Gregor Mendel

• “Father of modern genetics”• Researched with pea plants• Developed ideas about inheritance and

how parents pass the traits to their offspring.

Alleles• Mendel discovered that there are at least

2 different versions of each gene.• Today, we call those alleles. • Alleles can be dominant or recessive.• Dominant = allele that always shows its

trait• Recessive = allele that shows its trait only

if there are 2 copies

Alleles

• Alleles are represented by two-letter combinations. Example: RR, Tt, pp

• Dominant = capital letter (RR, Ww, Bb)• Recessive = lowercase letter (Rr, ww, mm)• The letter is often the first letter of the

dominant trait. (Ex. Round peas = Rr)• Dominant allele is always written first.

Dd, not dD

Genotype vs. Phenotype

• Genotype – Genes we inherit

from our parents– BB, Bb, bb

• Phenotype – Physical

characteristics– hair color, blood type

Phenotype

– Facial structure– Eyes– Smile– Ears– Nose

Notice the similarities:

Not all phenotypes are visible, BUT they are all determined by your genotype.

Mendelian GeneticsMendel studied a number of characteristics in pea plants including:

•Height - short or TALL

Mendelian GeneticsMendel studied a number of characteristics in pea plants including:

•Height - short or TALL•Seed color - green or YELLOW

Mendelian GeneticsMendel studied a number of characteristics in pea plants including:

•Height - short or TALL•Seed color - green or YELLOW•Seed shape - wrinkled or ROUND

Mendelian GeneticsMendel studied a number of characteristics in pea plants including:

•Height - short or TALL•Seed color - green or YELLOW•Seed shape - wrinkled or ROUND•Seed coat color - white or GRAY

Mendelian GeneticsMendel studied a number of characteristics in pea plants including:

•Height - short or TALL•Seed color - green or YELLOW•Seed shape - wrinkled or ROUND•Seed coat color - white or GRAY •Pod shape - constricted or SMOOTH

Mendelian GeneticsMendel studied a number of characteristics in pea plants including:

•Height - short or TALL•Seed color - green or YELLOW•Seed shape - wrinkled or ROUND•Seed coat color - white or GRAY •Pod shape - constricted or SMOOTH•Pod color - yellow or GREEN

Mendelian GeneticsMendel studied a number of characteristics in pea plants including:

•Height - short or TALL•Seed color - green or YELLOW•Seed shape - wrinkled or ROUND•Seed coat color - white or GRAY •Pod shape - constricted or SMOOTH•Pod color - yellow or GREEN•Flower position - terminal or AXIAL

Dominant vs. Recessive

•Two of the same alleles (RR or rr)• If they are both dominant, then they are homozygous dominant (RR). • What do you think it is called if they are both recessive?

•One dominant and one recessive allele (Rr)

Homozygous

Heterozygous

homozygous recessive

Predicting Inheritance

To determine the probability of inheriting a given trait, scientists use 4-square boxes called...

Punnett Squares

R rR RR Rrr Rr rr

Punnett Squares

R rR RR Rrr Rr rr

•Allows us to predict the possible genotypes and phenotypes of the offspring

•Show us the probability of getting a certain genotype or phenotype

Punnett Squares

Tt Tt

Tt Tt

The alleles from one parent go here.

The alleles from the other parent go here.

Punnett SquaresParents: TT x tt

T T

t Tt Tt

t Tt Tt

Punnett Squares

T T

t Tt Tt

t Tt Tt

Punnett Squares

T T

t Tt Tt

t Tt Tt

Punnett Squares

T T

t Tt Tt

t Tt Tt

Punnett Squares

T T

t Tt Tt

t Tt Tt

Punnett Squares

T T

t Tt Tt

t Tt Tt

Possible Genotypesof the Offspring

Interpreting the Results

T Tt Tt Ttt Tt Tt

The genotype for all the offspring is

The probability of offspring

genotype being Tt =

The phenotype for all the offspring is

The probability of offspring phenotype

being tall =

Tt

100%

tall

100%

Round or Wrinkled SeedParents: Rr x Rr

R r

R RR Rr

r Rr rr

Genotype Probabilities:

Phenotype Probabilities:

RR: 25% rr: 25% Rr: 50%

Round: 75%Wrinkled: 25%

Gray or White furParents: Gg x gg

G g

g Gg gg

g Gg gg

Genotype Probabilities:

Phenotype Probabilities:

Gg: 50% gg: 50%

Gray: 50%White: 50%

Red (dom) or Black EyesParents: Homozygous dom & homozygous

rec

R R

r Rr Rr

r Rr Rr

Genotype Probabilities:

Phenotype Probabilities:

Rr: 100% Red: 100%

Pink (dom) or white flowersParents: Homozygous dom & heterozygous

P P

P PP PP

p Pp Pp

Genotype Probabilities:

Phenotype Probabilities:

PP: 50% Pp: 50%

Pink: 100%