Gregor Mendel
 In 1865, Gregor Mendel published
studies of inheritance in pea plants.
 During sexual reproduction, male and
female gametes (reproductive cells; n)
join to form a zygote (2n) in the process
known as fertilization.
Heredity is…
1. Controlled by “factors.” (known as
genes on chromosomes)
2. Passed from generation to generation.
3. Predicted Outcomes through Probability
using Punnett Squares.
1902 – Chromosome Theory
of Heredity
 Genes (Mendels “factors”) are located on
chromosomes.
 The different forms (variations) of a gene
are called Alleles.
 This is the basis for the modern science of
Genetics.
Genetic Principles
 Dominant and Recessive
 Dominant = seen when present on the chromosome
 Recessive = only seen when dominant allele is not present on the chromosome
 Law of Segregation and Independent Assortment
 Randomness
 Placement of chromosomes within the process of meiosis
 Genotype and Phenotype
 Genotype – using letters to represent the trait’s alleles
 Ex: Flower Color - FF, Ff, ff
 Phenotype – using descriptive words (adjectives) to describe the trait’s alleles.
 Ex: Flower Color – Purple or white
 Homozygous and Heterozygous
 Homozygous = same allele variations (TT, tt)
 Heterozygous = difference allele variations (Tt)
Genes VS Alleles
Gene
Allele
Genes VS Alleles
Gene
Plant Height
Flower Color
Tall Plant
Purple Flowers
Short Plant
White Flowers
Allele
R.C. Punnett
 In 1905, R.C. Punnett introduced
the Punnett square.
 Punnett squares can be used to
predict and compare the genetic
variations that will result from a
cross (pg. 268).
Using Punnett Squares
Yellow pea seed (Y) is DOMINANT to green (y).
Genotype:
YY
x
yy
Phenotype:
Yellow
x
green
Parents (P)
y
y
Y
Y
Using Punnett Squares
Yellow pea seed (Y) is DOMINANT to green (y).
Genotype:
YY
x
yy
Phenotype:
Yellow
x
green
Parents (P)
y
y
Y
Y
Using Punnett Squares
Yellow pea seed (Y) is DOMINANT to green (y).
Genotype:
YY
x
yy
Phenotype:
Yellow
x
green
Parents (P)
Y
y
Yy
y
Y
Using Punnett Squares
Yellow pea seed (Y) is DOMINANT to green (y).
Genotype:
YY
x
yy
Phenotype:
Yellow
x
green
Parents (P)
Y
y
Yy
y
Y
Using Punnett Squares
Yellow pea seed (Y) is DOMINANT to green (y).
Genotype:
YY
x
yy
Phenotype:
Yellow
x
green
Parents (P)
Y
y
Yy
y
Yy
Y
Using Punnett Squares
Yellow pea seed (Y) is DOMINANT to green (y).
Genotype:
YY
x
yy
Phenotype:
Yellow
x
green
Parents (P)
Y
y
Yy
y
Yy
Y
Using Punnett Squares
Yellow pea seed (Y) is DOMINANT to green (y).
Genotype:
YY
x
yy
Phenotype:
Yellow
x
green
Parents (P)
Y
Y
y
Yy
Yy
y
Yy
Using Punnett Squares
Yellow pea seed (Y) is DOMINANT to green (y).
Genotype:
YY
x
yy
Phenotype:
Yellow
x
green
Parents (P)
Y
Y
y
Yy
Yy
y
Yy
Using Punnett Squares
Yellow pea seed (Y) is DOMINANT to green (y).
Genotype:
YY
x
yy
Phenotype:
Yellow
x
green
Parents (P)
Y
Y
y
Yy
Yy
y
Yy
Yy
Using Punnett Squares
Yellow pea seed (Y) is DOMINANT to green (y).
Genotype:
YY
x
yy
Phenotype:
Yellow
x
green
Parents (P)
Y
Y
y
Yy
Yy
y
Yy
Yy
All (100%) offspring in F1 Generation are Yellow hybrids.
