Mendel’s law of inheritance: Monohybrid cross

Red flowered and white flowered pea plants used in cross pollination are the plants of parental generation or P1 generation. The traits in pea plant depends on the factor called genes. Genes are always present in pair. At the time of gamete formation genes separates and segregates. This is known as law of segregation. In P1 generation two types of gametes are produced. One gamete possess the genes

• The first generation of monohybrid cross: The cross pollination of pea plants having red flower and white flowers produced pea plants with red flowers. As one parent had white flowers there must be some difference in red flowered plant of parental generation .
• The second generation of monohybrid cross: To find this difference Mendel self pollinated f1 generation plants, which produced second generation. Therefore, plants in second generation had both red as well as white flowered pea plants







• Mendel’s observation: In parental generation, the cross pollination between pea plant having dominant red trait with pea plant having recessive white trait that produces offspring with red traits. This is because dominant trait masks the recessive trait.
  Thus, plants in parent generation and first generation are phenotypically same but genotypically different. In second generation, during the gamete formation, pair of genes i.e. Rr segregates to form one type of gamete R or r. Here, phenotypically there are two types of plants that is red flowered and white flowered , however, genotypically there are three types of plants RR, Rr and rr. In this case pure dominant is RR and pure recessive is rr. RR and rr are homozygous and Rr is heterozygous.

Mendel’s law of inheritance: Dihybrid cross

In dihybrid crossing of plants, Mendel experimented on two pairs of contrasting characters that are shape of seed and color of seed. He crossed pea plants having round and yellow seeds with pea plants having wrinkled and green seeds. The parental generation possess, one parent with round and yellow seed or RRYY. And other parent have wrinkled and green seeds or rryy. During formation of gametes of plants in P1 generation each pair of genes separates and produces gametes RY and ry.

• First Generation: The cross pollination between the two parents produces genotype RrYy. Since, this genotype have dominant R and dominant Y therefore phenotype F1 generation is plant having round yellow seed. F1 generation plant produces four types of gametes RY, Ry, rY, ry. Here, RY and ry are similar to gametes of P1 generation called parental combination. And remaining two Ry, rY are called recombinations.







• Second Generation: Later, mendel self pollinated the plant of F1 generation to produce second generation. In this generations there are sixteen different combinations. However, in punnett square, male gametes are arranged on top and female gametes are arranged on side. In this experiment mendel produced 556 pea plants out of which 315 were round yellow seeds, 101 wrinkled yellow, 108 were round green and 32 wrinkled green seeds. Thus the phenotypic ratio is round yellow RY 9: wrinkled yellow rY 3: round green Ry 3: wrinkled green ry 1 and the genotypic ratio is 1 : 4 : 1 : 1 : 2 : 2 : 2 : 2 : 1