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Chapter 10
10.1 The Chromosome Theory of Heredity
Chromosomes are located in the nucleusFactors (genes) are found on chromosomesSutton discovered that genes are on chromosomes in 1902
Chromosome Theory of Heredity
States that genes are located on chromosomes and each gene occupies a specific place on a chromosomeOnly one allele is on a chromosome
Independent Assortment
Gene LinkageGenes on a chromosome are linked togetherInherited together – THEREFORE they do not undergo independent assortment
Linked Genes- genes on the same chromosome – inherited as a package
A
B
C
Height Gene
Flower color gene
Flower position gene
Thomas Hunt Morgan
Studied fruit flies – Drosophilia melanogaster
Fruit Flies are excellent for genetic studies because:
Reproduce quicklyEasy to raiseMany mutationsHave 8 chromosomes (n=4)
Fruit Fly Mutations
Thomas Hunt Morgan began to carry out experiments with
Morgan looked at TWO traits
Gray bodies – GNormal Wings - W
Black bodies – gSmall wings – w
The flies mated….
The female laid eggs
P1 GGWW x ggww
F1 GgWw
100%
Morgan then mated the F1 back to the recessive parent
GgWw x ggww
Expected ratio – 1:1:1:1
25% GgWw 25% Ggww25% ggWw 25% ggww
Morgan’s Actual Results
41.5% gray normal41.5% black small 8.5 % black normal 8.5% gray small
ConclusionGene for body size and wing color were somehow connected or linkedCan’t undergo independent assortment
Linkage GroupsPackage of genes that are always inherited togetherChromosomeOne linkage group for each homologous pairFruit flies – 4 linkage groupsHumans – 23 linkage groupsCorn – 10 linkage groups
So linkage groups explain the high percentages
(41.5%) but
What about the 8.5%??????
17% had new combinations
The combinations that were expected would be: Gray normal – GW
orBlack small - gw
G
W
G
W
P1
Mom
g
ww
g
Dad
G
W
F1
w
g
G
W
g
w
F1
g
ww
g
RecessiveFruit Fly
X
Heterozygous
F1 F1
G
W
g
w
g
ww
g
The Offspring of the Cross
F1 F1
and
41.5 % 41.5 %
G
W
G
W
g
ww
g
Genes of the Heterozygous Parent
The homologous pair copied
G
W
G
W
g
ww
g
The homolgous pairs pair up in Prophase and form a tetrad
When they are lined up they can become twisted
and switch genes
Crossing Over
G
W
G
w
g
wW
g
So you could then have …..
switch
G
w
g
w
g
wW
g
The other offspring of the cross
F1 F1
and
8.5 % 8.5 %
The 17% that had new combinations are known
asRecombinants – individuals with new combinations of genesCrossing Over – gives rise to new combinations – Prophase I
Gene MappingSturtevant – associate of MorganCrossing over occurs at randomThe distance between two genes determines how often they cross overGenes that are close do not crossover oftenGenes that are far apart – cross over often
So……If you know the frequency with which crossing over occurs then you can use that to map the position of the genes on the chromosome
Frequency of crossover exchange...
is GREATER the FARTHER apart 2 genes are
is proportional to relative distance between 2 linked genes
Relative distance is established as... 1% crossover frequency = 1 map unit of map distance
1% CrossOver Freq = 1 centiMorgan
Sex LinkageStevens – made observations of meal worm chromosomes
Sex ChromosomesOne pairFemale – XX
Male – XY
AutosomesAll the chromosomes except the sex chromosomes
Sex Determination
Genes on Sex Chromosomes
Sex chromosomes determine a person’s sexSex chromosomes also contain genes
Sex LinkedA gene located on a sex chromosomeUsually XExample – Fruit Fly Eye ColorSo the gene for
eye color is on the X chromosome and not the Y
Fruit Fly Sex Chromosomes
X X X Y
Females
XRXR
XRXr
XrXr
Males
XRY
XrY
Red Eyed
White Eyed
Mutations
A change in the DNA of an organismCan involve an entire chromosome or a single DNA nucleotide and they may take place in any cell
Germ Cell MutationOccur in an organism’s germ cells (gametes)- can only affect offpsring
Somatic MutationsTake place in an organisms body cells and can affect the organism
Lethal MutationCause death, often before birth
Good MutationsSome mutations can be beneficial – these organisms have a better chance to reproduce and therefore have an evolutionary advantageProvide the variation on which natural selection acts
Chromosome Mutations
Are either changes in the structure of a chromosome or the loss of an entire chromosome or an additionFour Types (duplication, deletion, inversion and translocation)
