Principle of complete dominance and Law of segregation of factors ( Mendel’s first law )
The dominant trait is the trait that appears in all the individuals of the first generation, The recessive trait is the trait that disappears completely in the individuals of the first generation.
Mendel repeated the same experiment on the seven traits of the pea plant and he got the same results, where he observed that one of the two traits appeared in all the offspring of the first generation and he named it the dominant trait, The other trait disappeared completely in the first generation and he named it the recessive trait.
The principle of complete dominance is the appearance of a dominant hereditary trait in the individuals of the first generation when two individuals are crossed, one of them carries the pure trait contrasting the trait carried by the other individual.
Mendel made several assumptions to explain the results he obtained in the experiments of the pea plant, these assumptions are:
In a living organism, every hereditary trait is controlled by two hereditary factors or (two genes) one from the father and the other from the mother, These factors are similar or homozygous if the trait is pure and they are different or heterozygous if the trait is impure.
The living organism that carries an impure trait is called Hybrid, The hereditary traits are transferred from the parents to the offspring by these factors ( the gens ) through the gametes.
Each gamete carries only one factor ( one gene ) for each hereditary trait because during gamete formation by meiosis the two hereditary factors ( genes ) for each trait is separated from each other.
Thus Mendel deduce what happens in his experiment as follows :
When the gametes carrying the yellow colour factor meet the gametes carrying the green colour factor , the yellow colour factor dominates , This leads to the production of only yellow seed in the first generation.
Pure Yellow + Pure Green → Hybrid Yellow
When the gametes of first generation are produced by meiosis, the hereditary factors are separated from each other, These factors meet again in the production of second generation.
Hybrid Yellow + Hybrid Yellow → Yellow + Green
If the yellow colour factor (homozygous) meets the green colour factors, the result is a yellow seed ( carries an impure trait hybrid plant ) , If the yellow colour factor ( homozygous ) meets another yellow colour factor ( homozygous ) , So , the result is the yellow seed ( carries the pure trait ).
If the green colour factor meets another green colour factor ( homozygous ), the result is a green seed ( carries a pure trait ), All these assumptions are the base of Mendel’s first law, and he named it law of segregation of factors.
Hybrid individual is the individual who carries a different ( contrasting ) pair of genes one is dominant and the other is recessive, while the pure individual is the individual who carries a similar pair of genes either dominant or recessive.
law of segregation of factors (Mendel’s first law)
When two individuals of any pair of hereditary traits are different from each other, The dominant trait appears in the first generation only, while the two traits appear in the second generation in a ratio of 3 dominant : 1 recessive, The dominant gene prevents the appearance of the effect of the other gene, The recessive trait is always pure.
The dominant trait is pure or hybrid, It appears when two similar factors of dominant trait aggregate or when one factors of dominant trait and the other of recessive trait aggregate, It appears with a ratio of 100 % in the first generation and a ratio of 75 % in the second generation.
The recessive trait is always pure, It appears only when two similar factors of recessive trait aggregate, It disappears in the first generation and it appears with the ratio of 25 % in the second generation.