GeneticsGenetics

Gregor MendelGregor Mendel

Gregor Johann Mendel Gregor Johann Mendel

► Gregor MendelGregor Mendel In 1865 turned the In 1865 turned the study of heredity into a science study of heredity into a science

► His work was so brilliant and His work was so brilliant and unprecedented at the time it unprecedented at the time it appeared that it took thirty-four appeared that it took thirty-four years for the rest of the scientific years for the rest of the scientific community to catch up to it.community to catch up to it.

► His short monograph, "Experiments His short monograph, "Experiments with Plant Hybrids," in which Mendel with Plant Hybrids," in which Mendel described how traits were inherited, described how traits were inherited, has become one of the most has become one of the most enduring and influential publications enduring and influential publications in the history of science. in the history of science.

Theory of Genetic BlendingTheory of Genetic Blending

► Before Mendel’s work, plant and animal Before Mendel’s work, plant and animal breeders based their work on the theory of breeders based their work on the theory of ““BlendingBlending”.”.

► This theory states that parents with different This theory states that parents with different traits produced offspring of intermediate traits produced offspring of intermediate appearance.appearance. Example: Red X White flowers = Pink flowersExample: Red X White flowers = Pink flowers

► When red or white flowers reappeared in future When red or white flowers reappeared in future generations it was due to the instability in the generations it was due to the instability in the genetic materialgenetic material

Mendel’s Experimental Mendel’s Experimental ApproachApproach

► Mendel Mendel experimented with experimented with pea plantspea plants

► This was a great This was a great model to do genetics model to do genetics onon Self fertilizingSelf fertilizing Cross fertilizeCross fertilize Breed trueBreed true Clear and contrasting Clear and contrasting

traitstraits

Genetic termsGenetic terms

► Genes:Genes: segments segments of DNA that provide of DNA that provide the information for the information for traits and traits and characteristicscharacteristics

► Locus: The location of Locus: The location of a specific gene on a a specific gene on a specific Chromosomespecific Chromosome

► Allele: alternative Allele: alternative forms of a gene.forms of a gene.

Locus, the locationfor a specific gene

A pair of alleles

Homologuous chromosomes

Three pairs of genes or Three pairs of alleles

Terms cont.Terms cont.

►Diploid is a 2n organism with two Diploid is a 2n organism with two complete sets of genetic information.complete sets of genetic information.

►Haploid is a 1n organism or cell with a Haploid is a 1n organism or cell with a single complete set of genetic single complete set of genetic informationinformation

►Homozygous is when both alleles for a Homozygous is when both alleles for a trait are the sametrait are the same

►Heterozygous is when the alleles for a Heterozygous is when the alleles for a particular trait are different.particular trait are different.

Terms cont.Terms cont.

► Dominant allelesDominant alleles: An allele that is : An allele that is expressed and hides or masks “expressed and hides or masks “RecessiveRecessive” ” alleles. alleles. Symbolizes as a capital letter.Symbolizes as a capital letter. Example “AA” in homozygous dominant.Example “AA” in homozygous dominant.

► Recessive alleleRecessive allele: An allele that is expressed : An allele that is expressed in the homozygous form only.in the homozygous form only. Symbolized as a lower case letterSymbolized as a lower case letter Example: “aa” in homozygous recessive.Example: “aa” in homozygous recessive.

Terms cont.Terms cont.

► Genotypes: The actual alleles that are Genotypes: The actual alleles that are present in an individual.present in an individual. Example:Example:

► Homozygous dominant = AAHomozygous dominant = AA► Heterozygous = AaHeterozygous = Aa► Homozygous recessive = aaHomozygous recessive = aa

► Phenotype: is the observable characteristic. Phenotype: is the observable characteristic. It is the combination of genes and It is the combination of genes and environmentenvironment

Terms cont.Terms cont.

►P P = = Parental generationParental generation

►F1 F1 = = The first generationThe first generation

►F2 = F2 = The off-spring from the The off-spring from the mating of mating of the first the first generation togethergeneration together

Mendel’s Monohybrid Mendel’s Monohybrid experimentsexperiments

►Realizing that the blending theory didn’t Realizing that the blending theory didn’t fit with what he was observing Mendel fit with what he was observing Mendel developed an alternative hypothesisdeveloped an alternative hypothesis

►Hypothesis: He said “Genes are Hypothesis: He said “Genes are particulate factors that passes unchanged particulate factors that passes unchanged from parent to progeny unchanged”.from parent to progeny unchanged”.

►Some traits mask or hide other traitsSome traits mask or hide other traits

Mendel’s Theory of SegregationMendel’s Theory of Segregation

► Monohybrid crossMonohybrid cross Crossing parents to Crossing parents to

observe a single trait observe a single trait

in the off-springin the off-spring

► Gene segregationGene segregation Each parent randomly Each parent randomly

contributes one set of contributes one set of

genetic information to genetic information to

the offspringthe offspring

Mendel’s Monohybrid CrossMendel’s Monohybrid Cross

Mendel’s Monohybrid CrossMendel’s Monohybrid Cross

Probability and Punnet Probability and Punnet SquaresSquares

► A possibility of outcomes and crossesA possibility of outcomes and crosses► Test crossesTest crosses

Unknown genotype crossed with Unknown genotype crossed with homozygous recessivehomozygous recessive

Independent AssortmentIndependent Assortment

► Do genes assort randomly in the Do genes assort randomly in the gamets?gamets? Do some genes prefer the company of Do some genes prefer the company of

other genes?other genes?

►Example: If one is good at music are Example: If one is good at music are also likely to be good a math as well?also likely to be good a math as well?

Fig. 11.9, p. 181

aB

AB

AB

abAb

Ab

aB

ab

1/4

1/4

1/4

1/4

1/4 1/4 1/4 1/4

AB ab

AABBpurple-

floweredtall parent

(homozygousdominant)

aabbwhite-

flowereddwarf parent(homozygous

recessive)

F1 OUTCOME: All F1 plants purple-flowered, tall(AaBb heterozygotes)

X

meiosis, gamete formation

AaBb AaBb

Possible outcomes of cross-fertilization

AaBb aabbAabb aaBb

AABB AABb AaBB AaBb

AABb AAbb AaBb Aabb

AaBb aaBB aaBbAaBB

1/161/161/161/16

1/161/161/161/16

1/161/161/161/16

1/16 1/16 1/16 1/16

ADDING UP THE F2 COMBINATIONS POSSIBLE:

9/16 or 9 purple-flowered, tall

3/16 or 3 purple-flowered, dwarf

1/16 or 1 white-flowered, dwarf

3/16 or 3 white-flowered, tall

Mendel’s Theory of Mendel’s Theory of SegregationSegregation

► Diploid cells have pairs of genes or Diploid cells have pairs of genes or

homologous chromosomeshomologous chromosomes

► During meiosis the two genes During meiosis the two genes

segregate and wind up in different segregate and wind up in different

gametesgametes

Independent AssortmentIndependent Assortment

Theory in Modern FormTheory in Modern Form

► Independent AssortmentIndependent Assortment

Gametes require genes Gametes require genes

independently of how other pairs of independently of how other pairs of

genes were sorted outgenes were sorted out

► Variety of OffspringVariety of Offspring

Dominance RelationDominance Relation

► Incomplete DominanceIncomplete Dominance Red snapdragon crossed with white Red snapdragon crossed with white

snapdragon snapdragon ---------> Pink F---------> Pink F11

► CodominanceCodominance Multiple allele systemMultiple allele system

►ABO Blood typesABO Blood types