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3.2.1. Mendel's First Law

Mendel's First Law or Law of Uniformity

When two purebred varieties that differ in one character are crossed, the offspring is uniform, also presenting the dominant character.

Animation: Mendel's Laws.

PARENTAL GENERATION (P) Guisante verde Guisante amarillo

green x yellow  

1st GENERATION SUBSIDIARY (F1 Guisante amarillo

100% yellow

When Mendel, avoiding self-pollination, crossed two purebred plants for each of the characteristics, such as yellow or green pea color, smooth or rough seed, etc. observed that all the descendant plants of this cross were equal to each other and equal to one of the parents or parents. He called these first descendants of the crossing the first filial generation or F1. Thus, he obtained these results:
Parental Smooth x rough seed 1 100% smooth seed
Yellow x green seed 100% yellow seed
Sheath swollen x not swollen 100% swollen sheath
Lilac x white flowers 100% Lilac flowers

He called this trait, present in one of the parents, which manifests itself in the descendants, a dominant character, and which, being present in one of the parents, did not appear in the descendants, a recessive character.

With these data, Mendel enunciated his First Law or Law of F1 uniformity: when two individuals of two pure races are crossed for a character, the first generation of the hybrids obtained is uniform for said character.

Mendel thought that parents passed on to their descendants hereditary factors that made them have a certain character, such as the color of the seeds, for example. He assumed there were two hereditary factors, one coming from the father and one from the mother.


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Biology and Geology teaching materials for Compulsory Secondary Education (ESO) and Baccalaureate students.