punnet squares “the law of independent assortment” alleles of different genes separate...
TRANSCRIPT
“The law of Independent Assortment”
Alleles of different genes separate completely from one another during
gamete formation
XY
X Y
Which means in Mendel’s flower experiments, he took a purebred purple
flower and crossed with a purebred white white flower.flower.
P = Purple color p = white color
=
=
PP Homozygous dominant
pp Homozygous recessive
A Punnett square - is used to predict the outcome of a genetic cross by considering all the possible combinations of alleles in the cross.
We are just going to concentrate on 1 trait for now.
So our cross would be PP x ppPurple White
A FEW RULES ABOUT PUNNET SQUARES
1. Alleles are listed alphabetically.
2. Dominant traits always are written before recessive for that allele. Rr rR
When using a punnett square, you take the first set of alleles and put them on top.
PP X ppPP
Then take the 2nd set of alleles and put them on the left side.
pp
Purple (P) is dominant over white (p)
So what is the phenotype of all the offspring?
P P
p
p
Pp Pp
Pp Pp
All Purple
P P
p
p
Purple (P) is dominant over white (p)
So what is the genotype of all the offspring?
Pp Pp
Pp Pp
All
Pp
When looking at Mendel’s flower experiment, he crossed the P1 purple with the P1 white flower and got 100% purple
flowers.
X
P1 F1
When doing the punnett square, we predicted they would all be 100%
purple. We were right!
P P
p
p
Pp Pp
Pp Pp
Then Mendel decided to cross a F1 purple flower with another F1 purple
flower.
So let’s try this with a punnett square. P P
p
p
Pp Pp
Pp Pp
What is the genotype for this cross in %?
P
P
p
p
PP Pp
Pp pp
25 % = PP (homozygous dominant) 1 out of 450% = Pp (heterozygous) 2 out of 425 % = pp (homozygous recessive) 1 out of 4
What is the Phenotype of the baby?
50% male 50% female
What is the Genotype of the baby?
50% XY 50% XX
Question: If the first baby is a boy, what is the chance the second child will be a boy?
X
X
X
Y
XX XY
XX XY
Answer: 50% - every time you have a baby , you start all over
Let’s practice a few Punnett squares to make sure you got it.
Do the following crosses on your paper and give me the phenotype and
genotype for the possible outcomes.
Tall plants (T) are dominant over short plants (t). Cross a purebred Tall with a
hybrid Tall. TT x Tt
T T
T
t
TT TT
Tt Tt
Phenotype 100% Tall
Genotype 50% TT
50% Tt
Green Peas (G) are dominant over Yellow Peas (g)
Cross a Hybrid Green Pea with a Hybrid Green Pea
G g
G
g
GG
Gg
Gg
gg
Phenotype
25% - GG 50% - Gg
25% - gg
Gg x Gg
3 Green: 1 yellow
Genotype
Black hair (B) is dominant to blonde hair (b) Cross a homozygous recessive blonde with a
heterozygous black haired person. bb x Bb
b b
B
b
Bb
bb
Bb
bb
50% black haired 50% blonde
50% - Bb 50% - bb
Phenotype
Genotype