Mendel's 3rd Law
If two individuals who differ in more than one character are crossed, these characters are transmitted independently of the rest.
The experiment is the same as with a character, but instead of observing a pair of chromosomes, we will have to do it with two pairs of chromosomes, such as a pair of chromosomes with the alleles that determine the color of the seed, and another pair of chromosomes with the alleles that determine the shape of said seed.
A = yellow seed a = green seed
B = smooth seed
b = rough seed.
F0 AABB
* aabb
Gametes
AB ab
F 1 AaBb
All individuals in the first generation will be diheterozygous (heterozygous for both characters). Its phenotype will be yellow and smooth seed, just like one of its parents.
By crossing the F1 individuals with each other, they will produce the following types of gametes:
F1 AaBb * AaBb
A a B b A a B b
Gametes: AB Ab aB ab
AB Ab aB ab
All the gametes of these individuals have the same possibility of forming, which is why the following zygotes can be obtained after fertilization:
|
AB
|
Ab
|
aB
|
ab
|
AB
|
AABB
|
AABb
|
AaBB
|
AaBb
|
Ab
|
AABb
|
AAbb
|
AaBb
|
Aabb
|
aB
|
AaBB
|
AaBb
|
aaBB
|
aaBb
|
ab
|
AaBb
|
Aabb
|
aaBb
|
aabb
|
In this way, the following phenotypic proportions are obtained:
9/16 A_B_ 3/16 A_bb
3/16 aaB_ 1/16 aabb
(9 smooth yellow seeds, 3 rough yellow seeds,
3 smooth green seeds, 1 green rough seed) .
If we analyze the results obtained in the Punnett square, we will observe that there are 16 different possible zygotes, although they only give rise to 9 different genotypes,
and these 9 genotypes only give rise to 4 different phenotypes:
16 types of zygotes
|
9 genotypes
|
4 phenotypes
|
Proportion
|
TOTAL
|
AABB
|
AABB
|
plain green
|
1/16
|
9/16
|
AABb
|
AABb
|
plain green
|
2/16
|
AABb
|
AaBB
|
AaBB
|
plain green
|
2/16
|
AaBB
|
AaBb
|
AaBb
|
plain green
|
4/16
|
AaBb
|
AaBb
|
AaBb
|
AAbb
|
AAbb
|
rough green
|
1/16
|
3/16
|
Aabb
|
Aabb
|
rough green
|
2/16
|
Aabb
|
aaBB
|
aaBB
|
yellow-smooth
|
1/16
|
3/16
|
aaBb
|
aaBb
|
yellow-smooth
|
2/16
|
aaBb
|
aabb
|
aabb
|
yellow-rough
|
1/16
|
1/16
|
That is, as the alleles go on different chromosomes, they separate in meiosis and combine in all possible ways, thus new phenotypes appear, which did not exist before.
Animation : Mendel's three laws.