Mendelian Genetics

• Austrian Monk

• Father of Genetics

• Pea Plant Experiments (1858-1868)

Site of Site of Gregor Gregor

Mendel’s Mendel’s experiment experiment - a garden - a garden

in the in the Czech Czech

RepublicRepublic

Mendel’s workplace

Fig. 2.5

Mendel stated that physical traits Mendel stated that physical traits are inherited as “particles”are inherited as “particles”Mendel did not know that the Mendel did not know that the “particles” were actually “particles” were actually Chromosomes & DNAChromosomes & DNA

Terms you need to know: TraitTrait - any characteristic that can - any characteristic that can

be passed from parent to offspring be passed from parent to offspring HeredityHeredity - passing of traits from - passing of traits from

parent to offspringparent to offspring Monohybrid crossMonohybrid cross - cross - cross

involving a single traitinvolving a single traite.g. flower color e.g. flower color

Dihybrid crossDihybrid cross - cross involving - cross involving two traits two traits e.g. flower color & plant heighte.g. flower color & plant height

Alleles Alleles - two forms of a gene (dominant & - two forms of a gene (dominant & recessive) recessive)

DominantDominant - stronger of two genes expressed in - stronger of two genes expressed in the hybrid; represented by a capital letter (R)the hybrid; represented by a capital letter (R)

RecessiveRecessive - gene that shows up less often in a - gene that shows up less often in a cross; represented by a lowercase letter (r) cross; represented by a lowercase letter (r)

GenotypeGenotype - gene combination for a trait (e.g. - gene combination for a trait (e.g. RR, Rr, rr) RR, Rr, rr)

PhenotypePhenotype - the physical feature resulting from - the physical feature resulting from a genotype (e.g. red, white)a genotype (e.g. red, white)

Terms you need to know:

Terms you need to know: Homozygous genotypeHomozygous genotype – –

When the two alleles are same When the two alleles are same (2 dominant or 2 recessive (2 dominant or 2 recessive genes) e.g. TT or tt; also genes) e.g. TT or tt; also called pure  called pure 

Heterozygous genotypeHeterozygous genotype – – When the 2 alleles are When the 2 alleles are different- one dominant & different- one dominant & one recessive allele    (e.g. Tt); one recessive allele    (e.g. Tt);

• Parental PParental P11 Generation Generation = the parental generation in a = the parental generation in a

breeding experimentbreeding experiment..

• FF11 generation generation = the first-generation offspring in a = the first-generation offspring in a

breeding experiment. (1st filial generation)breeding experiment. (1st filial generation)

– From breeding individuals from the PFrom breeding individuals from the P11 generation generation

• FF22 generation generation = the second-generation offspring in a = the second-generation offspring in a

breeding experiment. breeding experiment. (2nd filial generation)(2nd filial generation)

– From breeding individuals from the FFrom breeding individuals from the F11 generation generation

Mendel’s Pea Mendel’s Pea Plant Plant

ExperimentsExperiments

Why peas,Why peas, Pisum Pisum sativumsativum??

Can be grown in a small Can be grown in a small area area

Produce lots of offspring Produce lots of offspring Several generations in a Several generations in a

seasonseason Produce pure plants when Produce pure plants when

allowed to self-pollinateallowed to self-pollinate Can be artificially cross-Can be artificially cross-

pollinatedpollinated Many traits known Above all, easy to growAbove all, easy to grow

Reproduction in Flowering Reproduction in Flowering Plants Plants

• Pollen contains Pollen contains sperm. Pollen is sperm. Pollen is found on the stamenfound on the stamen

• Ovary contains Ovary contains eggs. Found inside eggs. Found inside the flower. Turns the flower. Turns into fruit after into fruit after fertilization of the fertilization of the egg cells by sperm.egg cells by sperm.

