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  Thomas Hunt Morgan (Columbia University 1910)  Fruit Flies (Drosophila) melanogaster) © 2007 Paul Billiet ODWSODWS

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 WILDTYPE MUTANT w+w+ w

The case of the white- eyed mutant CharacterTraits Eye colorRed eye (wild type) White eye (mutant) P Phenotypes Wild type (red-eyed) female x White-eyed male F 1 Phenotypes All red-eyed Red eye is dominant to white eye

Hypothesis A cross between the F 1 flies should give us: 3 red eye : 1 white eye F2F2 PhenotypesRed eyeWhite eye Numbers % % So far so good

An interesting observation F2F2 PhenotypesRed- eyed males Red- eyed females White- eyed males White- eyed females Numbers %58%18% 0% The F 2 generation showed the 3:1 red: white eye ratio, but only males had white eyes

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 white- eyed males This confirmed what Morgan suspected The gene for eye color is linked to the X chromosome

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 Sex- Linked Genes  The 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 (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 X n Females have to be X n X n to show sex-linked trait X n X Females do NOT show sex- linked trait Males have to be X n Y to show sex- linked 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 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

Another X-Linked Trait = Duchenne Muscular Dystrophy