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9.2.2.1. Mendel's first law or principle of uniformity in the first generation

Mendel's First Law

All hybrid offspring resulting from the cross between two homozygous individuals  are equal to each other.

When two pure races are crossed (a dominant homozygous with a recessive one) for a certain trait, all the descendants of the first generation (F1) will be equal to each other, phenotypically and genotypically, and phenotypically equal to one of the parents (genotype dominant), regardless of the direction of the cross. The corresponding character is called dominant (A = yellow), and recessive  (a = green) which is masked.

A = Yellow seed; a = green seed.           A > a

That is:

P          AA * aa        or       aa * AA

1         All    Aa

1        100%   All yellow seeds.

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Genetics problems solved: Practice with exercises in genetics of Mendel's first law (4th ESO). It contains genetic problems that have come out in EvAU exams from several Universities.

Madrid, July 2019, option A, question 1

Regarding Mendel's contributions to the study of heredity:

a) State Mendel's First Law. Explain what the genotype of the parents is like if, in an experimental cross between pea plants, it is observed that a recessive trait "a" is manifested in all the offspring (0.5 points).

b) Explain the different manifestation of a recessive character linked to the X chromosome in a man and in a woman, and cite an example (0.75 points).

c) Relate each concept in the left column with a definition in the right column (0.75 points).

1. Intermediate inheritance      A. Identical alleles for the same locus
2. Linked genes                          B. Diagram of the genetics of several generations of a family
3. Pedigree                                  C. Inherited alleles for a gene
4. Homozygous                          D. Representation of the possible offspring of a cross
5. Punnett square                       E. They are on the same chromosome
6. Genotype                                 F. Phenotype mixture of two purebreds