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big exception to mendel's ideas

chromosome theory and linkage


sutton/boveri chromosome theory

genes are located on chromosomes
-genes (alleles) and chromosomes both occur in pairs
-allele pairs and chromosome pairs both separate from each other and enter separate gametes
-separation of both allele pairs and chromosome pairs are independent


proof of chromosome theory

morgan created research group called Drosophila group
group showed that certain gene was on the X chromosome and therefore the chromosome theory must be true


morgan's observations

strange results from cross between white-eyed female and red-eyed male (all red-eyed females and all white-eyed males)
could only explain strange results if he assumed the white-eyed gene was on the X chromosome


bridge's final proof of chromosome theory

experiment with nondisjunction where he produced XXY female
in various sex chromosomes that resulted from these females, the X chromosome and the white eye gene seemed to be transmitted together
proof was first article in first volume of journal Genetics


X linkage in humans

genes on X chromosome=X linked
ex: hemophilia A and red green color blindness


sex linked genes

gene on X or Y chromosome


how many PARs on X and Y chromosomes

2 PARs
at tip of each telomere



more than 1 gene is located on each chromosome so 2 genes found on the same homologous pair of a chromosome will not obey mendel's law of independent assortment
alleles will move together (linked)
for a 2 point cross there are 2 possibilities


parental and recombinant gametes

gametes F1 individual makes are either:
-same combo of alleles as P generation
-opposite combo of alleles as P generation


independent assortment vs. linkage

if genes are on different chromosome pairs they will assort independently (# of P gametes will be ~equal to # of recombinant gametes)
if 2 genes are on same chromosome pair they will not assort independently but will be linked (# of P gametes will be significantly greater than # of recombinant gametes)


recombinant gametes and crossing over

for linked genes recombinant gametes are produced by crossing over between 2 genes
even if crossover occurs the products of a single meiotic event will be 2 parental and 2 recombinant alleles


predicting outcome of dihybrid cross involving linked genes

punnet square using probabilities
list gamete genotypes and probability of each gamete (probability calculated if you know distance between genes)
probabilities of each gamete pair multiplied together giving frequency of each genotype


gene mapping

for linked genes the distance between is proportional to the frequency of crossing over
further apart 2 genes are the more likely a crossover will occur between them
ex: 1% recombinant gametes=1 map unit or cM
max distance=50 cM


3 point test cross

used bc 2 point test crosses often overlook double crossovers
individual crossed with homozygous recessive
analysis reveals gene order, distance between genes, and the degree of interference


steps of 3 point cross

1. determine gene order
2. determine distance between genes
3. calculate coefficient of coincidence


what is the coefficient of coincidence

decimal that expresses what proportion of expected double crossovers actually happened


mapping in organisms with an ordered tetrad

ascomycetes (all products of single meiosis in sac)
some have cells maintained in order in which they are produced by meiosis I and II (possible to map distance between gene and its centromere)
4 of 1 kind on top and 4 of other kind on bottom


somatic cell hybridization

technique used in 1960s to map some human genes to chromosomes or even regions of a chromosome
fusion of human tissue culture cell to mouse cell
human cells preferentially lost so you can get cell with all mouse chromosomes but 1 human chromosome so if cell still produces specific human product then gene for that product must be on that chromosome



fluorescent in situ hybridization
technique involves hybridizing a probe DNA molecule (tagged with fluorescent marker) to a spread of chromosomes


SNPs and GWASs

single nucleotide polymorphisms and genome-wide associated studies
use common variants of a single DNA base pair as markers to hunt for genes related to a trait or disease


LOD score

log of odds favoring linkage
used in population genetics
pedigree analysis can be used to infer linkage of 2 genes
estimate probability that pedigree was produced by 2 genes linked at a given distance vs. produced by 2 non linked genes


discovery of nucleic acids

isolated new substance from nuclei of cells ("nuclein") which he realized was new class of organic material
knew it wasn't protein bc it had no S but had P
nuclein was probably mixture of DNA and RNA
"nucleic acids" today


DNA as genetic material of almost everything

avery, macleod, and mccarty
showed DNA was genetic material of bacterium that causes type of pneumonia in mice
based on griffith experiment with bacteria with/without capsule (S and R) ("transforming principle")


RNA as genetic material of some viruses

some viruses have no DNA but do have RNA
some RNA viruses replicate their RNA using RNA replicase or enzyme reverse transcriptase to make DNA intermediate which is then copied to RNA
viruses called "retroviruses"


DNA structure

polynucleotide (polymer of nucleotides)


DNA polynucleotide

polynucleotide made by joining many nucleotides together
3' pentose sugar bonded to 5' phosphate of next nucleotide
DNA has sugar-phosphate backbone with bases protruding off 1 side



each DNA (or RNA) nucleotide is composed of 3 subunits (phosphate, pentose, base)



pentose and base (no phosphate)



have a double ring structure and are larger than pyrimidines
adenine and guanine



single ring structure and are smaller than purines
cytosine and thymine (uracil replaces thymine in RNA)


the double helix

watson and crick proposed 3-D model for structure of DNA (double helix)
work based on x-ray crystallography work of Franklin and Wilkins and work of chargaff (A=T and G=C) and general understanding of structure of DNA polynucleotide


strands of DNA

2 antiparallel nucleotide strands running in opposite polarity


alpha helix

the 2 strands are coiled in an alpha helix (right handed helix)


specific base pairing

A always paris with T and G always pairs with C
base pairing is by hydrogen bonding involving N, O, and H
3 bonds that hold G to C
2 bonds that hold A to T


dimensions of the double helix

3.4 A thick and stacked internally
with of molecule is 20 A and makes 1 complete turn ever 34 A
10 base pairs per turn of the helix


eukaryotes--the unineme theory of histones

prokaryotic chromosome is 1 circular DNA molecule
eukaryotic chromosome DNA is linear
each chromosome has 1 long DNA molecule (unineme theory)
wound around clusters of 8 histones