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Gene (trait)
Alleles (variations of trait)
-Flower Color
-Hair type
-Plant Height
-Purple, blue, white
-curly, straight, wavy
-Short, tall
Create a gene and a corresponding
allele on your whiteboard
Gene: _____________
Allele: ________, _________
 Phenotype
 Physical appearance terms
 ex. Tall, short, brown hair etc.
 Genotype
 Genes in cell, written with letters that represent
the genes (ex: “T” for tall, or “t” for short)
• Punnett Squares: diagram that determines the probable
distribution of inherited traits in the offspring.
• UPPERCASE letters are used to represent dominant
genes (traits).
• lowercase letters are used to represent recessive
genes (traits).
Homozygous: Both alleles are the same. Either both
capitols (dominant) or both lower-cased (recessive)
Example:
•BB (homozygous dominant)
•bb (homozygous recessive)
Heterozygous: Both alleles are different. One capital
letter and one lower-cased letter.
Example:Bb
1.
RR=
2.
Ll=
3.
Yy
4.
Ee
1.
A fish is homozygous recessive for fin color
2.
A cat is heterozygous for tail length.
3.
A turtle is homozygous dominant for shell
color.
4.
A polar bear is homozygous recessive for
claw length.
Sample Problem
• T represents the gene for TALL in pea
plants
• t represents the gene for short in pea plants
So:
TT & Tt both result in a TALL plant, because
T is dominant over t.
t is recessive.
What will tt result in?
Remember there are two genes for every trait!






Write the 3 possible genotypes and
phenotypes for the following:
Green eyes are dominant to blue eyes
(use the first letter of the dominant trait)
GG=
Gg=
gg=






Write the 3 possible genotypes and
phenotypes for the following:
Curled toes are dominant to straight toes
(use the first letter of the dominant trait)
CC=
Cc=
Cc=
Let’s cross a homozygous tall plant (TT) with a short
plant (tt).
Each plant will give only one of its’ two genes to the
offspring.
TT x tt
T
T
t
t
Each gene (allele) separates from the other so that the
offspring get only one gene from each parent for a given
trait.
TT x t t
T
T
t
t

Predict the offspring if a tall plant (TT) is
crossed (mates) with a short plant (tt)
TT x tt
Tt
Tt
The genes from one parent go here.
The genes from the other parent go
here.
Tt
t
T
T
t
Tt
Tt
t
Tt
Tt
If a box receives a
capital letter and a lowercased letter we write the
capitol letter first
T
T
t
Tt
Tt
t
Tt
Tt
Possible offspring Genotype =
Possible offspring Genotype = Tt.
Possible offspring Genotype = Tt.
What percent of offspring will have the
following genotypes:
TT=
Tt=
tt=
Possible offspring Genotype = Tt.
What percent of offspring will have the
following genotypes:
TT= 0%
Tt= 100%
tt= 0%
Possible offspring Genotype = Tt.
What percent of offspring will have the
following genotypes:
TT= 0%
Tt= 100%
tt= 0%
Possible offspring phenotype =
Possible offspring Genotype = Tt.
What percent of offspring will have the
following genotypes:
TT= 0%
Tt= 100%
tt= 0%
Possible offspring phenotype = tall.
Possible offspring Genotype = Tt.
What percent of offspring will have the
following genotypes:
TT= 0%
Tt= 100%
tt= 0%
Possible offspring phenotype = tall.
What percent of offspring will have the
following phenotypes:
Tall=
Short=
Possible offspring Genotype = Tt.
What percent of offspring will have the
following genotypes:
TT= 0%
Tt= 100%
tt= 0%
Possible offspring phenotype = tall.
What percent of offspring will have the
following phenotypes:
Tall= 100%
Short= 0%
Black rabbits are dominant to brown. Two
heterozygous rabbits have offspring. Predict what
percentage of their offspring will be brown.
Black rabbits are dominant to brown. Two
heterozygous rabbits have offspring. Predict what
percentage of their offspring will be brown.
Step 1: Determine the genotype of parents. Always
use the first letter of the dominant trait.
____________x _____________
Black rabbits are dominant to brown. Two
heterozygous rabbits have offspring. Predict what
percentage of their offspring will be brown.
Step 1: Determine the genotype of parents. Always
use the first letter of the dominant trait.
____________x _____________
Step 2: Set up a Punnett Square
Step 2: Set up a Punnett Square
Step 3: Complete Punnett Square
Step 4: Predict offspring phenotype.
Black rabbits are dominant to brown.
Two heterozygous rabbits have
offspring. Predict what percentage
of their offspring will be brown?
Your Turn!!
Tall plants are dominant to short. Cross two
heterozygous pea plants.
1. Determine parent’s genotype and write on
whiteboard



Set up a punnett square and fill it in.
Hold up whiteboard
T
t
T
TT
Tt
t
Tt
tt

Determine the genotype percentages and
write on whiteboard
 TT=
 Tt=
 Tt=
Genotype percentage : TT – 25%
T
t
T
TT
Tt
t
Tt
tt
Genotype percentage: TT – 25%, Tt – 50%
T
t
T
TT
Tt
t
Tt
tt
Genotype percentage: TT – 25%, Tt – 50%, tt – 25%
T
t
T
TT
Tt
t
Tt
tt

Determine the Phenotype percentages and
write on whiteboard
 Tall=
 Short=
Genotype percentage: TT – 25%, Tt – 50%, tt – 25%
T
t
T
TT
Tt
t
Tt
tt
Phenotype percentage: Tall – 75%
Genotype percentage: TT – 25%, Tt – 50%, tt – 25%
T
t
T
TT
Tt
t
Tt
tt
Phenotype percentage: Tall – 75%, short – 25%
Round seeds are dominant to wrinkled seeds.
 Rr x rr What percentage of the offspring will
be round? _______________

Classwork: Punnett Packet Pages 1-?

Homework: Punnett Packet Pages ?-? (due
tomorrow)