Duplication – segment of a chromosome is repeatedDeletion – the loss of a chromosome or part due to chromosomal breakage – that information is lost
Inversion – a chromosomal segment breaks off and then reattached in reverse orientation to the same chromosomeTranslocation – a chromosome breaks off and reattaches to another nonhomologous chromosome
NondisjunctionSome chromosome mutations alter the number of chromosomes found in a cell
Nondisjunction – the failure of a chromosome to separate from its homologue during meiosis
Gene Mutations
May involve large segments of DNA or a single nucleotide within a codonInvolve individual genes
Point Mutations – 3 typesThe substitution, addition or removal of a single nucleotide
1. Substitution – a point mutation where one nucleotide in a codon is replaced with a different nucleotide, resulting in a new codonEx. Sickle Cell Anemia – sub. Of A for T in a single codon
2 & 3. Insertion and Deletions – one or more nucleotides is lost or added – have more serious effects
Frameshift MutationWhen a nucleotide is lost or added so that the remaining codons are grouped incorrectlyInsertions and deletions are frameshift mutations
THE FAT CAT ATE THE RAT
PolyploidyCondition in which an organism has an extra set of chromosomes3N, 4NUsually fatal in animalsPlants – usually more robustCaused by - Nondisjunction
10-3 Regulation of Gene Expression
As biologists have intensified their studies of gene activity, it has become clear that interactions between different genes and between genes and their environment are critically important
Gene InteractionsGene – piece of DNA – DNA codes for proteinsIn many cases the dominant allele codes for a protein that works and the recessive allele codes for a protein that does not work
Incomplete DominanceWhen offspring have a phenotype that is in-between the two parentsOccurs when two or more alleles influence the phenotypeExample – flowers – four o’ clocks, snapdragonsAlleles – R/R’, R/r, R/W, FR F r
RedFlower
WhiteFlower
PinkFlower
Red mixed with white makes pink
Incomplete Dominance Example #2
Incomplete dominance is a half way between point.
Halfway to dark blue is light blue.
Incomplete Dominance is not a blending.
RR rr Rr
Phenotypic Ratio: 1:2:1
Genotypic Ratio:
1:2:1
CodominaceOccurs when both alleles for a gene are expressed in a heterozygous offspringNeither allele is dominant or recessiveExample – horse coat color
Horse Coat ColorRed – HR HR
White – HWHW
Roan – HR HW
Roan – red and white hairs
Blue roan - The coat has white hairs and blue hairs
Polygenic InheritanceTraits controlled by two or more genesExamples – height, skin color, coat patternsPhenotypes are seen in a range
AB Ab aB ab
AB AABB AABb AaBB AaBb
Ab AABb AAbb AaBb Aabb
aB AaBB AaBb aaBB aaBb
ab AaBb Aabb aaBb aabb
Polygenic Inheritance
Gene Expression in Prokaryotes
Genes serve as a pattern for the production of mRNAmRNA serves as the instructions to make a proteinAll the genes of an organism can’t be active all the time
When a cell needs a product it must be able to make it fastWhen the product of a gene is being made we say the gene is being expressed
Genes are:Rarely expressedConstantly expressedTurn on and off
The Operon
Genes that work together are clustered togetherSome genes in the cluster do not code for proteins instead they are involved in regulation and expression
OperonGenes that work togetherOperatorPromotorThere is slight overlap between the operator and the promotor
InducerMolecule that causes the production of a protein
To Make a ProteinRNA polymerase must attach to the promoter (“Start here”)Moves along to the genes mRNA
The RepressorSpecial proteinAttaches itself to the RNA between the promotor and the genesDoes not let RNA polymerase make a proteinTurns off genes
Each repressor has a special shape that allows it to attach to a specific piece of RNA
Gene ActivationWhen an inducer enters a cell it binds to the repressorThe repressor changes shape and can no longer bondRNA polymerase can then attach
Proteins eats up the inducer repressor attaches againEx. Lactose – sugar – food for bacteria
Gene Expression in Eukaryotes
More complex than EukaryotesMore DNA in a nucleus1976 – Sharp and BergetDiscovered mRNA produced during transcription may be altered before it is used to make a protein
DNA mRNA not an exact copy as was thought – not complementary
ExonsSequences that code for a proteinExpressed sequences
IntronsSegments that do not code for a proteinIntervening sequencesIN the way