Which of the law was discovered first by Mendel:

The correct option is B

Law of segregation

The correct option is B:

Explanation of correct option:

1. The law of segregation states that at the time of gamete formation, the pairs of alleles have to undergo separation or segregation.
2. Gamete formation occurs before fertilization.
3. Hence, it was the first law proposed by Gregor Mendel.

Explanation of incorrect options:

Option A:

1. The law of dominance states that the trait that appears in the heterozygous state that masks the expression of other traits is called dominant, whereas the trait whose expression is masked is called recessive.
2. Law of dominance was observed after the first filial generation was obtained.

Option C:

1. Law of independent assortment states that in a dihybrid condition, during gamete formation, the alleles of two different genes get assorted independently without the effect of one allele over the other.
2. This law was postulated during the dihybrid cross.

Option D:

1. Law of sex determination is not a Mendelian law.
2. Mendel's contribution to plant inheritance led to the discovery of sex determination in human beings.
3. Humans have XX/XY sex-determination system.

Final answer: The law that was discovered first by Mendel was the law of segregation.