THOMAS MORGAN

Morgan’s DISCOVERY of LINKED GENES Characteristics of linked genes GENES that are CLOSE ON CHROMOSOMES are often INHERITED TOGETHER LINKED GENES usually don’t SEPARATE during Crossing Over Prophase I (Meiosis)

How Morgan Discovered Linked Genes USED DROSOPHILA Genus name common name Fruit flies WHY? They Mature in 2 weeks They Produce large numbers of offspring They only have 4 pair of chromosomes One pair are the Sex CHROMOSOMES

The Experiment for Linked Genes Part I Thomas crossed PURE BRED fruit flies for two traits Homozygous Dominant GRAY BODIES & NORMAL WING SIZE (GGWW) with Homozygous Recessive flies that had BLACK BODIES & SMALL WINGS (ggww) GG WW X gg ww WHAT Genotype DID HE EXPECT IN THE OFFSPRING? GgWw WHICH IS WHAT HAPPENED Okay Big deal! What do you think he did next?

MORGAN’S Experiment Part II GgWw x ggww GW gw Gw gW   He then crossed an F-1 of the GgWw hybrid offspring w/a recessive ggww What would you expect? Fill out this punnett- he did not get as expected… ¼ GRAY NORMAL ¼ GRAY SHORT ¼ BLACK NORMAL ¼ BLACK SHORT INSTEAD, he got…

Experiment’s Results for Part II gg ww alleles Gg Ww alleles Expected Actual Results 2300 total offspring

MORGAN’S RESULTS Animation 41.5% GRAY body/Normal wings 41.5% BLACK body/small wings 8.5% GRAY body/Small wings 8.5% BLACK body/Normal wings MORGAN’s Conclusion The genes for wing size and body color were so commonly inherited as only two combinations either gray body/normal wing or black body/small wing that they had to be … on the same chromosome! This indicated that the genes for body color and wing size were… LINKED onto one chromosome. http://www.springer.com/cda/content/document/cda_downloaddocument/1002s.swf?SGWID=0-0-45-753210-0

Crossing Over Explains the other 8 Crossing Over Explains the other 8.5% combinations of either Black Body/Normal Wing or Gray body/Small Wing Chromosome combinations for gametes Homologous chromosomes

This means that even though genes can be linked, They can sometimes separate from one another during Crossing Over in Meiosis

NETTIE STEVENS Discovered Sex Chromosomes She worked with MEALWORMS DISCOVERED they had 20 chromosomes. MALES had 19 regular size & 1 small one. The Females all the same size chromosomes SHE SAID “THE 19 that were the same are AUTOSOMES (body chromosomes), & the other set were SEX CHROMOSOMES” FRUIT FLIES same for male/females Males have XY Females have XX

GENES ON SEX CHROMOSOMES Morgan’s Sex linked experiment SEX-LINKED GENE-gene on a a SEX CHROMOSOME MORGAN DISCOVERED the 1ST SEX-LINKED gene in fruit flies He crossed a PUREBRED Dominant RED-EYED FEMALE W+W+ with a PUREBRED recessive WHITE-EYED WW MALE Let’s take a look at the 1st cross. http://www.dnaftb.org/10/animation.html

Morgan’s 1st Sex Linked Cross All offspring had red eyes. No surprise.

MORGAN’S 2nd CROSS from F-1 , W+W RED-EYED female X RED-EYED W+ MALE 3:1 RATIO of Red eye to White eye, but only MALES had WHITE EYES! Why? no FEMALES had WHITE EYES Morgan hypothesized that EYE COLOR must be a SEX-LINKED gene, on Trait must be on the X CHROMOSOME • The RECESSIVE TRAIT White eyes, is inherited more often in males that receive the rallele on their one and only X CHROMOSOME

MORGAN’S EXPERIMENTAL CROSS White eyed male Red eyed female All females had red eyes All males had red eyes 3:1 ratio red eyes to white eyes Only males had white eyes