PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Sex:--- 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-cutdifference between males and females and the1:1 sex ratioCosts 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 teamsBenefits of sex:(1) Reduces mutational load(escape “Muller’s ratchet” -- irreversible loss of genes)(2) Free good mutations from bad genetic backgrounds(3) Help to keep ahead of parasites(there is no “optimal” genotype in the real world)perhaps males particularly useful (rationale for “maladaptations” from sexual selection)“Sex determination genes” determine two qualitativelydifferent things (a distinction not often appreciated, even by thosewho study the genetic programming of sex):population sex ratiosexual dimorphism (developmental differences)Bonellia viridisFemale: 100 mmMale: 1 mmlarva lands on rocklarva lands on adult femaleAn extreme example of sexual dimorphismESD: environmental sex determinationrelevant variables for ESD:Host (Bonellia)Temperature (turtles, alligators)Neighbor density (parasitic wasps)“Presence of male” (tropical fish)vs. GSD: genotypic sex determinationSegregation of alleles (genes) determines sexbest for generating 1:1 sex ratiosapparant paradox:Since females are rate-limiting for reproduction, why see 1:1 sex ratio so often?In the aggregate, both sexes contribute equallyto the next generation (every female needs a male)hence, any minority sexon average will make a disproportionate contribution per individualNatural selection will favor generation of the minority sex.At 1:1, no minority sex!(as usual, Darwin had the answer first)Calvin Bridges (1916):Known for fruit flies: XX females XY maleswhite daughers (fertile)red sons (sterile)(primary)white daughers (fertile)red sons (fertile!)expected:w -/w+ (red) daughtersw -/Y (white) sonsXXXYXXYX(O)XXYXY(±Y)progeny are“secondary” exceptionsx red XYw -/w - (white eyed) Females X Males (red eyed) w+/Y“exceptions”:…but what really determines fly sex?(xxx & o/Y die)for fruit flies:normal: XX females XY malesabnormal: XXY females XO malesX chromosome number determines sexY chromosome does not detemine sex(but is required for male fertility)Sex-chromosome difference CAUSES(triggers) different sexual developmentXX females XY malesWhat 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 answerParental types:px + & + spNonparental types:(recombinant)( 6.5 cM)+ + & px sppx bw ++ bw sppx + spFemales MalesXexpected PROGENY:(autosomal genes)ALSO: one unusually large ++ femalepx bw ++ bw sppx + sp(1) Three, not two, parental types recovered:(2) many intersexual(sterile) progenyX px bw sp MaleXXXAAAXXYAAA(3) normal and jumbo femalesXX AA X:A = 1, femaleX AA X:A = 0.5, maleXX(±Y) AAA X:A = 0.67, intersexXXX AAA X:A= 1, female (large)X A X:A=1,(dead) femaleXX AA zygote --> XXAA cells / X AA cellsX-chromosome loss generates “gynandromorphs”GENETIC MOSAICS (XXAA)Female(X AA) Male(XXAA)Female(X A) FemaleXXAA zygote -->XXAA cells/XA cells(“loss” of an entire haploid set)(XA never reachesadult stagebut mosaics do)XX AA X:A = 1, femaleX AA X:A = 0.5, maleXX(±Y) AAA X:A = 0.67, intersexXXX AAA X:A= 1, female (large)X A X:A=1,(dead) femaleGSD by X:A ratio (balance)The worm:XX self-fertilizing hermaphroditeXO 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 -> 50:50The worm:XX self-fertilizing hermaphroditeXO male (heterogametic sex)XX AAA X:A= 0.67 = maleXXX AAAA X:A = 0.75 = hermaphroditeGSD by X:A ratioHUMANS:XX female XY maleXXY Kleinfeler Syndromesterile male (1:1000 men)XO Turner Syndromesterile female (1:2000-5000)GSD by Active Ydominant masculinizerHOUSE FLIES:m/m female M/m maleGSD by dominant masculinizing allele M(one of three different GSD systemsin the same species!)Birds, moths and butterflies:ZZ male ZW femalefemale 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)HaploidGSD by “haplodiploid” systemBut is the relevant variable ploidy?Let’s encourage inbreeding among the honeybees:increased homozygositysuddenly: DIPLOID MALES!a1/a2 heterozygotes: females (queens and workers)a1 or a1/a1 hemizygotes and homozygotes: malesa1/a2 Queen X a1 Drone --> a1/a1 & a2/a1diploiddrones(fertilization)GSD by a multiple allele system--- highly “polymorphic” sex gene (many
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