If you analyze the data in the table qualitatively, and quantitatively you will find that the ratio of dominant to recessive characteristic is 3:1 in the f2.
Why did the recessive characteristic in each pair disappear completely in the F1 generation, then reappear in the F2 generation??
Why did the two characteristics appear in the F2 generation, dominant: recessive in the ratio 3:1??
The answer these questions as follows:
The appearance or disappearance of characteristics (in F1) or their appearance together (in F2) is controlled by factors (now called genes). Mendel assumed that these factors exist in pairs. The factors of each pair separate (segregate) during the formation of gametes (pollen grains, and ovules). Mendel assumed that each individual carries pairs of factors for controlling the allelomorphic characteristics (a pair of factors for controlling each allelomorphic characteristic).
Law of Segregation of Factors:
When two pure members that differ in any pair of allelomorphic characteristic are crossed, only the dominant characteristic appears in the F1 generation, while the two characteristics appear in the F2 generation in the ratio of 3 dominant : 1 recessive.
N.B.:
1. The factors controlling the allelomorphic characteristics exist in pairs in each individual.
2. The organism is said to be Homozygous (pure) for a characteristic if both factors for a given characteristic, taken each from one parent are exactly alike.
3. The organism is said to be Heterozygous (hybrid, or impure) for a characteristic if the two factors for a given characteristic are different; a dominant factor from one parent, and a recessive factor from the other.
4. Using the symbols:
The factor of the dominant characteristic is represented by the first letter of the English name and is written in the capital form, whereas the factor of the recessive characteristic is represented by the same letter but in its small form.
Ex.: For the height of the stem in Pea plant:
T: Represents the factor for tall stem in Pea (which is the dominant characteristic).
t: Represents the factor for short stem in Pea (which is the recessive characteristic).
So, Pure (homozygous) tall-stemed Pea plant is symbolized by TT.
While the hybrid (heterozygous) tall-stemed Pea plant is symbolized by Tt.
And the the short-stemed Pea plant is symbolized by tt.
5. The individual that carries the recessive characteristic is always pure (homozygous).
6. There are letters and signs that are usually used when representing genetic crosses such as:
(X): The sign of the occurrence of mating (conjugation or cross).
(P): The sign of parentental individuals.
(G): The sign of the parental gametes.
(F1): Members of the first filial generation.
Genetic representation:
Using Punnett square: This square resembles the chess square. On its top, the female gametes are put, while the male gametes are put on the left of the square in a perpendicular way, or vice versa.
Using the symbols.
Example:
Experiment 1:
If crossing takes place between a pure tall-stemed pea plant and another short-stemed pea plant, the F1 plants of this cross will be all tall-stemed pea plants, as shown in the following 2 figures:
Short-stemed pea plant Tall-stemed pea plant
P1 tt X TT
G1
♂ ♀
|
T |
T |
t |
Tt |
Tt |
t |
Tt |
Tt |
F1 Tt
Tall-stemed pea plants
100%
N.B.:
- The genotypes of the parents: tt and TT.
- The phenotypes of the parents: Short-stemed pea plant and tall-stemed pea plant.
- The genotype of all the progeny is Tt.
- The phenotype of all the progeny is tall-stemed pea plant.
Experiment 2:
When the individuals of F1 are left for self-pollination, fertilization takes place, and F2 plants are obtained. They are tall-stemed pea plants, and short-stemed pea plants in the ratio 3 tall-stemed : 1 short stemed as follows in the following 2 figures:
Tall-stemed pea plant Tall-stemed pea plant
P2 Tt X Tt
G2
TT
Tt Tt
tt
25
50 25
F2 Tall-stemed Short-stemed
Pea plants pea plants
3 : 1
♂ ♀
|
T |
t |
T |
TT |
Tt |
t |
Tt |
tt |
Many experiments and researches continued the study of the inherited characteristics. Theses included animals, plants, and Man. It was found that Mendel’s first law is applicable to most of the inherited characteristics. The following represents the inheritance of some of these characteristic in animals and Man:
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