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Sex: --- understanding its biological significance

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

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Sex: --- understanding its biological significance

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  1. 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

  2. (1) Males dilute females’ genetic contribution (the couple is the unit of reproduction) Costs of sex: (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

  3. (3) Help to keep ahead of parasites (there is no “optimal” genotype in the real world) 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

  4. “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)

  5. An extreme example of sexual dimorphism Bonellia viridis larva lands on rock Female: 100 mm Male: 1 mm larva lands on adult female ESD: environmental sex determination

  6. 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

  7. 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!

  8. w -/w -(white eyed) Females X Males (red eyed)w+/Y XX w -/w+(red) daughters expected: XY w -/Y (white) sons “exceptions”: XXY white daughers (fertile) X(O) red sons (sterile) xredXY XXY white daughers (fertile) progeny are “secondary” exceptions XY(±Y) red sons (fertile!) Known for fruit flies: XX femalesXY males …but what really determines fly sex? Calvin Bridges (1916): (primary) (xxx & o/Y die)

  9. Y chromosome does not detemine sex (but is required for male fertility) Sex-chromosome difference CAUSES X chromosome number determines sex (triggers) different sexual development for fruit flies: normal: XX femalesXY males abnormal: XXY femalesXO males

  10. 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? XX femalesXY males What about X-chromosome number matters? …again, genetic exceptions to the rule provide the answer

  11. px bw + X Females Males px + sp + bw sp px + & + sp Parental types: Nonparental types: + + & px sp (recombinant) ( 6.5 cM) X px bw sp Male Three, not two, parental types recovered: px bw + + bw sp px + sp (autosomal genes) expected PROGENY: XXX AAA ALSO: one unusually large ++ female (2) many intersexual (sterile) progeny XXY AAA (3) normal and jumbo females

  12. 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) X A X:A=1, (dead) female

  13. X-chromosome loss generates “gynandromorphs” XX AA zygote --> XXAA cells / X AA cells (X AA) Male (X A) Female (XXAA) Female (XXAA) Female GENETIC MOSAICS XXAA zygote --> XXAA cells/XA cells (“loss” of an entire haploid set) (XA never reaches adult stage but mosaics do)

  14. 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) X A X:A=1, (dead) female GSD by X:A ratio (balance)

  15. (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) Origin of males: (1) Spontaneous X-chromosome nondisjunction (rare) to make “O” eggs (+ X self sperm)-> XO male

  16. 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

  17. HUMANS: XX femaleXY male XXY Kleinfeler Syndrome sterile male (1:1000 men) XO Turner Syndrome sterile female (1:2000-5000) GSD by Active Y dominant masculinizer

  18. HOUSE FLIES: m/m femaleM/m male GSD by dominant masculinizing allele M (one of three different GSD systems in the same species!)

  19. female is the heterogametic sex (compare: XY males) Birds, moths and butterflies: ZZ maleZW female GSD by feminizing W or Z:A ?

  20. 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?

  21. (fertilization) a1/a2 Queen X a1 Drone --> a1/a1 & a2/a1 diploid drones 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 GSD by a multiple allele system --- highly “polymorphic” sex gene (many alleles)

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