View
229
Download
2
Tags:
Embed Size (px)
Citation preview
MORGAN & Drosophila• To Explore How Eye Color is Inherited in Flies Morgan
crossed:
Red-eyed Female
White-eyed Male
X
THE DISCOVERY OF SEX CHROMOSOMESNettie Stevens:Observed differences in chromosomesbetween male & female beetles
• Chromosomes named X and Y:– Half male gametes contain Y, other
half X
– All female gametes contain X
THE DISCOVERY OF SEX CHROMOSOMES
Nettie Stevens:
• Developed hypothesis about sex determination:
• Male is formed when
• Female is formed when
XX
Y
BACK TO MORGAN…Morgan guessed that
D. melanogaster (like T.
molitor) had chromosomes
that differ between sexes:
sperm sperm
Gamete Formation In a Male Fly
MORGAN & DrosophilaThe X Linked Hypothesis: • Morgan hypothesized that the gene
controlling eye color
PRACTICE SEX LINKED PROBLEMCross a heterozygous red-eyed female with a
red-eyed male
• R = red eyes, r = white eyes
• Genotype of female parent =
• Genotype of male parent =
PRACTICE SEX LINKED PROBLEM
PARENT: XR / Xr
PA
RE
NT
: XR /
Y
** hint: gametes along top, “whole people” on inside
• Red eyed
female
crossed
with Red
eyed male
•R = red
•r = white
Morgan’s work on Drosophila provided evidence that:
• The X chromosome contains genes the Y doesn’t:• Inheritance patterns of sex-linked genes vary between sexes
• Genes are located on chromosomes
MORGAN & Drosophila
GENE LINKAGELinkage:
– Autosomal Genes: – Reside on the autosomal chromosomes
– In humans: genes located on chromosome #
– Sex-Linked Genes: – Found on sex chromosomes
– In humans: genes found on #
MORGAN & LINKED GENES• First examples of linked genes were found
on X chromosome of Drosophila:
• Morgan established that eye color & body color are linked traits
» Both found on X chromosome of fruit fly
MORGAN & LINKED GENES
• Morgan re-evaluated Mendel’s Principle of Independent Assortment
– Morgan predicted:• Linked genes should be transmitted together
during gamete formation
LINKAGE HYPOTHESIS
R R
g g
r r
G G g g
R R
g gGG
r r
Hypothesis (Morgan): When two genes occur on ONEchromosome, INDEPENDENTASSORTMENT DOES NOT OCCUR
AFemale(2X chromo)
White eyes
Gray body
Red eyes
Yellow body
Gam
etes
rG Rg
Meiosis II
Meiosis I
ONLY 2 gamete types
Eye Color: Red = wild type (R) White = mutant (r)
Body Color:Gray body = wild type (G)Yellow body = mutant (g)
r r
G G
R R
r R
G g G g
R Rrr
G g gG
Crossing overduring meiosis I
Meiosis II
R Rr r
G G g g
rG rg RG RgRecombinant chromosomes
Ga
met
es
GENETIC RECOMBINATION: PRODUCTION OF NEW COMBINATION OF TRAITS
Linked genes can become unlinked through cross over
RECOMBINATION PROBABILITY
• The farther apart two genes, the higher the probability they will be separated during crossover:
RECOMBINATION PROBABILITY
• A & D are more likely than B & C to become separated (unlinked)
• B & C more likely to be inherited together (stay linked)
GENE MAPPING
• Maps of genes can be constructed from recombination data
Linkage map: genetic map based on recombination frequencies
Crossing Over
Gene 1Gene 2Gene 3Gene 4Gene 5Gene 6Gene 7Gene 8Gene 9Gene 10Gene 11Gene 12
Crossing over rarelyoccurs betweenadjacent loci, recombinationsare rare.
Crossing overalmost alwaysoccurs betweendistant loci, recombinationsare frequent.
0 : Yellow body0 : Yellow body1.4 : White eyes
Map units
0 : Yellow body1.4 : White eyes
20 : Cut wings
Chromosomes are composed of genes
The physical distancebetween loci determines the frequency of crossing over
Frequency of crossover can be used to map physical between loci
Linkage map
GENE MAPPING% recombinant gametes Reflects distance between 2 loci