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.