Unformatted text preview: IB35ac Class 20: Sex & gender What are the genes and genetic mechanisms that underlie the differences between men and women and people who aren't really either one? We'll talk about the ultimate battle between the sexes, the difference between sex and gender, and some other interesting aspects of, perhaps, the most important variation within our species. IB35 Human Biological Variation Class #20 The Chromosomes 23 pairs Autosomes / Sex chromosomes Topic for today:
Sex versus Gender In females, only 1 X chromosome is active, the other is inactive as a Barr body Biological Sex Mammals Female is XX Male is XY The X Chromosome X-chromosome inactivation Barr bodies Dosage compensation between males and females Bi d Birds Female is ZW Male is ZZ New term: heterogametic ->Genetic regulatory mechanism ->Operates to equalize the phenotypic expression of characteristics determined by genes on the X chromosome so that they are equally expressed in the human XY male and the XX female.
15% of the inactivated X-chromosome does not inactivate and stays on Heterozygous for sex chromosomes E.g. XY, ZW Father's X Mother's X The X Chromosome
Mitosis ~1,098 genes Contains 4% of all human genes Accounts for 10% of diseases we've recognized th t h i d that have a M d li pattern Mendelian tt of inheritance Pseudoautosomal region with the Y chromosome Barr body Males only have 1 X chromosome so any recessive alleles on the X chromosome will be expressed 1 PSEUDOAUTOSOMAL REGIONS ->Genes located within PA region are inherited like any autosomal genes ->Crossing over between X and Y chromosome limited to PA region ->PA genes exhibit an autosomal, rather than sex-linked, pattern of inheritance ->Function of PA regions is to allow X and Y chromosomes to pair and properly segregate during meiosis in males The X Chromosome Evolutionary history Evolved from a pair of autosomes 300 million years ago Major characteristics of our X chromosome today evolved before the radiation of mammals ~105million years ago Mammals formed 195 mya Pseudoautosomal region ->SRY gene turns on 6 weeks after conception ->Signals production of testosterone ->Testosterone masculinizes embryo/foetus The Y Chromosome The gene-coding portion of the X chromosome is 6 times longer than that of the Y chromosome Has homologues for only 54 of the 1,098 genes on the X chromosome Y essentially does not exchange genes with the X save for a small region Y has gained some genes of its own ~15 genes on the Y chromosome have no detectable X chromosome homologue Chimpanzee versus Human Y Chromosome The human Y chromosome has 54 functional genes that pair with a gene on the X chromosome Comparison of 16 of these The chimpanzee Y chromosome has lost 5 of these 16 (inactivated) compared to the human Y chromosome How did this happen? Polyandry versus polygyny/harems/pair-bonding Considerable decay in the non-recombining regions Sperm competition in multiple-male mating All 15 genes related to sperm function So why do the X and Y have so little recombination? X and Y conflict Sexually antagonistic genes Examples from your reading The butterfly Acrea encedon DAX and SRY d Wolf spiders Maternal-fetal conflict ->Mother gives offspring just enough support for it to survive as she needs to conserve energy/time for subsequent pregnancies ->Offspring genetically predisposed to demand care 2 Alfred Kinsey 1948 Sexual Behavior of the Human Male 1953 Sexual Behavior of the Human Female Kinsey scale of sexual orientation 0 = maximally heterosexual 6 = maximally homosexual Evidence for a genetic contribution for homosexuality in humans: Brothers of male homosexuals were 4 times as likely to be homosexual as well compared to controls Monozygotic twins are concordant for male homosexuality 52% of the time dizygotic twins time, only 22% of the time Monozygotic twins are concordant for female homosexuality 48% of the time, 16% for dizygotic twins Key point: more than half of the monozygotic twins were discordant in spite of identical genes and similar rearing... other factors contribute Is there a "gay" gene? 1993 Dean Hamer Linkage on the tip of the long arm of the X chromosome Evidence that homosexuality reduces evolutionary fitness in humans: Homosexuals in San Francisco had 1/5 the number of children as heterosexuals Openly homosexual men had 1/10 the number of children as their heterosexual counterparts Correlation with birth order H-Y genes (produce antigens) 2005 study by Mustanski et al. Linkage on the long arm of chromosome 7 ->Social pressures in the past would have led to homosexuals getting married and having offspring anyway ->Male foetus seen as foreign to mother's body ->Production of antigens ->Every subsequent son leads to a bigger immunization reaction Evolution 2-6% of males are homosexual How does it stay at such a high frequency? P Possible evolutionary scenarios ibl l ti i Reproductive advantage for the heterozygote Altruism / kin selection Linkage to the X chromosome Do other animals exhibit homosexual behaviors? Bonobos (Pan paniscus) 3 Nature versus Nurture Sex = the genetic make-up of the individual Gender = behavioral and other characteristics that are often associated with a particular sex Is gender biologically determined? Sociobiology Edward O. Wilson 1975 Sociobiology: The New Synthesis Standard Social Science Model Embodies the idea that the human mind has no inborn tendencies or inclinations. language Nature versus Nurture False dichotomy Sex = the genetic make-up of the individual G d = b h i l and other Gender behavioral d th characteristics that are often associated with a particular sex Is gender biologically determined? "As Nature Made Him" Book by John Colapinto David Reimer's story Genetic Imprinting Gene expression depends on whether the allele is inherited from the mother or father differential genomic expression. Example: Prader Willi syndrome and Prader-Willi Angelman syndrome Chromosome 15 deletion 4 Angelman syndrome Prader-Willi syndrome Turner's Syndrome Monosomy of the X chromosome Phenotypically female Affect on behavior depends on parental origin of th X i i f the Paternally-derived Maternally-derived Only functioning copy is from the father Deleted in Angelman syndrome Only functioning copy is from the mother Symptoms: 1- Developmental delay 2- Speech impairment 3- Puppet-like movements 4- Seizures of laughter 5- mental retardation Symptoms: 1- Weak as infants, trouble suckling 2- By age 4-5, uncontrollable appetite 3- obesity 4- mental retardation STUDY of 80 KIDS WITH TURNER'S SYNDROME ->55 w/ X chromosome from mother had poor social skills ->25 w/ X chromosome from father had better social skills Summary There is evidence for a genetic contribution to human behavior But... behavior is a complex trait K Keep in mind th environmental context i i i d the i t l t t in which our behavioral phenotypes evolved But remember, the key to human evolution = plasticity 5 ...
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- Spring '08
- Angelman Syndrome, X-inactivation, human y chromosome