BSCI222 Lecture 9 missed Lecture 8 10 3 13 Second lecture on Mendellian genetics missed the first one Chapter 4 Sex determination and sex linked characteristics Drosophila are the model system for studying sex genetics o Small genome easy to culture short generation time large chromosomes o Salivary gland has many replications copies of DNA without cell division of the centromere called polytene chromosomes o First experiments collected mutants looking for genetic traits White eye mutant wild type has red did cross between white eyed male and wild type female found that F1 all had red eyes so red is dominant to white Intercrossing F1 to make F2 get several flies with white eyes BUT all the females had red eyes and some of the males had white thus not a standard recessive trait It s a sex linked trait Autosomal recessive Have to do reciprocal crosses doing it the other way red female with white male get red eyed F1 white female with red male red eyed female white eyed males The males of the second pairing are hemizygous the Y chromosome doesn t have a copy of the gene so since the X from mom was w they had to be white eyed the females got an X w from the mom but got a normal X from the dad because he s red eyed Two red eyed parents from red female and white male gave red females and some white males white female with red male half white and half red across both sexes Clues that it s sex linked reciprocal crosses don t give you the same results and different phenotypic ratios in the two sexes Hemizygous only have one copy of the gene BUT when they crossed homozygous white female with red male saw some white females even though should have had all red females and all white males Hypothesized that the females were XXY causing a nondisjunction which is when 2 copies of the X chromosome in one gamete and a Y in the other gamete or XY in one gamete and X in the other The homozygous female is already rare difficult to make and has this problem causing a bigger Punnet square Eight possible children YY and X XwXw will die Nondisjunction happens when meiosis goes wrong and both homologs wind up in the same daughter cell Can end up with an XX gamete and an O gamete blank or in a male can have an XY gamete and an O gamete o Happens in Meiosis I because that s where homologs separate MII sister chromatids are what is separating o To determine sex in drosophila compare ratio of X chromosomes to Autosomes XX females have 1 0 XY males have 0 5 In humans it s Y that determines sex but in Drosophila it s the X A ratio An XO fly s ratio is 0 5 and is male even though doesn t have a Y chromosome An XXY fly s ratio is 1 0 will be female XX and XY is used for sex determination in all mammals Human sex chromosomes o Birds are different The females are heterogametic not the males o Females are WZ or ZW males are ZZ o Peacock locus Ca wild type is blue capital letter indicates its dominant ca recessive is brown Sex linked Show via recessive cross blue female Z Ca W and cameo male Z ca Z ca or cameo female Z ca W with blue male Z Ca Z Ca Get blue males and cameo females get all blue progeny Intercross first one get half blue half brown across intercross second get blue males half cameo females half blue o A Y chromosome originated from the X chromosome genes were gradually lost until the chromosome became much smaller o Still pairs with the X chromosome during meiosis only at the tips called pseudoautosomal regions undergoing recombination just in those small regions In the female the X chromosomes are recombining the whole time the males just get the small amount at the tips o SRY gene of Sox family derived by duplication of a gene that s found on the X chromosome called Sox3 Is found in the non recombining region of the Y chromosome never on the X o A small amount of Y is recently X transposed a lot of it is X degenerate and then lots of it is ampliconic where they group together highly repetitive units 60 of the region shows over 99 9 similarity o Because the Y has lost so many genes so have sex linked inheritance because recessive alleles are exposed when they re the only copy o Having the ratios of gene expression the same between all the different genes in the genome is pretty critical for development o Can have more expression of one gene than another as long as the ratio remains constant o The big X chromosome has about a thousand genes has gradually lost genes on the Y the Y only has 78 or 79 active genes When the pairs started out millions of years ago as an autosomal pair they both had equal amounts of genes Became sex chromosome Y started to lose genes ratio went wild Evolution of X inactivation o There starts to be selection for the males to coax more expression from the genes on the X chromosome creating a problem for females because when you start increasing expression accumulating mutations starts expressing too much in the females Conflict between preferred expression levels between males and females New mechanism to balance it evolved females developed a mechanism to silence many of the genes on the second X chromosome essentially turn one of their X s off called X inactivation One copy of the X will be all balled up into inactive chromatin unable to express anything called a Barr body after the scientist who discovered them o There are some genes that become expressed because of the X inactivation because it makes the female essentially hemizygous and now recessive alleles on the active X chromosome will be revealed absence of sweat glands in females o Tortoiseshell cat fur some orange some black Gives you some idea of when the X went inactive patches derived from one particular cell in the embryo that continued to divide o Nondisjunction happens in humans too Turner Syndrome XO only 45 chromosomes and Klinefelter Syndrome XXY 47 chromosomes could be nondisjunction in either male or female parent Even with extra X s only one will be expressed others become Barr bodies Sex and gender o Athletes o Androgen Insensitivity Syndrome XY insensitive to androgens phenotypically female Sex determination in other species o Hymenoptera bees wasps and ants ploidy level whether they re haploid bee drones males or diploid bee queen determines the sex If she lays an egg that she doesn t allow to be fertilized it will be a drone male but if she fertilizes the egg she lays it will be female Single locus where if the locus is heterozygous it makes females and if it s hemizygous unfertilized eggs or