GENETICS
Gregor Mendel: The Father of Genetics
PUNNETT SQUARES AND HEREDITY
MENDEL
• Austrian Monk• Interested in Biology
• Bred Pea Plants for specific traits• Was able to show that certain traits
were “inherited”—meaning they were passed down from one
generation to the next
Genetics Heredity
BIOLOGICALSTUDY
ofHeredity
Passing on of a characteristic from parents to offspring.
Pea Plants
Reproduce Sexually, which means egg and sperm.
Mendel studied seven traits of pea plants
Setting up a Cross
C C
C C
A A
B
B
A=Parent 1 Alleles
C=Offspring Genotype
B=Parent 2 Alleles
Setting up a CrossBasic Terminology
P1 = Parents
F1 = 1st Generation of Offspring
F2 = 2nd Generation of Offspring
Setting up a CrossBasic Terminology
Dominant Allele:Only need only copy to be expressed phenotypically.
Uppercase letter = dominant allele
Recessive Allele:
Need two copies to be expressed phenotypically.
Lowercase letter = recessive allele
NOTE: Use letter of dominant trait for
cross.
Homozygous Dominant x Homozygous Recessive
Dominant is Round Recessive is Wrinkled
Monohybrid Cross: Seed Shape
R R
R R
r
r
r r
Rr Rr
Phenotype(s):
Parent 1:
Parent 2:
Offspring:
Genotype(s):
Parent 1:
Parent 2:
Offspring:
Round Seed
Wrinkled Seed
Round Seed
R R
r r
R r
Y
Y
y
y
y y
Y y y
Phenotype(s):
Parent 1:
Parent 2:
Offspring:
Genotype(s):
Parent 1:
Parent 2:
Offspring:
Yellow Seed
Green Seed
½ green, ½ yellow
Y y
y y
½ Y y, ½ y y
Heterozygous x Homozygous RecessiveDominant is Yellow Recessive is
Green
Monohybrid Cross: Seed Color
y
y
y
G
G
g
g
Gg g
Phenotype(s):
Parent 1:
Parent 2:
Offspring:
Genotype(s):
Parent 1:
Parent 2:
Offspring:
Green Pod
Green Pod
¾ Green, ¼ Yellow
G g
G g
¼ GG, ½ Gg, 1/4 gg
Heterozygous x HeterozygousDominant is Green Recessive is
Yellow
Monohybrid Cross: Pod Color
g
g
GG G
I
I
I
i
I i i
Phenotype(s):
Parent 1:
Parent 2:
Offspring:
Genotype(s):
Parent 1:
Parent 2:
Offspring:
Inflated Pod
Inflated Pod
Inflated
I i
I I
¼ I I, ¾ I i
Heterozygous x Homozygous DominantDominant is Inflated Recessive is
Constricted
Monohybrid Cross: Pod Shape
i
I
II I
A few things to keep it simple….• Homozygous Recessive (2 recessive alleles)• Homozygous Dominant (1 recessive allele)• Heterozygous (1 dominant, 1 recessive
allele)• Letter for cross is determined by the first
letter of the dominant allele.• Capital Letter always comes first.• Parent #1 Goes across the top of the punnett
square and is mentioned first in the cross.• Parent #2 Goes down the side of the punnett
square and is mentioned second in the cross.