Mendel Genetics and Punnett
Squares
Origin of Genetics
Steps of a Monohybrid cross
Terminology
Practice
Punnett Squares
Determining Unknown
Genotypes
The Origin of Genetics
• Heredity- the passing of characteristics
from parents to offspring
• Genetics- the branch of Biology that
focuses on heredity
• Cross- combining traits from two
parents by breeding (sexual
reproduction)
The Origin of Genetics
• Gregor Mendel
– Austrian monk
– Studied theology, science and
mathematics
– Noticed patterns in the pea plants he
was growing
– Mendel crossed pea plants and
studied the genetic turn-outs
– Was the first to develop rules that
accurately predict patterns of
heredity
– Father of genetics
The Origin of Genetics
Monohybrid cross- a cross that involved
one pair of contrasting traits
Ex. Crossing a white flower with a
purple flower.
Steps of a Monohybrid cross
1. Ensure each variety is true-breeding
for a particular characteristic
Ex. A true-breeding purple flower will
produce only purple flowers
•
These are the 1st two individuals crossed
- P generation (parental generation)
Steps of a Monohybrid Cross
2. Cross-pollinate two P-generation
plants
Ex. A true breeding purple flower
crossed with a true-breeding white
flower
•
The offspring are called F1 generation
(filial generation)
Steps of a Monohybrid Cross
3. Self-pollinate the F1 generation to create the F2
generation (second filial generation)
– This creates a 3:1 ratio
Self pollinate
Self pollinate
Cross
Self pollinate
True breeding
P generation
F1 generation
F2 generation
Terminology
• Alleles- alternative versions of genes
– The gene for hair color can have multiple
alleles (blonde hair allele, brown hair
allele, etc)
• Dominant- the expressed (visible) trait
• Recessive- the trait that is not
expressed or observable
Terminology
• Homozygous- when two alleles of a particular
gene are the same
– (PP), (pp), (RR), (rr), (VV), (vv), etc
• Heterozygous- when two alleles of a
particular gene are different
– Only the dominant allele is expressed while the
recessive is present, but not expressed
– (Pp), (Rr), (Vv), etc
Terminology
• Genotype- the set of alleles an individual has
for a trait
– think “genes” and “letters”
– (Rr), (RR) or (rr)
– (Vv), (VV) or (vv)
• Phenotype- the physical appearance of a trait
– Think “face” and “colors”
– White, purple, red, blonde, freckles, blue eyes, etc
Practice
(assume that P is purple and p is white)
(Pp):
What is the genotype?
What is the phenotype?
Is it homozygous or heterozygous?
Practice
(assume that P is purple and p is white)
(pp):
What is the genotype?
What is the phenotype?
Is it homozygous or heterozygous?
Practice
(assume that P is purple and p is white)
(PP):
What is the genotype?
What is the phenotype?
Is it homozygous or heterozygous?
Punnett Squares
• Punnett square- is a diagram that
predicts the outcome of a genetic cross
by considering all possible
combinations
Punnet Squares
• Ex. Having freckles (F) is dominant to not
having freckles (f). Parent 1 is heterozygous
for freckles and Parent 2 is also heterozygous
Parent 1
F
Parent 2
f
F
F F
Ff
f
Ff
ff
Phenotype
ratio?
3:1
Punnett Squares
Box 1 is homozygous for….
Box 2 and 3 is…..
Box 4 is….
F
F
f
FF
Ff
1
f
Ff
3
2
ff
4
Punnett Squares
• Ex. A homozygous yellow pea plant (Y) is
dominant over a homozygous green pea
plant (y).
Y
Y
What’s the genotype of
all offspring?
Yy
y
Yy
Yy
What’s the phenotype of
all offspring?
yellow
y
Yy
Yy
What’s the phenotypic
ratio?
4:0
Determining Unknown
Genotypes
• When a phenotype is dominant but the
genotype is not known, a test cross is
performed.
– You cross the unknown genotype with a
homozygous recessive individual
Determining Unknown
Genotypes
What does that mean?
You can physically see that the pea plant is
yellow, but you don’t know if they are (YY) or
(Yy).
So, you cross the yellow pea plant with a
recessive pea plant (yy) that is green.
If the offspring are all yellow the genotype is
(YY)
If the offspring are ½ yellow and ½ green the
genotype is (Yy)
Determining Unknown
Genotypes
• Doesn’t make sense? Use the Punnett
Square and work backwards!
Known, recessive genotype
y
Y
Unknown
genotype
???
y