Question

Mendel’s first law is
(a)Law of dominance
(b)Law of variation
(c)Law of independent assortment
(d)Law of segregation

Answer 1

Mendel’s first law is associated with the expression of alleles when they are present in a heterozygous condition. This law was proposed based upon the observations of the monohybrid cross.

Complete answer:
Mendel’s first law is the Law of Dominance. This law states that characters are controlled by discrete units called factors that were later found out to be genes. These factors (genes) occur in pairs. In the case of the heterozygous condition, where two dissimilar genes (alleles) are present, the gene which is dominant will express itself. This explains the phenotype of the $F_1$ generation where all the offsprings were dominant or the phenotypic ratio of 3:1 in the $F_2$ generation.

Additional Information: Let us look at the exceptions of the Law of Dominance.
Codominance: This is the phenomenon in which during the heterozygous condition of alleles, both the alleles express themselves. It can also be said that both the alleles are dominant on each other and express their phenotype. A classic example of this is the AB blood group in which the alleles for A and B antigen express themselves together.
Incomplete dominance: It is the phenomenon in which the dominant allele is not fully dominant over the recessive allele and thus a phenotype that is intermediate of the dominant and recessive phenotypes is seen in the F1 generation. This type of dominance is seen in the inheritance pattern of flower color in dog flowers. Red and white flowers are crossed to produce offsprings with pink (intermediate phenotype) colored flowers.
So, the correct option is ‘Law of dominance’.

Note: -The Law of segregation was also made on the basis of the observation of the monohybrid cross whereas the Law of Independent Assortment was based on the dihybrid cross.
-The Law of Segregation states that during gamete formation, the factors or alleles segregate from each other in a way that the gamete has only one allele present in it.
-The Law of Independent assortment states that genes assort independently of each other during gamete formation.