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 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 An extreme example of sexual dimorphism Bonellia viridis Female 100 mm Male 1 mm 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 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 Known 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 PROGENY px sp Parental types Nonparental types recombinant 6 5 cM px sp XXX AAA ALSO one unusually large female X px bw sp Male 1 Three not two parental types recovered px bw bw sp px sp 2 many intersexual XXY sterile progeny AAA 3 normal and jumbo females 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 XA X A 1 dead female GENETIC MOSAICS X chromosome loss generates gynandromorphs XX AA zygote XXAA cells X AA cells XXAA X AA Female Male XXAA zygote XXAA cells XA cells loss of an entire haploid set XXAA X A Female Female XA never reaches adult stage but mosaics do 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 XA X A 1 dead female GSD by X A ratio balance 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 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 XX AAA X A 0 67 male XXX AAAA X A 0 75 hermaphrodite GSD by X A ratio HUMANS XX female XY male XXY Kleinfeler Syndrome sterile male 1 1000 men XO Turner Syndrome sterile female 1 2000 5000 GSD by Active Y dominant masculinizer HOUSE FLIES m m female M m male GSD by dominant masculinizing allele M one of three different GSD systems in the same species Birds moths and butterflies ZZ male ZW female female is the 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 Diploid royal jelly Eggs not fertilized Drones males Haploid GSD by haplodiploid system But is the relevant variable ploidy 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 fertilization a1 a2 Queen X a1 Drone a1 a1 a2 a1 diploid drones GSD by a multiple allele system highly polymorphic sex gene many alleles
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