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 .