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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%
