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Genes and ChromosomesGene Linkage, Crossing Over, Codominance and Incomplete Dominance
Mendel and Chromosomes
Mendel never asked “Where in the cell are the factors that control heredity?”i.e. where are the genesHe couldn’t have answered the question though His promotion to abbot kept him too busy
Where are they?
By 1900 cell biologists had discovered most organelles and the process of mitosis and meiosis Nucleus was logical place
Why the Nucleus?
Central LocationLarge sizeLots of activity during mitosis and meiosis
Chromosome Theory of Inheritance Walter Sutton – Columbia University
Genes are located on chromosomesEach gene occupies a specific place on a chromosome A gene may exist in several forms, or alleles Each chromosome contains one allele for each gene
Thomas Morgan
Embryologist at Columbia UniversityWorked with fruit flies
Fruit Flies?! …Yuck!
Drosophila melanogaster
Prolific reproducers: a single cross will produce 100s of offspring4 pairs of chromosomes Larva hatch within two weeks
Genes on a Chromosome
Each chromosome has hundreds or thousands of genesGenes on the same chromosome tend to be inherited togetherRemember: a chromosome is passed along as a single unit during meiosis
Thomas Morgan
First to associate a specific gene with a specific chromosomeCrossed gray, normal winged homozygous parents with black, vestigial wing parents. – GGNN x ggnn = GgNn– Then test crossed GgNn x ggnn
Let’s do the Punnet Square:
Expected Punnet Square
GN Gn gN gngn GgNn Ggnn ggNn ggnngn GgNn Ggnn ggNn ggnngn GgNn Ggnn ggNn ggnngn GgNn Ggnn ggNn ggnn
Expected:
25% Gray, Normal wing25% Gray, Vestigial wing25% Black, Normal wing25% Black, Vestigial wing
Actual Phenotypes:
41.5 % Gray, Normal41.5% Black, Vestigial8.5% Gray, Vestigial8.5% Black, Normal
If they are linked…
Let’s let G = G and NLet’s let g = g and nSo GG would be GGNN and … Gg would be GgNn and … gg would be ggnn.
If they are linked…
G g
g Gg gg
g Gg gg
If they are linked…
Don’t forget that Gg is GgNn and gg is ggnnSo we have 50% Gray bodied, Normal wingAnd 50% Black bodied, Vestigial wingThis explains the variation from the original expected.
Morgan’s Hypothesis
Body color (gray or black) and wing shape (normal or vestigial) are usually inherited together in a specific combination due to GENE LINKAGE
Gene Linkage
Genes located on the same chromosome tend to be inherited together because they are part of a single chromosome that is passed along as a unit.
Morgan’s Crosses
83% of Morgan’s flies had characteristics of their parents. (gray/normal wings or black/ vestigial wings.)(41.5% gray/normal + 41.5%
black/vestigial =83%)
17% of the flies had eithera)Gray, vestigial wingsb)Black, normal wings (8.5% gray, vestigial + 8.5% black,
normal = 17%)
Morgan and Sturtevant (his associate) hypothesized that linkages could sometimes be brokenGray/normal and black/vestigial genes can occasionally be separated
INTERESTING QUESTION:
If body color and wing shape genes are linked, how come all of the offspring were not like their parents (gray/normal and black/vestigial)?????????????
CROSSING OVER IS TO BLAME!!!!!
CROSSING OVER
DURING MEIOSIS, HOMOLOGOUS CHROMOSOMES EXCHANGE PORTIONS OF THEIR CHROMATIDS
CROSSING OVER
GENE MAPPING
Distance between chromosomes determines how often crossing over occurs Further away = more crossing overFrequency of crossing over allows mapping of position on chromosomes
Gene Mapping Diagram
Genes A & B are very close togetherC & D are farther apartA random crossover event is much more likely to separate genes C & D than A and B (which are closer together).
Sex Linkage
1 exception to rule that every chromosome has a corresponding homologous chromosome … 1905 American – Nettie Stevens – mealworms–Female contains 20 large chromosomes– male – 19 and one small
These are sex chromosomes– Female: XX– Male: XY
Because X and Y determine sex, genes located on one of these chromosomes are called “Sex-Linked genes”
HERE’S AN EXAMPLE:
Genes associated with vision in humans are located on the X chromosome.– Males only have ONE X chromosome!!!!
This can be a problem if the X chromosome given to him by Mom has a genetic disorder!
Red-Green Color Blind?
Red-Green Color Blind?
Colorblindness:
Your mother unknowingly has the genotype XCXc. Dad is not colorblind and therefore has the genotype XCY.Will any of the children be colorblind??????
Answer: Yes! One of the sons will be colorblind because he will have the genotype XcY.
XC Xc
XC
Y
XCXC XCXc
XCY XcY
Revenge against Men!
Males have just one X chromosome. Therefore, if the mom gives them a recessive allele on her X chromosome, it will be expressed in the son EVEN though the mom’s phenotype did not express the allele!
INCOMPLETE DOMINANCE
Josef Kolreuter – crossed white carnations (rr) with Red (RR) carnations.Got all Pink!!! (Rr)
INCOMPLETE DOMINANCE
Did red and pink blend together????
DID THE COLORS BLEND????
NO!!!!!!! How do we know????–When crossed Rr x Rr (F1 x F1)…….–¼ Red–½ Pink–¼ WhiteLet’s do the Punnett Square
PUNNET SQUARE
25% RR: Red50% Rr: Pink25% rr: White
R r
R
r
RR Rr
Rr rr
INCOMPLETE DOMINANCE
When mom makes a birthday cake and wants to frost it, how does she do it?
White frosting Wants to make red frosting
no red food color makes white little red food color makes pink lots of red food color makes red
INCOMPETE DOMINANCE
Same with genesGenes code for a proteinIf the protein is a pigment, like carnations:– no dose = white– single dose = pink– double dose = red
CODOMINANCE
What does dominant mean?In codominance, both alleles are expressed
CODOMINANCE EXAMPLE
On sports teams, often have two captains. Do both captains have equal representation?Sure they do!!!!
CODOMINANCE
The same is true for codominant alleles…both are equally expressed within a phenotype.Written as a capital letter with a superscriptHR or HW means the trait is hair color and red and white are both dominant.
CATTLE:
Cattle – Red hair (HR) and White hair (HW) are codominant –HRHR= Red hair–HW HW= White hair–HR HW = roan or combination – rust
colored (Red and white)
Try This!
What would you get if you crossed a Roan with a White haired cow?Do the punnett square:
PUNNET SQUARE
25% ROAN50% WHITE
HR HW
HW HR HW HWHW
HW HR HW HWHW
NONDISJUNCTION
Whole chromosomes or sets of chromosomes fail to separate normally during meiosis Example: Down’s syndromeWhen all chromosomes fail to separate, can cause 3N or 4N organisms.
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