INTRODUCTION TO LINKED GENES AND SEX LINKAGE AP Biology/ Ms. Gaynor Chapter 15 (Part 5) INTRODUCTION TO LINKED GENES AND SEX LINKAGE AP Biology/ Ms. Gaynor

Locating Genes on Chromosomes Are located on chromosomes Code for protein that Figure 15.1

Chromosome Theory of Inheritance The chromosome theory of inheritance states that Genes have specific loci on chromosomes Chromosomes undergo segregation and independent assortment during meiosis

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 linked 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 factors (alleles)

Sex Linkage Explained Thomas Hunt Morgan (Columbia University 1910) 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 WILDTYPE w+ w MUTANT

The case of the white-eyed mutant Character Traits Eye color Red eye (wild type) 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 Red eye White eye Numbers 3470 82% 782 18% So far so good…

An interesting observation F2 Phenotypes Red-eyed males Red-eyed females White-eyed males White-eyed females Numbers 1011 2459 782 24% 58% 18% 0% © 2007 Paul Billiet ODWS

Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) The F1 generation all had red eyes The F2 generation showed the 3:1 red: white eye ratio (as expected), but only males had white eyes

Morgan determined that white-eye mutant allele must be located on the X chromosome Figure 15.4 The F2 generation showed a typical Mendelian 3:1 ratio of red eyes to white eyes. However, no females displayed the white-eye trait; they all had red eyes. Half the males had white eyes, and half had red eyes. Morgan then bred an F1 red-eyed female to an F1 red-eyed male to produce the F2 generation. RESULTS P Generation F1 X F2 Morgan mated a wild-type (red-eyed) female with a mutant white-eyed male. The F1 offspring all had red eyes. EXPERIMENT

Morgan Also Tried A Reciprocal Cross … Morgan crossed 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.

Gene are LINKED to chromosomes Morgan’s discovery transmission of X chromosome in fruit flies correlates with inheritance of the eye-color trait 1st solid evidence that a specific gene is associated with a specific chromosome

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 = Hemophilia About 85% of hemophiliacs suffer from classic hemophilia 1 male in 10 000 They cannot produce factor VIII The rest show Christmas disease where they cannot make factor IX The genes for both forms of hemophilia are sex linked Hemophiliacs have trouble clotting their blood

Another X-Linked Trait = Duchenne Muscular Dystrophy Recessive disorder 1 male in 3,600 Defective muscle protein called dystrophin Fatigue Muscle weakness (legs, pelvis, arms, neck) Difficulty with motor skills Frequent falls Progressive disease  trouble walking leads to wheelchair bound patients

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 called x-linked)

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 sex-linked trait Males have to be Xn Y to show sex-linked trait **Most sex-linked disorders affect males; sometimes females

X inactivation in Female Mammals In mammalian females One of the two X chromosomes in each BODY cell is usually randomly inactivated in EACH cell during embryonic development Inactive X condenses into a Barr body

Inactivation of X is Random The inactivation is random and approximately ½ the X’s from Mom and ½ the X’s from Dad will inactivate

Females usually have a mosaic of cell types for characteristics on X chromosome Ex: Tortoiseshell and Calico cats Two cell populations in adult cat: Active X Orange fur Inactive X Early embryo: X chromosomes Allele for black fur Cell division and X chromosome inactivation Black Figure 15.11