Monohybrid Cross
Yy
x
Yy
Yellow
x
Yellow
F1 Generation
Y
y
Monohybrid Cross
Genotype:
Yy
Phenotype: Yellow
F1 Generation
Y
y
Y
x
Yy
x
Yellow
y
Monohybrid Cross
Genotype:
Yy
Phenotype: Yellow
F1 Generation
Y
y
Y
x
Yy
x
Yellow
y
Monohybrid Cross
Genotype:
Yy
Phenotype: Yellow
F1 Generation
Y
Y
YY
y
x
Yy
x
Yellow
y
Monohybrid Cross
Genotype:
Yy
Phenotype: Yellow
F1 Generation
Y
Y
YY
y
x
Yy
x
Yellow
y
Monohybrid Cross
Genotype:
Yy
Phenotype: Yellow
F1 Generation
Y
Y
YY
y
Yy
x
Yy
x
Yellow
y
Monohybrid Cross
Genotype:
Yy
Phenotype: Yellow
F1 Generation
Y
Y
YY
y
Yy
x
Yy
x
Yellow
y
Monohybrid Cross
Genotype:
Yy
Phenotype: Yellow
x
Yy
x
Yellow
F1 Generation
Y
y
Y
YY
Yy
y
Yy
Monohybrid Cross
Genotype:
Yy
Phenotype: Yellow
x
Yy
x
Yellow
F1 Generation
Y
y
Y
YY
Yy
y
Yy
Monohybrid Cross
Genotype:
Yy
Phenotype: Yellow
x
Yy
x
Yellow
F1 Generation
Y
y
Y
YY
Yy
y
Yy
yy
Monohybrid Cross
Genotype:
Yy
Phenotype: Yellow
x
Yy
x
Yellow
F1 Generation
Y
y
Y
YY
Yy
y
Yy
yy
Genotype Ratio:
25% (1/4) offspring are YY, Ratio is 1:4
(homozygous dominant : total)
50% (2/4) offspring are Yy, Ratio is 1:2
(heterozygous : total)
25% (1/4) offspring are yy, Ratio is 1:4
(homozygous recessive : total)
Phenotypic Ratio:
75% (3/4) offspring are Yellow; Ratio
is 3:4 (Yellow : total)
25% (1/4) offspring are green; Ratio
is 1:4 (green : total)
Dihybrid Cross
 Round pea seed (R) is dominant to
wrinkled pea seed (r).
 FOIL: YyRr

YR, Yr, yR, yr
 (These are the possible genotypic
combinations)
Dihybrid Cross
Round pea seed (R) is dominant to
wrinkled pea seed (r).
Possible
Combinations
YR
Yr
yR
yr
YR
Yr
yR
yr
Dihybrid Cross
Round pea seed (R) is dominant to
wrinkled pea seed (r).
Possible
Combinations
YR
Yr
yR
yr
YR
YYRR
YYRr
YyRR
YyRr
Yr
YYRr
YYrr
YyRr
Yyrr
yR
YyRR
YyRr
yyRR
yyRr
yr
YyRr
Yyrr
yyRr
yyrr
Possible
Combos
YR
Yr
yR
yr
YR
YYRR
Yellow, Round
YYRr
Yellow, Round
YyRR
Yellow, Round
YyRr
Yellow, Round
Yr
YYRr
Yellow, Round
Yyrr
Yellow, wrinkled
YyRr
Yellow, Round
Yyrr
Yellow, wrinkled
yR
YyRR
Yellow, Round
YyRr
Yellow, Round
yyRR
green, Round
yyRr
green, Round
yr
YyRr
Yellow, Round
Yyrr
Yellow, wrinkled
yyRr
green, Round
Yyrr
green, wrinkled
What is the probability that the plant will produce:
Yellow, Round Peas:
Yellow, Wrinkled Peas:
Green, Round Peas:
Green, Wrinkled Peas:
Possible
Combos
YR
Yr
yR
yr
YR
YYRR
Yellow, Round
YYRr
Yellow, Round
YyRR
Yellow, Round
YyRr
Yellow, Round
Yr
YYRr
Yellow, Round
Yyrr
Yellow, wrinkled
YyRr
Yellow, Round
Yyrr
Yellow, wrinkled
yR
YyRR
Yellow, Round
YyRr
Yellow, Round
yyRR
green, Round
yyRr
green, Round
yr
YyRr
Yellow, Round
Yyrr
Yellow, wrinkled
yyRr
green, Round
Yyrr
green, wrinkled
What is the probability that the plant will produce:
Yellow, Round Peas:
9/16
Yellow, Wrinkled Peas: 3/16
Green, Round Peas:
3/16
Green, Wrinkled Peas:
1/16
To find the probability that
two different events will
happen at the same time,
multiply the probabilities
of each event happening
separately.
To find the probability that two different events will
happen at the same time, multiply the probabilities of each
event happening separately.
Probability of offspring having YELLOW pea
seeds:
¾
Probability of offspring having ROUND pea
seeds:
¾
Probability of offspring having BOTH YELLOW
AND ROUND PEAS:
¾ X ¾ = 9/16
Trihybrid Cross
 Tall Stem (T) is dominant to short stem (t).
 The probability of producing tall stems in a
monohybrid cross is:
 What is the probability that the offspring in a
trihybrid cross will be Yellow, Round, and Tall?
Trihybrid Cross
 Tall Stem (T) is dominant to short stem (t).
 The probability of producing tall stems in a
monohybrid cross is:
¾
 What is the probability that the offspring in a
trihybrid cross will be Yellow, Round, and Tall?
¾ X ¾ X ¾ = 27/64