All the problems we have encountered so far have involved the inheritance of only one trait.
Mendel discovered that one trait (seed color for instance) does not affect the other (seed shape for example).
This is the Law of Independent Assortment: that the “genetic factors”
involved in heredity move independently during meiosis.
Genes “R” and “Y” separate independently of each other
We now know this is true only if the genes are located on different chromosomes: “unlinked”
Eye Color
Hair Color
Eye Color and Hair Color are UNLINKED because they are on different chromosomes
Now we will examine cases in which two traits are involved
We will assume that the genes for these traits are carried on different chromosomes
Two traits:
Each gene coded for by a gene on a different chromosome
Each gene has two alleles
Gene | Dominant Allele | Recessive Allele |
(R) | R - round | r - wrinkled |
(Y) | Y - yellow | y - green |
The genotype for pure breeding round yellow is written as RRYY, while pure breeding wrinkled green is written rryy.
HOMOZYGOUS
Round Yellow (RRYY) | Wrinkled green (rryy) |
Only 1 allele of each gene ends up in the gametes because of the (Law of Segregation during meiosis)
End of M2 - haploid unreplicated
Review of Meiosis
Germ Cell
G1 - diploid unreplicated
G2 - diploid replicated
DNA replication during S phase
Meiosis 1
Meiosis 2
Meiosis 2
Gamete
Gamete
Gamete
Gamete
End of M1 - haploid replicated
End of M2 - haploid unreplicated
Only 1 allele of each gene ends up in the gametes because of the (Law of Segregation during meiosis), so the gametes of the pure breeding round yellow must be RY and for the pure breeding wrinkled green ry.
Round Yellow (RRYY)
Germ Cell
G1 - diploid unreplicated
G2 - diploid replicated
DNA replication during S phase
Meiosis 1
Meiosis 2
Meiosis 2
Gamete
Gamete
Gamete
Gamete
R
R
R
R
R
R
All the gametes are RY
R
R
R
R
R
R
R
R
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
End of M1 - haploid replicated
End of M2 - haploid unreplicated
Germ Cell
G1 - diploid unreplicated
G2 - diploid replicated
DNA replication during S phase
Meiosis 1
Meiosis 2
Meiosis 2
Gamete
Gamete
Gamete
Gamete
r
r
r
r
r
r
All the gametes are ry
r
r
r
r
r
r
r
r
y
y
y
y
y
y
y
y
y
y
y
y
y
y
End of M1 - haploid replicated
Wrinkled green (rryy)
Only 1 allele of each gene ends up in the gametes because of the (Law of Segregation during meiosis), so the gametes of the pure breeding round yellow must be RY and for the pure breeding wrinkled green ry.
When the pure breeding smooth RRYY are crossed with the pure breeding wrinkled green rryy, all the F1 offspring are round and yellow and have the genotype RrYy
RRYY
rryy
RrYy
WHY?
Because yellow is dominant over green…
and smooth is dominant over wrinkled.
Because the genes are not on the same chromosome, you can treat each trait independently.
Fertilization
meiosis
meiosis
ry
gamete
RY
gamete
Then the F1 plants were crossed (F1 x F1)
Round Yellow (RrYy)
Round Yellow (RrYy)
X
There are four possible unique gametes created from a RrYy plant.
The variation in gametes arises due to independent assortment during meiosis
independent assortment
Trick: the “foil” method to determine gamete genotypes
The potential genotypes of offspring of a RrYy x RrYy cross can be shown in a 16 box Punnett Square
The results in the F2 generation show a 9:3:3:1 ratio of phenotypes
TRY COUNTING YOURSELF!
New combinations of traits different from those shown in the parent line are called recombinants
In the RrYy x RrYy cross, the parents were round and yellow.
So, the recombinant offspring would be the ones that are:
The Punnett Square steps for multi-gene crosses are the same as for single gene crosses:
Example problem:
Seed color and seed traits are coded for by genes on different chromosomes.
R= round r = wrinkled
Y= yellow y = green
Cross a wrinkled-green pea with a round-yellow pea that is heterozygous for both traits.
X
Genes and alleles are given in the problem
Parent genotypes:
Green-wrinkled parent = rryy
Yellow-round parent = RrYy
Example problem:
Seed color and seed traits are coded for by genes on different chromosomes.
R= round r = wrinkled
Y= yellow y = green
Cross a wrinkled-green pea with a round-yellow pea that is heterozygous for both traits.
X
Green-wrinkled parent (rryy) can only make ry gametes
Yellow-round parent (RrYy) can make RY, Ry, rY and ry gametes
Example problem:
Seed color and seed traits are coded for by genes on different chromosomes.
R= round r = wrinkled
Y= yellow y = green
Cross a wrinkled-green pea with a round-yellow pea that is heterozygous for both traits.
X
Example problem:
Seed color and seed traits are coded for by genes on different chromosomes.
R= round r = wrinkled
Y= yellow y = green
Cross a wrinkled-green pea with a round-yellow pea that is heterozygous for both traits.
X
Example problem:
Seed color and seed traits are coded for by genes on different chromosomes.
R= round r = wrinkled
Y= yellow y = green
Cross a wrinkled-green pea with a round-yellow pea that is heterozygous for both traits.
X
Example problem:
Seed color and seed traits are coded for by genes on different chromosomes.
R= round r = wrinkled
Y= yellow y = green
Cross a wrinkled-green pea with a round-yellow pea that is heterozygous for both traits.
X
¼
¼
¼
¼
Example problem:
Seed color and seed traits are coded for by genes on different chromosomes.
R= round r = wrinkled
Y= yellow y = green
Cross a wrinkled-green pea with a round-yellow pea that is heterozygous for both traits.
X