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Lecture #30 Introduction to Sex Linkage Honors Biology Ms. Gaynor Gene Linkage Linked genes Usually inherited together because located near each other on the SAME chromosome Genes closer together on the same chromosome are more often inherited together Each chromosome Has 100’s or 1000’s of genes Sex-linked genes exhibit unique patterns of inheritance; genes on the X or Y chromosome Morgan’s Experimental Evidence Thomas Hunt Morgan Provided convincing evidence that chromosomes are the location of Mendel’s heritable alleles Sex linkage explained http://nobelprize.org/nobel_prizes/medicine/articles/lewis/index.html Thomas Hunt Morgan (Columbia University 1910) Fruit Flies (Drosophila) melanogaster) © 2007 Paul Billiet ODWS Morgan’s Choice of Experimental Organism Morgan worked with fruit flies Lots of offspring A new generation can be bred every two weeks They have only 5 pairs of chromosomes Morgan and Fruit Flies Morgan first observed and noted Wild type (most common) phenotypes that were common in the fly populations Traits alternative to the wild type are called mutant phenotypes w+ WILDTYPE w MUTANT The case of the whiteeyed mutant Character Eye color type) Traits Red eye (wild White eye (mutant) P Phenotypes Wild type (red-eyed) female x White-eyed male F1 Phenotypes All red-eyed Red eye is dominant to white eye Hypothesis A cross between the F1 flies should give us: 3 red eye : 1 white eye F2 Phenotypes Numbers So far so good Red eye White eye 3470 82% 782 18% An interesting observation The F2 generation showed the 3:1 red: white eye ratio, but only males had white eyes Phenotypes F2 Numbers Redeyed males Redeyed females Whiteeyed males Whiteeyed 1011 2459 782 0 24% 58% 18% 0% females A reciprocal cross Morgan tried the cross the other way round white-eyed female x red-eyed male Result All red-eyed females and all whiteeyed males This confirmed what Morgan suspected The gene for eye color is linked to the Morgan’s Discovery: Sex Linked Traits Eye color is linked on X Chromosome Females carry 2 copies of gene; males have only 1 copy If mutant allele is recessive, white eyed female has the trait on both X’s White eyed male can not hide the trait since he has only one X. The Chromosomal Basis of Sex An organism’s sex Is an inherited phenotype determined by the presence or absence of certain chromosomes XX = girl XY = boy Inheritance of SexLinked Genes sex chromosomes Have genes for many characters unrelated to sex (especially the X chromosome) A gene located on either sex chromosome Is called a sex-linked gene The (Usually on X chromosome) What genes are on the X chromosome? carries a couple thousand genes but few, if any, of these have anything to do directly with sex determination Larger and more active than Y chromosome What genes are on the Y chromosome? Gene called SRY triggers testis development, which determines male sex characteristics This gene is turned “on” ~6 weeks into the development of a male embryo Y-Chromosome-linked diseases are rare Sex-linked genes follow specific patterns of inheritance Fathers pass sex-linked alleles to ALL their daughters but NONE to their sons XY (Father) XX (daughter) XY (Father) XY (son) Mothers can pass sex-linked alleles to BOTH sons and daughters XX (Mother) XX (daughter) XX (Mother) XY (son) Sex Linkage If sex-linked recessive on Xn Females have to be Xn Xn to show sex-linked trait Xn X Females do NOT show sexlinked trait Males have to be Xn Y to show sexlinked trait **Most sex-linked disorders affect males; sometimes females Sex-Linked Disorders Some recessive alleles found on the X chromosome in humans cause certain types of disorders Color blindness Duchenne muscular dystrophy Hemophilia Male pattern baldness X-Linked Trait = Male Pattern Baldness Baldness Another X-Linked Trait = Hemophilia About 85% of hemophiliacs suffer from classic hemophilia 1 male in 10 000 cannot produce factor VIII The rest show Christmas disease where they can’t make factor IX The genes for both forms of hemophilia are sex linked Hemophiliacs have trouble clotting their blood Another X-Linked Trait = Red-Green Colorblindness Normal vision Color blind simulation http://www.onset.unsw.edu.au/issue1/colourblindness/colourblindness_print.htm Another X-Linked Trait = Duchenne Muscular Dystrophy