POST MENDELIAN GENETICS

EXTENDING MENDEL

• Thomas Hunt Morgan:

MORGAN & Drosophila

Morgan Revealed

Extensions of Mendel’s

Rules:

MORGAN’S WORK WITH Drosophila

RedEyesIn FliesIs Normal(wild)

White Eyes Is Rare (mutation)

MORGAN & Drosophila• To Explore How Eye Color is Inherited in Flies Morgan

crossed:

Red-eyed Female

White-eyed Male

X

MORGAN & Drosophila• Next, Morgan crossed F1:

Red-eyed Female

X

Red-eyed Male

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

NORMAL SEX CHROMOSOMES

A Male

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

GENE MAPPING• If % of recombinant gametes is high, 2 genes

are assumed to be far apart on a chromosome

– Map Unit refers to distance between 2 loci

• Function of % recombinants