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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 education .. .from a genetical and evolutionary (= genetical) perspective 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 Forces in evolution (and evolution is what genetics -- and life -- is really all about): (1) Natural selection: reproduction of the fittest …remember, responses to changes in biological environment (including parasites) are as important as responses to changes in physical environment. (this term is in your glossary) (2) Sexual selection: reproduction of the sexiest (this term is NOT in your glossary) runaway sexual selection generally leads to maladaptive ("unfit") sexually selected traits human brain = peacock tail ? …but a tail with the capacity to change the way life evolves Sex: Sexual reproduction vs. Asexual reproduction Coming together of genetic material from (two) individuals to form progeny that combine genes from all (both) parents symmetrical sex: us asymmetrical sex: bacteria equal genetic contribution from each partner qualitative exceptions: mitochondria & Y chromosome unequal genetic contribution from each partner Can there be sex without gender? (males & females) Recall: Sex: Sexual reproduction vs. Asexual reproduction Coming together of genetic material from (two) individuals to form progeny that combine genes from all (both) parents GAMETES (sex cells) Isogamous sex systems: gametes from each parent are of equal size Anisogamous sex systems: gametes from each parent are of different sizes Males: sperm/pollen (smaller) 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: no meiosis (genes are missing) unusual distribution of DNA variation suggests no gene mixing for ages 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 including those that determine behavioral differences) An extreme example of sexual dimorphism Bonellia viridis Female: 100 mm Male: 1 mm sex is determined by whether: 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, 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! 1:1 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 testcross PROGENY phenotypes: px + & + sp Parental types: XXX AAA Nonparental types: (recombinant) ( 6.5 cM) + + & px sp 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 For fruit flies: X(±Y) AA XX AA 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 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)