1. THOMAS MORGAN

2. 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)

3. 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

4. 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?

5. 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…

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

7. 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.

8. 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

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

10. 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

11. 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.

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

13. 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

14. 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