Mendel’s Experimental MethodsMendel’s Experimental Methods• Mendel hand-pollinated Mendel hand-pollinated

flowers using a paintbrush.flowers using a paintbrush.• He could snip (cut) the He could snip (cut) the

stamens to prevent self-stamens to prevent self-pollinationpollination

• He kept detailed records to He kept detailed records to trace traits through the trace traits through the several generationsseveral generations

How Mendel BeganHow Mendel BeganMendel Mendel produced produced purepure strains by strains by allowing allowing the plants the plants to to self-self-pollinatepollinate for for several several generatiogenerationsns

Eight Pea Plant TraitsEight Pea Plant Traits• Seed shapeSeed shape --- Round (R) or Wrinkled (r) --- Round (R) or Wrinkled (r)• Seed ColorSeed Color ---- Yellow (Y) or  Green (y) ---- Yellow (Y) or  Green (y)• Pod ShapePod Shape --- Smooth (S) or wrinkled (s) --- Smooth (S) or wrinkled (s)• Pod ColorPod Color ---  Green (G) or Yellow (g) ---  Green (G) or Yellow (g)• Seed Coat ColorSeed Coat Color ---Gray (G) or White (g) ---Gray (G) or White (g)• Flower positionFlower position---Axial (A) or Terminal (a)---Axial (A) or Terminal (a)• Plant HeightPlant Height --- Tall (T) or Short (t) --- Tall (T) or Short (t)• Flower color --- Flower color --- Purple (P) or white (p)Purple (P) or white (p)

Monohybrid Monohybrid CrossesCrosses

Punnett SquarePunnett SquareUsed to help Used to help

solve genetics solve genetics problemsproblems

• Trait: Seed ShapeTrait: Seed Shape• Alleles: Alleles: RR – Round – Round rr – Wrinkled – Wrinkled• Cross: Cross: RoundRound seedsseeds xx Wrinkled Wrinkled seedsseeds• RR RR xx rr rr

PP11 Monohybrid Cross Monohybrid Cross

R

R

rr

Rr

RrRr

Rr

Genotype:Genotype: RrRr

PhenotypePhenotype: RoundRound

GenotypicGenotypicRatio:Ratio: All alikeAll alike

PhenotypicPhenotypicRatio:Ratio: All alike All alike

• Trait: Seed ShapeTrait: Seed Shape• Alleles: Alleles: RR – Round – Round rr – Wrinkled – Wrinkled• Cross: Cross: RoundRound seeds seeds xx Round Round seedsseeds• RrRr xx Rr Rr

FF11 Monohybrid Cross Monohybrid Cross

R

r

rR

RR

rrRr

Rr

Genotype:Genotype: RR, Rr, RR, Rr, rrrr

PhenotypePhenotype: Round Round && wrinkled wrinkled

G.Ratio:G.Ratio: 1:2:11:2:1

P.Ratio:P.Ratio: 3:1 3:1

FF11 Monohybrid Cross Monohybrid Cross

ReviewReview Heterozygous x heterozygousHeterozygous x heterozygous Offspring:Offspring:

25% Homozygous dominant25% Homozygous dominant RRRR50% Heterozygous50% Heterozygous RrRr25% Homozygous Recessive25% Homozygous Recessive rrrr

Offspring calledOffspring called FF22 generation generation Genotypic ratio isGenotypic ratio is 1:2:11:2:1 Phenotypic RatioPhenotypic Ratio is 3:1 is 3:1

What Do the Peas Look Like?What Do the Peas Look Like?

• Did the observed ratio match the theoretical ratio?Did the observed ratio match the theoretical ratio?

The theoretical or expected ratio of The theoretical or expected ratio of plants producing round or wrinkled seeds plants producing round or wrinkled seeds is 3 round :1 wrinkledis 3 round :1 wrinkled

Mendel’s observed ratio was 2.96:1Mendel’s observed ratio was 2.96:1

The discrepancy is due to statistical errorThe discrepancy is due to statistical error

The larger the sample the more nearly The larger the sample the more nearly the results approximate to the the results approximate to the theoretical ratiotheoretical ratio

Dihybrid CrossYellow, Round x Yellow, Round

• YyRr X YyRr• Each parent can produce 4 types of gametes.

• YR, Yr, yR, yr

• Cross is a 4 X 4 with 16 possible offspring.

Dihybrid CrossDihybrid CrossRYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry

Dihybrid CrossDihybrid Cross

RRYY

RRYy

RrYY

RrYy

RRYy

RRyy

RrYy

Rryy

RrYY

RrYy

rrYY

rrYy

RrYy

Rryy

rrYy

rryy

Round/Yellow: 9

Round/green: 3

wrinkled/Yellow: 3

wrinkled/green: 1

9:3:3:1 phenotypic ratio

RYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry