and energy -- and is dangerous
unusual distribution of DNA variation
suggests no gene mixing for ages
(3) Sexual conflicts arise
(remember the Haig hypothesis for imprinting)
(4) Sex and its consequence, recombination,
break up winning gene teams
2
Benefits of sex:
(1) Reduces mutational load
(escape “Muller’s ratchet”
-- irreversible loss of genes)
perhaps males particularly useful
“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
(rationale for “maladaptations” from sexual selection)
including those that determine behavioral differences)
(2) Free good mutations from bad genetic backgrounds
(3) Help to keep ahead of parasites
(there is no “optimal” genotype in the real world)
relevant variables for ESD:
An extreme example of sexual dimorphism
Host (Bonellia)
Bonellia viridis
Female: 100 mm
Male:
1 mm
sex is determined by whether:
Temperature (turtles, alligators)
larva lands on rock
Neighbor density (parasitic wasps)
larva lands on adult female
ESD: environmental sex determination
“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:
1:1 for fruit flies: XX females XY males
…but what really determines fly sex?
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, regardless of population sex ratio
(every female needs a male)
hence, the 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!
Calvin Bridges (1916):
w -/w -
(white eyed)
expected:
“exceptions”:
(primary)
Females X Males (red eyed) w +/Y
w -/w + (red) daughters
w
-/Y
(white) sons
white daughers (fertile)
red sons (sterile)
x red X Y
progeny are
“secondary”
exceptions
white daughers (fertile)
red sons (fertile!)
XX
XY
XXY
X(O)
(xxx & o/Y die)
XXY
XY(±Y)
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for fruit flies:
XX females XY males
normal: XX females XY males
What about X-chromosome number matters?
abnormal: XXY females XO males
absolute number:
Sex-chromosome difference CAUSES
(triggers) different sexual development
1=male, 2 or more = female
odd vs. even (paired?)
XX X=male?
number relative to ploidy (non-sex chromosomes)?
Y chromosome does not detemine sex
(but is required for male fertility)
X AA male, but X A female?
X chromosome number determines sex
…again, genetic exceptions to the rule provide the answer
(autosomal genes)
px bw +
+ bw sp
Females
X
Males
For fruit flies:
px + sp
X(±Y) AA
XX AA
expected testcross PROGENY phenotypes:
XXX
AAA
Parental types:
px + & + sp
Nonparental types:
(recombinant)
( 6.5 cM)
+ + & px sp
XX(±Y) AAA
X:A = 0.5, male
X:A = 1, female
X:A = 0.67, intersex (phenotypic mosaic)
XXX AAA X:A= 1, female (jumbo)
XA
X:A=1,
(dead) female
ALSO: one unusually large ++ female X px bw sp Male
(1) Three, not two,
parental types recovered:
px bw +
+ bw sp
px + sp
highest freq.
&= numbers
(2) many intersexual
XXY
(sterile) progeny
AAA
(3) normal and jumbo daughters
X:A ratio sex-determination mechanism
What is it about ploidy changes that affects the way that
X-chromosome dose determines sex?
(1921
2007, and not what Bridge's thought or what the textbooks say)
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