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