Presentation is loading. Please wait.

Presentation is loading. Please wait.

Genetic mosaics have zillions of uses besides just facilitating mutant isolation …and geneticists have ways of controlling exactly when and where FLPase.

Similar presentations


Presentation on theme: "Genetic mosaics have zillions of uses besides just facilitating mutant isolation …and geneticists have ways of controlling exactly when and where FLPase."— Presentation transcript:

1 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

2 Sex education...from a genetical and evolutionary (= genetical) perspective

3 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

4 Forces in evolution (and evolution is what genetics -- and life -- is really all about): (1) Natural selection: reproduction of the fittest (2) Sexual selection: reproduction of the sexiest runaway sexual selection generally leads to maladaptive ("unfit") sexually selected traits …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) (this term is NOT in your glossary)

5 peacock tail …but a tail with the capacity to change the way life evolves human brain = ?

6 Sex: Sexual reproduction vs. Asexual reproduction Coming together of genetic material from (two) individuals to form progeny that combine genes from all (both) parents

7 symmetrical sex: asymmetrical sex: equal genetic contribution from each partner unequal genetic contribution from each partner qualitative exceptions: mitochondria & Y chromosome us bacteria

8 Can there be sex without gender? (males & females)

9 Sex: Sexual reproduction vs. Asexual reproduction Coming together of genetic material from (two) individuals to form progeny that combine genes from all (both) parents Recall:

10 Isogamous sex systems: gametes from each parent are of equal size Anisogamous sex systems: gametes from each parent are of different sizes GAMETES (sex cells) 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)

11 Sex is ancient and ubiquitous no meiosis (genes are missing) unusual distribution of DNA variation suggests no gene mixing for ages Evidence: bdelloid rotifers: no sex for millions of years Nevertheless:

12 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

13 Benefits 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)

14 “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 )

15 Bonellia viridis Female: 100 mm Male: 1 mm larva lands on rock larva lands on adult female An extreme example of sexual dimorphism ESD: environmental sex determination sex is determined by whether:

16 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

17 apparant paradox: Since females are rate-limiting for reproduction, why see 1:1 sex ratio so often? 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! (as usual, Darwin had the answer first)

18 Calvin Bridges (1916): 1:1 for fruit flies: XX females XY males white daughers (fertile) red sons (sterile) (primary) white daughers (fertile) red sons (fertile!) expected: w - /w + (red) daughters w - /Y (white) sonsX XY XXY X(O) XXY XY(±Y) progeny are “secondary” exceptions x red X Y w - /w - (white eyed) Females X Males (red eyed) w + /Y “exceptions”: …but what really determines fly sex? (xxx & o/Y die)

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

20 XX females XY males What about X-chromosome number matters? absolute number: 1=male, 2 or 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

21 Parental types: px + & + sp Nonparental types: (recombinant) ( 6.5 cM) + + & px sp px bw + + bw sp px + spFemalesMales X expected testcross PROGENY phenotypes: (autosomal genes) ALSO: one unusually large ++ female px bw + + bw sp px + sp (1)Three, not two, parental types recovered: (2) many intersexual (sterile) progeny X px bw sp Male XXY AAA (3) normal and jumbo daughters highest freq. &= numbers XXX AAA

22 XX AA X:A = 1, female X(±Y) AA X:A = 0.5, male XX(±Y) AAA X:A = 0.67, intersex (phenotypic mosaic) XXX AAA X:A= 1, female (jumbo) X A X:A=1, (dead) female For fruit flies: X:A ratio sex-determination mechanism What is it about ploidy changes that affects the way that X-chromosome dose determines sex? ( , and not what Bridge's thought or what the textbooks say)


Download ppt "Genetic mosaics have zillions of uses besides just facilitating mutant isolation …and geneticists have ways of controlling exactly when and where FLPase."

Similar presentations


Ads by Google